Animals make choices based upon their knowledge of the environment and their own phenotype to maximize their ability to reproduce and survive. ~ English zoologist George Lovell
Too much has been made by evolutionary biologists that life’s purpose is propagation. The repertoire of animal behaviors and their motives are too expansive to characterize life so singularly.
That said, breeding plays an outsized role in every individual inclined toward sex, precisely because the desire is so keenly felt. Biology impels the mind. Many animals sacrifice their lives to bring forth the next generation.
The desire for a mate is especially compelling for animals where the partnership extends beyond mating. Pet fish left alone in a tank after a partner dies have been known to commit suicide by leaping from the water and dying on the floor.
Monogamy-minded zebra finches are choosy about their mates. Like humans, finch mating preferences are individual, based upon life experiences and personality makeup.
Zebra finch fathers participate in childcare. Successful upbringing of offspring depends upon a couple being happy together. This too resembles the human experience.
Then there is the joy of sex, which varies widely. For many, however driven to enact it, sex is a mirthless moment. At the other end of the spectrum, the compelling stimulus to sex can be a biochemical reward: a pleasure that often accompanies the act, or at least its prelude. Sex is fun. This especially tends to be the case in gregarious animals.
Biochemical structures evolved many hundreds of millions of years ago that provide positive reinforcement for life-engendering behaviors. Emotional payoff for sex is adaptive to species lifestyle; just another life-history variable. (A life-history variable is a trait which may be considered a trade-off from an evolutionary perspective.)
Homosexuality is common among animals that are not innately bisexual, especially social species. Both males and females engage in same-sex bonding. Homosexuality has been observed in 1,500 species, from gut worms to fish to birds to primates.
Homosexuality only sometimes excludes heterosexual activity. There are numerous exceptions. Monogamous species, such as penguins, form exclusive same-sex pair bonds.
Rabbitfish reproduce in mass aggregations, but they still pair up for companionship. Not all pair bonds are heterosexual.
25% of black swan pairings are homosexual. The parenting urge remains. Male pairs may steal nests or form a temporary threesome with a female to obtain eggs. Once she has laid, the mated male pair drive the female off.
10% of male domestic sheep refuse to mate with ewes, but readily do so with other rams. Their sexual orientation is quite decided.
For all the cunning involved to getting sex, it is a primal urge, not unlike the drive to eat. The two may mix.
Some birds trade sex for food. Male hummingbirds may keep a female from select flowers until she offers herself, whereupon a few sips are on the house before she is sent on her way.
Roadrunner females squat in the mating position for a mate returning home with a lizard, but squat not for an empty beak coming back. 3 out of 4 roadrunner matings involve a delivered meal.
A female osprey incubating nest eggs relies upon her mate to bring her food. If the mate is late, for a passing stranger she might trade a trout for a fornication.
Food, sex, and bonding are not an uncommon combination. For enjoyment, mated blue jays pass tidbits beak to beak. Or they may just symbolically touch beaks.
Sometimes the lure of food is a ruse to attract the opposite sex. Male water mites attract mates by vibrating their legs at a frequency matching the small copepods that are favored fare. To enhance their allure, male orchid bees bathe themselves in the flower scents frequented for nectar.
Swordtail characins live in the rivers of Trinidad, eating hapless insects that plop into the water from riverbank vegetation. Male characins have a thin cord that extends from their gills. The cord ends in an ornament of sorts. A male dangles this ornament in front of female, who mistakes it for a bite to eat. When she gets close enough to bite the lure, the male is in a good position to inseminate her.
Mating selection varies considerably among animals. Females as the choosy sex is contravened in several species, commonly because only dominant males have mating rights.
Such is the case with humans. High-status males have more offspring, at least in less economically developed societies.
The link between a man’s status and reproductive success is broken only in societies where women are granted social rights and access to contraception. Then the lower socioeconomic strata breed more fecundidly than the upper classes, and male social status in the lower strata may factor in.
There is a biological basis for striving for status: it’s universally rewarded with the only currency that matters in biology – children. ~ Swiss anthropologist Adrian Jaeggi
Less often, females compete for a male mate. This typically occurs when males are heavily vested in parental care.
In seahorses and pipefish, females deposit their eggs in a special brooding pouch that a male has. As parenting is entirely the male’s responsibility, a male chooses his mate.
Some species of male poison frogs care for offspring until the little ones old enough to be on their own. Females compete for the opportunity to leave their eggs with one of a limited number of males.
Male wattled jacanas care for eggs that have been laid: incubation, defending the nest, feeding offspring. This extended male parental engagement means that the right to lay eggs in an established nest is extremely competitive.
In most species, mating is occasional, often seasonal. Exceptions include hyenas and humans, whose females are fecund year-round.
A prelude to mating often involves profound physiological changes, especially in females, chemically if not overtly.
Female mammals typically become sexually receptive (estrus) when the timing of birth coincides with habitat conditions that are conducive to rearing young, which is often late spring. If conditions are unfavorable, many mammals can postpone pregnancy by delaying embryo implantation.
Males too may change for the mating season. Male moose and deer prepare for competitive mating rituals by growing antlers, which peak in time for mating in autumn, and are shed for winter.
Sexual reproduction is but one facet of animal mating. Sex is often the quickest and easiest part. Much goes into choosing a mate, courtship, and ensuring that sex results in a next generation.
Spider mating and sex are often tricky business. Females are often several times larger than males.
A female nursery-web spider insists that a potential mate bring her a tasty treat: a juicy, freshly killed arthropod wrapped in silk.
Some males play dead while holding the wedding present in their mouths. When a female approaches to take the bait, the male springs up and tries to mate.
Several spider and fly species curry to nuptial gifts as a mating right. A redback spider male takes gift-giving to the extreme: somersaulting into his partner’s mouth during copulation; risking death for the sake of prolonging sex.
Widow spiders are so named for the feminine practice of making a meal of her mate after mating. A male widow approaches a prospective mate’s web warily: producing whispery low-amplitude vibrations by shaking its abdomen. The quality of this courtship calling card differs from clumsy prey caught in the web.
The nutritional benefit of a female eating her mate is advantageous to future offspring. Moreover, male sacrifice often offers the opportunity for extended copulation that allows more sperm to be passed on. Males that try to escape suffice with a shorter sex session.
Male cannibalism is practiced by a few spiders. The males of the ground spider Micaria sociabillis sometimes eat females they do not care to mate with. This happens more often with males born in the summer, which are usually larger than spring-born spiders. Older females are typically targeted as a meal ticket.
In spiders, males generally experience a high risk of sperm competition because females can store fertile sperm after copulation for a prolonged time. ~ Czech arachnologist Lenka Sentenská et al
Male Micaria sociabillis spiders insert mating plugs into females they have sex with so that their sperm is not overridden by a subsequent suitor. Copulation duration and plug production are controlled by the female. Both sexes of this spider are choosy about who they couple with.
There has been much to debate about how animals choose a mate, most notably the degree to which mating is instinctual or a conscious decision. Biological bias plays a strong hand in selection, including in humans.
Across the animal kingdom, familiarity between individuals affects mating preferences. Familiarity can breed contentment with a partner.
Or not. Guppy females prefer novel males over ones familiar to them, thereby promoting genetic diversity. A single brood of guppies may be sired by up to 12 fathers.
Conversely, a female Japanese rice fish prefers a mate she knows. So too with many rodents.
Prairie voles mate for life. A vole lady prefers a male she knows, in the hope that he may be steady on.
Courtship behaviors are among Nature’s greatest dramas. Sandhill cranes perform an elaborate ballet. Bald eagles display stupendous flying cartwheels.
Where choice is abundant, conventional thinking is that a female chooses a mate based on attributes that the next generation of females will find favorable. Peahens presumably choose peacocks with colorful plumage in their long tails or pay the price that their offspring will not attract the females that ensure a succeeding generation in the family line.
Whatever signals convey fitness in a species are commonly compelling to females with a choice. Surprisingly, in numerous species females don’t always make the obvious choice.
Further, in many species, females have little or no choice. Males competitively determine relative fitness, and thereby earn the right to breed with as many mates as can be guarded. Females submit.
In some species, female submission to the dominant male comes with the caveat of extra-pair mating if the opportunity arises. All told, mating selection is seldom simple.
The diversity of mating systems is an evolutionary consequence of several factors, ranging from the spacing of resources, genetic relatedness, and the odds of offspring survival under various conditions.
Monogamy is the state of having a single mate. In contrast, polygamy involves multiple mates. The common form of polygamy is polygyny, where a male mates multiple females.
In polyandry, a female mates with more than 1 male. Polyandry is rare because males typically can make the most of their reproductive potential by having multiple mates, or at least one to oneself.
Oversexed Marsupial Mice
When practiced, polyandry can be exhausting. The marsupial mouse is a small shrew-like marsupial endemic to southeast Australia.
Males mate with many females in marathon sessions. The task is so taxing that all mating males die before their young are born.
A female stores sperm in a specialized container in her ovary. She does not ovulate until the end of the breeding season. Her brood is typically sired by several fathers.
The typical biological goals of male and female differ at least somewhat. Males, able to sow their seeds far and wide, have an innate preference for quantity over quality. As bearer and caregiver, a female is more quality oriented. This innate conflict between males and females affects mate selection and parental care.
Fathers are biological necessities but social accidents. ~ American cultural anthropologist Margaret Mead
A female sage grouse picks a mate from a lek. Once mated, she has no need for the either the male or his territory. She leaves to rear her young alone.
A lek is a gathering of males for competitive courtship display; a marriage market. During breeding season leks may be a daily occurrence.
In a lekking reproductive system, males provide no resources to females or parental care for their offspring. Species adopt leks when females are dispersed, and when males offer little more to the females than the prospect of a decent genetic package. In some species, the males at a lek are related.
There are 2 types of lek: classical and exploded. In a classical lek, males gather within sight of each other. Physical contests are a frequent mating ritual, such as with shorebirds and game birds. Peacocks lek to display their tails.
Freshwater African cichlid fish lek for a construction contest. Males build sandcastles that can take up to 2 weeks to build. Lekking males with tall mounds win a mate.
Exploded leks rely on vocal displays. A lek may be more geographically dispersed.
The voice box of a male hammer-headed bat takes up more over half its body cavity. In breeding season, male bats gather for several hours at dawn and dusk, honking for females to fly by and pick a mate.
Túngara frog males aggregate into a choral lek and croak for sex. The larger the chorus, the more females approach. Fortunately for the frogs, frog-eating bats, which pick up on the chorus calls, are less likely to attack a lek than soloists.
Kakapo are a unique flightless New Zealand parrot, also called the owl parrot. Kākāpō is Māori for night parrot.
Kakapo are one of the longest-living birds, with an average life expectancy of 58 years, and a maximum of ~90 years.
The Kakapo is the heaviest and only flightless parrot: herbivorous, nocturnal, a low basal metabolic rate, quite sexually dimorphic, no paternal offspring care, and the only parrot with a polygynous lek mating system. Kakapo males position themselves kilometers apart and lek with booming calls.
Kakapo are on the cusp of extinction. 82 mya, New Zealand broke off from Gondwana; 1 of 2 supercontinents at the time (the other was Laurasia).
The kakapo speciated ~70 mya. Having no predators, the parrot lost its ability to fly.
Polynesian and European colonization introduced kakapo predators: rats, cats, stoats, and ferrets, which nearly wiped the kakapo out. Early conservation efforts by humans, from the 1890s into the 1980s, were ineffectual. More recent plans have only slowed the kakapo’s inevitable demise. As of spring 2018, fewer than 150 kakapos were alive.
A mung is where females congregate for males to pick a mate. Yellow-spotted millipedes mung.
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The limited signal lekking reproductive system presents biologists with a paradox: how females can tell the best mates. Consistent female choice for a valued trait should erode genetic variance, which removes the genetic benefit of choice. Further, hypothetically, males could cheat: if courtship displays confer no survival advantage, a good show signifies nothing.
Another theory explains the paradox by treating a fine display as an honest signal of virility and robustness: croaking well means you won’t croak easily. This theory at least confounds the charlatan speculation. The mystery remains as to mate selection and genic quality, but the system works.
Female insects commonly have multiple mates. The last mate is mostly likely to fertilize a female’s eggs.
It was long taken for granted that insects stay together after mating so that a male can prevent a female from taking another partner. That is not true for crickets. A cricket male stays with its mate to protect her and die for her if necessary.
Such chivalry is not without self-interest. The female carries the family lineage. Nevertheless, it also a strong statement of mate commitment, and of deep caring, that has been long presumed to be beyond insects.
Some species of mantis shrimp are monogamous: partnering for their entire adult lives of up to 20 years. They share the same burrow and coordinate their daily lives. Both partners take turns taking care of eggs. In some species the female lays 2 clutches of eggs: one for dad to care for and one for mom.
Social monogamy tends to evolve when both parents are needed for offspring care. By contrast, when a single mother may fend for herself and her offspring, a male may benefit from desertion and mating with another.
As crickets and mantis shrimp illustrate, that is not always the case; but it remains as something of an idealized norm.
Adult sex ratio may have an important role in shaping mating behaviour in a wide range of organisms. ~ Hungarian ornithologist András Liker
Even among nominally monogamous animals that share parental responsibilities, cheating and divorce often occur, depending upon social circumstance. The following examples involving birds are merely exemplary. The same dynamics apply to several other animals, including humans.
A major factor in couples staying together is the sex ratio. If there are a lot of unpaired members of the opposite sex, the rarer sex has more opportunity to attract partners.
A female bias in sex ratio destabilizes the pair-bond system, as males are more easily enticed away.
Conversely, extra-pair mating becomes more common when there are more males than females. Females may shop around for a better mate. Rape may become more prevalent if males are unable to secure a permanent mate, as men have amply demonstrated.
Birds lay eggs which are incubated outside the female’s body. The young are often fed by laborious food foraging. Incubation and infant feeding can be done by a male or female but are more effective if both parents are involved.
As rearing a clutch is demanding, ~90% of bird species are monogamous, at least for the duration of bringing up a brood, if not for life. As with other monogamous animals, there is often genuine affection between avian mates. Pair-bond behaviors, such as mutual preening, maintain a loving relationship.
The largest exception to monogamy comes when hatchlings can quickly feed themselves. Precocial birds, such as chickens, ducks, and geese, tend to polygyny because hatchlings are covered in down and capable of being led to food. Males add little to that situation, as contrasted to altricial offspring, which need the foraging efforts of 2 birds if hatchlings are to survive.
Females may choose males because of the resources they defend, often territory. Even if a male does not directly contribute to an offspring’s care, his defending the nest against predators or intruders of the same species is helpful.
This does not necessarily lead to monogamy. One male may host several female nestings in his territory and look out for them all.
The great reed warbler is migratory. Males arrive at the breeding grounds before females and stake out territories. The females are attracted by the quality of a male’s real estate. A successful male may nest multiple females in his territory. Alas, a polygynous male may not live up to his advertising. 2nd- and 3rd-arriving females have less breeding success. An already-mated male may deceive follow-on females: placing them in reed beds in his large territory that hide the different mates from one another.
A male pied flycatcher defends multiple territories simultaneously, placing a female in each, deceiving his mates that each pairing is monogamous. The male may benefit, but the female is worse off for not having chosen an unmated male.
Males may adjust the amount of care they give offspring depending upon genetic relatedness. Dunnock males feed their nestlings more when they are the likely father.
Both starling parents care for their young. While the pair are socially monogamous, starlings are not sexually exclusive. 3 or 4 males may be fathers of a single brood.
Climate affects an avian female’s choice of mate and her fidelity. Female lark bunting preferences vary considerably from year to year, depending upon climactic conditions and associated assessment as to what male traits may help ensure healthy offspring.
The more that climate appears unpredictable, the more inclined a female is to infidelity, as the improvement in genetic diversity gives her better odds of lasting reproductive success.
Boobies are large tropical seabirds, closely related to gannets. A half-dozen booby species range throughout the Atlantic, Pacific, and Indian oceans.
Boobies were named for their fearlessness, which meant that mariners could easily capture them. Instead of giving the birds credit, the Spanish named them bobo: slang for stupid. So much for sailor smarts.
The blue-footed booby lives in the eastern Pacific, from southern California to northern Peru, and on the Galápagos Islands.
Booby bills are sleek; their bodies cigar shaped. Booby wings are long, narrow, and angular: perfect for circling high above the sea, looking for shoals of fish, and schools of squid. Once a prey is sighted, a bird folds in its wings and plunges headlong into the water; its bill a missile honing in.
If an attempted snag is unsuccessful, a booby may swim in pursuit. Blue-footed boobies can also dive from a sitting position on the water’s surface.
Since the blue-footed booby dives headlong into the water for its meals, its nostrils are permanently shut. The bird must breathe through the corners of its mouth. Boobie skulls have special air sacs that protect their brains from the severe impact of hitting the water at 97 km per hour.
A booby may hunt alone, in pairs, or among a flock. A social outing may comprise a dozen or so boobies. When a lead booby spies a fish shoal, it signals the group and they all dive in unison, their bodies a synchrony of arrows.
Male and female blue-footed boobies fish differently. This may explain why blue-foots, unlike other boobies, raise multiple chicks.
A male blue-foot is slightly smaller, but with a longer tail. This enables males to swim in shallow waters as well as deep. The larger female can carry more food.
When fishing for themselves, boobies tend to go alone or in pairs; usually in the early morning or late afternoon. They eat while still underwater.
Hunting groups head out midday to forage for takeout. Both male and female boobies feed their chicks through regurgitation.
Boobies nest in colonies but have a highly developed sense of territory. Many ritualized displays are used to signify and defend individual turf within a large breeding colony, including head nodding and jabbing.
Courtship also involves a whimsical-looking dance that is anything but. A male raises his feet, one after the other several times, then gives a sky-pointing gesture: wings extend horizontally and toward the tail, raising its bill and emitting a long, drawn-out whistle.
If a female is interested, she responds similarly. The 2 blue-foots dance together.
Both male and female blue-footed boobies are choosy about their partner. The one trait they fixate on is how blue their potential partner’s feet are. The best blue is turquoise. It comes from extracting the carotenoids in the fish they eat and concentrating the pigment in their feet to render a dazzling aquamarine.
Unsurprisingly, blue-footed booby eyes are keenly sensitive to the blue-green part of the light spectrum; for good reason. The hue of the foot reveals health and hardiness.
Many boobies change mates from season to season but there are great benefits to fidelity. Boobies in lasting relationships raise 35% more offspring than newlywed couples. The key to a successful long-term partnership is equitable sharing of nest duties. While biparental care is the rule of booby chick rearing, longtime mates have perfected the art of taking turns. They spend equivalent time and effort brooding and feeding their young.
Another mating pattern works nearly as well for boobies as high fidelity: a couple where one partner is young and the other much older (the so-called May–December effect). It does not matter whether the mother is younger than the father or vice versa. The progeny of age-mismatched parents are more likely to become parents than are offspring of pairs whose ages are similar.
Boobies usually lay 2 eggs several days apart. This gives the older chick an enormous edge over its younger sibling.
In some booby species, this difference is often fatal. The only hope for a junior booby is the death of the 1st-born.
Among blue-foots, sibling violence is provisional. As long as parents are ample providers, a senior chick tolerates the younger hatchling. Woe to the 2nd-born if the 1st is not well fed.
Youth is still tortuous for the younger. An elder fledgling takes all the food that it can, and chronically pecks to demand submissive behavior, like facing away with its bill tucked down. If a 2nd-born makes it through its first months, its prospects are just as good as its sibling.
Bullying in infancy does not make wimpy adults. ~ Mexican ornithologist Oscar Sánchez-Macouzet
No matter how badly treated as a chick, upon maturity, a once-persecuted booby is surprisingly unflappable. 2nd hatchlings are just as capable of attracting partners, repelling rivals, and raising families as their domineering elder peers. Fearless indeed.
Many avian females solicit copulations from alluring males. They are often trading up.
Females are likely to have twice as many grandchildren if they mate with an extra-pair male than if they remain truly monogamous. ~ English ornithologist Nicole Gerlach
Blue tit females mated to attractive high-quality males are less inclined to extra-curricular copulations than females mated with lower-quality mates.
Blue tits are territorial. A female may find the best males taken and so must settle for less in a nest mate. Extra-pair mating may get her quality offspring and a supportive caretaker.
2nd-rate males are more often abandoned by their own female when she is fertile, receive fewer visits by neighboring females, have fewer offspring, and themselves survive less well. Whereas females fare well by divorcing their mates, males rarely ever wind up doing better.
By contrast, females in lekking species rarely copulate with more than 1 male. These females have free choice of a mate, regardless of what other females are doing.
With many birds, availability breeds lust. Avian extra-pair matings shoot up with population density. But not for burrowing owls. In the wild, burrowing owl nests are typically 15 km apart. Human urbanization brings nearby nesting opportunities. Nests may be as close as 10 meters.
Despite close proximity, burrowing owls remain monogamous. Other factors besides opportunity determine their mating practices. The need for parental care from both parents is a strong inhibitor of cheating.
Like burrowing owls, barn owls are monogamous. But 25% of barn owls eventually divorce, when breeding does not prove successful. The marital goal of a barn owl remains to find a compatible mate and stay together.
Swans are the largest of the waterfowl family, and among the biggest flying bird. The largest swan species – including mute, trumpeter, and whooper – can reach 1.5 m in length, with 3-meter wingspan, and 15 kg girth. Males are generally larger and heftier than females.
The fidelity of swans is legendary. They are monogamously mated year-round for much of their 20-year life. The longer a swan pair are together, the better they get at raising chicks. Unlike geese and ducks, swan males help build the nest, which is fiercely guarded by both.
Many seabirds form lasting relationships, almost always after considerable acquaintance and assessment. Repetitive mating rituals serve this courtship purpose, as well as forming emotional bonds between a new pair.
Albatrosses are as faithful as swans. Giant petrels are mobsters in their social habits but have marriages of model monogamy.
There can be marked mating contrast between similar birds. Gannets have long-term relationships whereas somewhat similar frigatebirds change partners each bout of breeding.
Gannets colonially nest in crowded conditions on rocky seaside cliffs. The timing of breeding is critical to chick survival. A good nesting space is hard to find, so partners return to the same spot year after year. Monogamy suits their situation, to the point of having developed elaborate bonding rituals.
Frigatebirds too have elaborate rituals, though for mating courtship, not partner bonding. That’s the closest similarity between gannets and frigatebirds in the pairing department.
Frigatebirds nest on tropical islands. There are no clear seasons. Food supply is uncertain. Parenting is arduous. It takes more than a year to rear a chick.
So frigatebirds find a mate when conditions are favorable and get on with the business of breeding the next generation. There are no loving displays at a frigatebird nest: they generally ignore each other.
Besides birds, several monkey species are monogamous. In long-lived animals, a male is prone to abandon a female if the paternity of a brood is in question. Where paternal care is important, this reinforces monogamy.
With their mammaries, female mammals have infant nursing built in. Only where a male can contribute substantially – in protection and/or paternal care – does monogamy appear in mammals.
House mice employ a complex olfaction-based communication network. Even without direct encounter, a mouse knows the sex and social status of another from the scent of its urine.
A pregnant female will spontaneously abort a litter she is carrying if a strange male mouse is in the house. However maladaptive this may seem, it is instead making the most of unfortunate choice.
Male mice tend to kill newly born mice that they did not father. Rather than risk carrying a litter to term that will be killed, a female reabsorbs her embryos; starting anew with a new male; thus raising the odds of offspring surviving.
Owing to chronically limited food supplies, jackals are territorial. They are also monogamous, as defending territory is much easier by working together. Jackal males contribute parentally by feeding pups through regurgitation.
Prairie voles too are territorial and monogamous. The bond is lasting. Couples pair off and live together throughout their adult lives. The huddle and groom each other. They share parental care. While pair-bonded females are typically aggressive toward unfamiliar males, some will have a fling if the opportunity arises. Other vole species vary in their mating systems: some tending toward monogamy, others not.
Monogamous monkeys include the titi, night monkey, and the gibbon. The tiny tarsier is the standout monogamous primate.
Tamarins are squirrel-sized New World monkeys, arboreal inhabitants of tropical rainforests. They live in groups of up to 40, comprising 1 or more families, though small groups of 3 to 9 are more frequent. Each family group centers on a single dominant breeding female. Young females reaching sexual maturity either leave the group or remain and don’t breed. Multiple males mate with the dominant female. Twins are typical with tamarins. A few days after birth, the babies are carried by the males in the group and only handed back to mom for nursing.
Marmosets, closely related to tamarins, are similar in mating and parenting practices. Marmosets live in extended family groups of 3 to 15, including unrelated companions. Their mating system is variable, altering between monogamy, polygyny, and occasionally polyandry.
Marmosets practice alloparenting. Females other than the mother and males carry young through months of dependency. Marmosets scent-mark home ranges and defend them, but are not entirely territorial, as home ranges often overlap between groups.
In mammals, female gestation and lactation means that a male may have little to contribute in improving the chance of his offspring surviving. His reproductive interest may be in seeking more mates, not in parental care. Hence, promiscuity and polygyny are the most common mating system in mammals.
Polygyny predominates in primates. Males defend a small harem of mates whose long gestation and well-spaced receptiveness render them easily controlled.
A dominant male’s harem may be chipped away by promiscuity. Macaque females copulate on the sly with lower-ranking males when an opportunity arises.
Other than protection, primate mothers rear offspring unassisted. This typifies lemurs, capuchins, howler and leaf monkeys, macaques, and gorillas.
Several primates besides humans are quite promiscuous, including patas monkeys, baboons, and chimpanzees. A receptive female Barbary ape may be mated by as many as 10 males.
Cooperative offspring care is not the norm, but neither is it unusual. Conversely, cooperative breeding is rare.
The essentials of cooperative breeding evolved early. Recall the common soil amoeba Dicty, which aggregate into a fruiting body, whereupon only a few amoeba spore.
The Tasmanian native hen is a flightless rail endemic to Tasmania. Males face stiff competition for females as juvenile males outnumber females. Yet some males cooperate rather than compete. A native hen nesting arrangement is either of a mated pair or a trio, typically comprising 2 brothers and a female.
Sibling cooperation pays off. Trios produce larger clutches, and successfully rear a higher percentage of chicks, than mated pairs.
Wild turkeys brothers in Texas pair up to get a mate for the better of the two. Brothers help each other in the fierce competition for females.
When 6 to 7 months old, brothers leave their brood flock together. The relative status between the siblings is then decided by series of fights, some lasting 2 hours, where they peck at each other and strike with their wings. The winner becomes the dominant one for life.
Next, brother pairs challenge each other for dominance over the winter flock. Usually the largest pair wins.
Different flocks compete whenever they meet, settling dominance at that level. The outcome determines the one male above all others in mating precedence.
Females and males gather at the mating grounds in February for a lek. Each brother pair competitively struts and fantails synchronously in the direction of observant females.
When a female is ready to mate, the subordinate yields to the dominant tom. Likewise, subordinate brotherhoods yield to the dominant pair of the flock. As a result, only a tiny fraction of males mate: 4% or less.
Birthing basically bifurcates into laying eggs (oviparity) or bearing live young (viviparity). The mode of birth greatly influences pre-birth and post-natal parental care.
Most fish, amphibians, reptiles, all birds, and a few mammal species are egg layers. Fertile eggs have their own energy supply. Yet egg-laying provides opportunities for parental care, both before and after eggs are laid. Pre-birth care can be particularly important for egg layers.
There’s no sexual jealousy in fish. ~ Australian mathematical ecologist Hanna Kokko
25% of fish species practice protective care of eggs. When there is no trade-off between mate attraction and paternal care, males are more likely to look after eggs. Female fish often prefer to mate with a male already looking after eggs, as it demonstrates caring credentials.
Several fish carry their eggs in their mouths. A male seahorse carries eggs in his brood pouch; eggs are laid there by his lifelong mate for safekeeping.
Fish fatherly care of an emergent brood is common in nest preparation and protecting laid eggs. Protecting free-swimming fry often finds dad on duty.
Lusitanian toadfish males chorus like frogs to attract females to spawn with. They then spend 3–4 weeks caring for the resultant eggs, which are attached to the ceilings of their nests. These toadfish are devoted fathers. Aside from aerating eggs and warding off predators, they splash water on the eggs to keep them hydrated if the nest becomes exposed at low tide. It is a full-time job. A hungry dad occasionally eats some eggs to keep going.
Parental care in tailless frogs and toads is rare: only 10% of species care in any way for eggs or offspring. 18% of the 555 salamander species extend parental care.
Amphibian parental care includes creating foam nests to protect eggs, guarding eggs and moistening them to prevent desiccation. Some frogs carry eggs on their backs. A few tote their tadpoles.
Unlike the desiccation-prone eggs of fish and amphibians, reptiles and other egg-laying species lay eggs on land that are toughened by a sturdy membranous sac which encloses an embryo, encased in a hardened shell. Eggs may be laid and left; exactly what many reptiles, and some birds, do. That is not poor parenting, as protective provisions were made for the eggs, by careful placement to avoid predation.
Reptiles typically tend toward solitude. After mating, they separate and continue their independent lives.
Left their own devices, eggs are not entirely helpless. Some fish eggs hatch only when light and temperature conditions are favorable.
Signals sent by predators can prompt embryos to break out early or delay hatching. Frog, lizard, and snake embryos sense chemical and physical surroundings, including suggestive vibrations, to decide when to hatch.
Some avian embryos within eggs are aware of their situation. Franklin’s gull is a small migratory gull that nests in wetland vegetation in central Canada. A growing embryotic chick knows when in the nesting period it was laid by the nature of the nutrients in its egg and the amount of daylight it experiences. The embryo adjusts its development so as to hatch and be ready to migrate on schedule.
Parental care by insect males is rather rare. Like reptiles, the parental care for many insects extends only as far as carefully laying eggs. There are exceptions.
Giant water bug males have eggs laid directly on their back. A brooding male spends hours perched near the surface of the local pond, doing pushups to keep well-aerated water flowing over the eggs.
Giant water bugs are among the world’s largest insects. They specialize in grabbing and stabbing sizable prey (from an insect perspective): frogs and fish, as well as tasty tadpoles.
After hatching, an insect grows through immature stages in successive molts. When an immature insect molts, it acquires a new skin into which it quickly grows. Growth stops when an insect reaches sexual maturity.
At each stage of development, the respiratory system of an insect is fixed in size. This limits oxygen intake. Suffocation drives molting and metamorphosis.
No immature insect grows more than 50–60% in a single molt. The giant water bug family evolved with a maximum 5 to 6 molt sequence.
A newly hatched giant water bug (the first instar) needs to be pretty big to tackle a tadpole. Given the evolutionary parameters, that requires a large egg. Not surprisingly, the eggs of giant water bugs are themselves giant compared to other insect eggs.
Being large requires a higher metabolism, which, for an egg, means needing a lot of oxygen. Being large also means a low surface-area-to-volume ratio. Therefore, the egg getting enough oxygen is a potential problem. Large eggs have a built-in oxygen deficit.
Oxygen diffuses through air more readily than through water, but eggs without a hard shell quickly desiccate.
Giant water bugs were bound by biophysical constraints in solving the adaptive problem of ensuring that their large eggs got enough oxygen. The solution was dad doing pushups at water’s edge with eggs on his back.
To produce her clutch of large eggs, a female water bug has a much higher basal metabolism than a male. Hence parental care has a greater fitness cost for females than males. This biases adaptation to paternal care.
It’s not all drudgery for a brooding male. Carrying a clutch of eggs can be good advertising of diligent paternal care. Males with backside to spare sometimes attract a 2nd female.
All sorts of nests are constructed for eggs. Megapodes and crocodiles build nest mounds.
The megapode is a precocial chicken-like bird native to the Australasian region. Megapodes have large feet and small heads (mega = large + poda = foot).
Megapode hens don’t incubate their eggs with their own body heat. Instead they bury their eggs in a massive mound of decaying vegetation. The male tends the nest mound, regulating internal temperature by adding or removing nest litter. The sex of the chick depends upon nest temperature, as it does with crocodiles.
Megapodes are superprecocial. A hatched chick digs its way out of the nest mound without parental assistance. It takes but a day before a megapode chick takes wing and flies off.
Other megapode species have different incubation strategies, employing geothermal heat or Sun-warming sand. Some vary their incubation strategy depending upon local conditions.
Despite their massive girth and natural aggressiveness, crocodiles and alligators are tender, loving parents; demonstrating the most highly developed parental care of all reptiles. A crocodile parent, mom or dad, guards the nest for the 70 days it takes an egg to hatch. A hatchling having trouble escaping its eggshell is given an assist. One or both parents help hatchlings from the nest, which are then helpfully carried to nearby water in huge jaws.
Pythons are the only ectothermic vertebrate that directly warms eggs: coiling around them to incubate them. Other egg-laying reptiles go to considerable care to find a place to bury their eggs, protected from the elements and predation. Of the snakes, only the king cobra makes a nest that is aggressively defended until the eggs hatch.
In endothermic birds and monotreme mammals, eggs are incubated through warmth. Most of these animals feed hatchlings.
Exposure to hormones during early development is important in shaping the development and personality traits of offspring. ~ English zoologist Carl Soulsbury
When laying eggs, a mother bird may tweak the character of her chicks by adjusting energetics which materialize in yolk hormones. This produces distinct personalities as an adaptive hedge in unpredictable environments.
While birds are well known for their nests, some do not bother. Several species of seabirds lay their eggs on a bare rock ledge or on a sandy beach.
The size and shape of bird eggs is related to a bird’s body and lifestyle: how a bird flies, how far, and how much dietary calcium is available. Environmental factors also weigh in, including need for resistance against impacts.
To maintain sleek and streamlined bodies for flight, birds lay eggs that are more asymmetric or elliptical. With these egg shapes, birds can maximize egg volume without increasing the egg’s width – this is an advantage in narrow oviducts. ~ American evolutionary biologist Mary Caswll Stoddard
Japanese quail lay speckled eggs which vary greatly in appearance between individuals. A female picks a location on the ground to lay her eggs that best matches the color of her own eggs, thus providing the most effective camouflage.
Avian egg color strikes a balance between 2 opposing, and potentially damaging, effects of sunlight: light transmission into light-colored eggs, and heating dark-colored eggs. While camouflage from predators is the key factor in dull or mottled egg colors, the hues of brighter-colored eggs are adapted to their exposure to the Sun.
Animals which give birth to live young have 4 main birthing methods: 2 each for ectotherms and endotherms. The ectothermic techniques assume embryos developing inside eggs kept in a mother’s body until ready to hatch.
Several invertebrates, bony fish species, a few amphibians, and many snakes and lizards practice ovoviviparity, where there are no trophic interactions between mother and developing embryo other than gas exchange (respiration). The egg is nourished entirely by egg yolk.
The Australian yellow-bellied 3-toed skink practices oviparity or ovoviviparity depending upon habitat. In the warm costal lowlands, the skink lays eggs. At higher elevation, in the colder mountain ranges, the skink holds her eggs internally, birthing live young.
In viviparity, the other ectotherm method, the embryo develops inside a mother with her nourishment, but without placenta. Viviparity is practiced by some fish, amphibians, and reptiles.
Mammal birthing takes 2 routes to bear live young. The earlier evolutionary development lacks a placenta. Marsupials connect to an internal embryo but give birth to premature babies that are nursed in a mother’s pouch.
A later-evolved technique employs a placenta: a sack-like organ with fetus inside, connected to mom’s uterus by an umbilical cord for respiration, nutrient uptake, and waste elimination.
Although quite caring after birth, external pre-birth care is largely absent in mammals. Pinnipeds, such as sea lions, give birth without preparation beyond migrating a vast distance to reach the beach where they were born. Most ungulates make no preparations for a birth other than some separation from the herd just prior to giving birth.
Except for nest-building prosimians, female primates simply give birth, then pick up the offspring, which instinctually clings to its mother.
The young of most species are quickly mobile within a few hours of birth, whereupon they must keep up with mom.
All vertebrate classes provide examples of post-natal care. The varieties and depths of parental commitment to offspring vary widely.
The degree of precociality or altriciality is the most obvious difference. Whereas precocial animals don’t need much help getting a start in life, altricial ones would die without extended parental care.
The ability to get on with living is not the only factor. Sociality also matters. Innate knowledge may be inherited, but many social animals rely upon cultural knowledge, and family ties are often significant in social status. Hence, after-birth relations can vary by social circumstance. Many mammals maintain relationships with their parents long after being weaned.
Typically, the major disparities in behaviors are between ectotherms and endotherms. But, as with so much of zoology, there are many exceptions.
There are insects, fish, and a few amphibians that care for their young. Among reptiles, only crocodiles and alligators are doting parents.
Most baby birds are parentally fed or at least shown where to forage. All infant mammals must suckle sustenance for some duration.
Some endotherms bring food to the mate caring for the offspring. Most often the male provides for his mate.
Many vertebrate animals are good parents, and the responsibilities of parenthood take priority over all other aspects of their behavior. Parental care becomes the dominant force in their lives. ~ English zoologist Clive Roots
Parental duties are typically twofold: food and protection. The extent of post-natal care varies considerably by species, from days to a decade or more.
Animals have been taking care of their young for at least 450 million years. Ostracods (seed shrimp) are the earliest known example of brood care. Early in dinosaur descent, some diapsids provided post-natal parental care.
The general rule that parental care increases with complexity, size, lifespan, or whatever is so full of exceptions to not be a rule at all. Hooded seal pups are nursed for only 4 days on average, whereas parental cichlid fish care may last months.
Mortality is typically reduced by parental care, and affords the opportunity of learning from parents, including cultural transmission.
European earwig mothers lay a clutch of ~50 eggs in an underground nest during autumn, caring for them by cleaning the eggs to avert fungal growth. When the eggs hatch in the spring, good mothers guard and feed their offspring until they reach maturity: about a month.
Some European earwig moms are less fastidious in their maternal care: leaving their offspring to fend for themselves. Loss of maternal care influences neither developmental time nor survival rate for the 1st neglected generation. Indeed, orphaned earwigs may grow to be bigger adults.
There is a transgenerational cost: maternal care makes better moms in the next generation. Orphaned females tend to neglect caring for their eggs and their juveniles, feeding and guarding them less. Survival suffers.
Cichlids provide parental care for both eggs and larvae. Mouthbrooders incubate eggs in their mouth as soon as they are laid. Some, including the discus (pictured), feed their hatchlings by skin secretion from mucus glands.
Free-swimming cichlid young may be cared for weeks or months. Some species practice communal parental care: multiple monogamous pairs care for a mixed school of offspring. Fairy cichlids school and employ a collective nursery: fry are protected by older juveniles from previous spawns as well as by adults.
Strawberry Poison Frogs
With a blazing red-orange body that brazenly warns would-be predators, Central American strawberry poison frogs outsource their toxicity production. They garner their alkaloid poison from the mites, ants, and other arthropods they eat, storing it in their skin glands.
A female lays her eggs in leaf litter, leaving them in papa’s care for a week or so. He keeps the eggs safe and pees on them to keep them moist.
Mother then returns and moves the eggs into a pool of water she forms in the foliage.
When the eggs hatch, she feeds each her offspring daily for 6 weeks with unfertilized eggs until the tadpoles transform into frogs. Besides being nutritious, the unfertilized eggs possess enough poison to protect the tadpoles from predators.
Parenting is often part of an animal’s life cycle; its form related to the mode of birthing, number and type of offspring, habitat, lifestyle, and sociality, among other things. There are confluences in these factors, forming a species survival strategy.
Animals either lay eggs or bear live young. The young may be precocial or altricial.
The number of eggs laid in a clutch varies from a very few to tens of thousands. The more eggs, the more likely that: 1) offspring are precocial, and 2) post-hatch parental care is limited, if there is any at all.
Large numbers of hatchlings make post-natal care impractical. Parental care in such case is pre-birth: placing eggs in a safe place and possibly watching over them until they hatch.
Some precocial species show parental care, particularly protection of hatchlings. Grebes and loons are precocial, but parents protectively carry their young on their back. The number of offspring per brood are small in both instances. Species of both birds are gregarious. Most grebes flock to migrate. Some loons, though territorial, have ritualized social gatherings.
Some beetle mothers take prodigious care of their little ones. Hatchlings are slow-moving, so leaf beetle moms, like many others, look out for predators and parasites. This is not the only maternal care that beetles engage in. Some leaf beetles prepare for bearing live offspring by readying leaf resources. Once born, mothers herd their hatchlings among leaves that maximize nourishment.
However birthed, having only a small number of offspring eases parental care. The degree to which post-natal care occurs and its duration often coincides with the sociality of the species. Most viviparous species are quite social.
Live birth is necessarily of a very few offspring at a time: 1 is common. Offspring are typically altricial. Little if any pre-natal care, and considerable post-natal parental care, are the norm.
However crucial to successful species propagation, parental care is but one part of life’s package deal. At the onset of breeding, mating style and parenting practice are related in many species.
Post-natal parental care appears in 3 forms: maternal, paternal, or biparental. Protective care by elder youngsters is common in social species. A few species practice familial upbringing.
Parental care is a major factor in social organization. While mothers are a default caretaker, dads get involved in relatively few species. In 90% of mammalian species, a male’s role in reproduction ends with fertilization. Contrastingly, many fish and birds, including colonials such as gulls, have fathers that extend prodigious care and feeding to their progeny. Paternal care occurs for a notable few mammals, including marmosets and gibbons.
For emperor penguins, a father fasts for months while holding an egg between its legs to keep it warm. In contrast, baboon fathers do not assist in offspring upbringing. Instead, they guard the entire troop, along with other adult males.
Humans are not very typical mammals, but they are quite typical birds. Birds have pair bonds, extra-pair copulations and divorce. They have all kinds of complicated social relationships, not so unlike humans. ~ Hanna Kokko
Biparental care is most common in birds. 90% of avian species are socially monogamous (but only 3% of mammals). Both bird parents share incubation and brooding duties.
For some birds, paternal interest is more or less enthusiastic, depending the partner. A male blue tit is a more diligent father when he considers his mate attractive.
Reproductive success requires a high degree of cooperation by each mate, as they switch duties. ~ American wildlife biologist John Piatt on murres
The common murre lives its life at sea, fishing in the frigid waters of the northern oceans. It spends time on land only to breed, in a high-density colony on rocky shoreline cliffs.
A common murre female lays a single egg per breeding season. Parenting is shared between mates.
Communication between mates allows murres to negotiate the level of effort that each member of the pair puts into the breeding effort. ~ John Piatt
Mom and dad take turns watching over the chick while the other goes fishing. When one parent returns with a meal, the couple preen each other and switch roles. This swapping ceremony typically transpires 3–4 times daily.
Role flipping does not always go smoothly. In some 20% of the switching ceremonies, one parent is slow to preen its mate, and then refuses to go fishing; forcing the one who just returned with a fish to go back out.
Tension also rises when the one with fishing duty comes back empty-beaked, which happens about 10% of the time. The failed fisher quickly starts to preen its mate; but the eager preening is only slowly reciprocated, or not at all.
The brooder is basically communicating: “The chick still needs a fish; you better go get one.” ~ Canadian psychologist and ethologist Carolyn Walsh
The swapping ceremony is a way for these seabirds to communicate their well-being, and to negotiate. Staying in the nest allows a worn-out murre to recover its strength. It may well need the respite.
Flying out to sea to fish is taxing for murres, as these birds are not very aerodynamic. Though fast fliers, murres are not very agile.
There is a correlation between feeling well and swapping ceremony performance. While being a good mate sometimes requires forbearance, chronic shirking gets old.
Murres usually mate for life, but pairs sometimes divorce. Whether a couple is heading toward a split is a tale foretold in its role-swapping negotiations.
2% of bird species have a propensity to pilfer. Certain bird families are more prone than others. Birds of prey and seabirds tend to thievery, while songbirds much less so.
How birds make their living extends to their social graces (or lack thereof). Care in parenting may be altogether quite different.
Terns are prone to relieve larger species of their catches, and well as con one another. A female tern flirted with fish-toting males just long enough to sidle close and nab their food. Such antics are no reflection on parenting. A study found that tern thieves “were far and away the best parents.”
Many birds vocalize at their young. This often starts before hatching. A mallard mother-to-be mutters to her eggs. The tiny ducklings within respond when near hatching time, uttering sounds of their own. This part of the imprinting process, by which ducklings know to follow their mother.
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Eusocial species have an entire colony as an extended family. Alloparenting is de rigueur.
Various gregarious species practice alloparenting: individuals other than biological parents act parentally. Females may forgo breeding to help others in the group rear younglings. The biologic for cooperative breeding is in raising the odds for successfully rearing offspring.
Some human tribes, such as the Efé of Ituri Forest in the Congo, practice alloparenting. Sustained intimate group cooperation is not typical of this social-but-selfish species.
Spiders are not known for their sociality, but of the 43,678 known species, at least 2 dozen are gregarious. Spider sociality independently evolved several times.
2 closely related cobweb spiders cohabit in a colony. They don’t hunt together. Instead, the colony is a colossal crèche. These spiders practice protective alloparenting. A female looks after any young within her purview.
Meerkats are outgoing mongooses that live in close-knit groups. Individuals know each other, both by looks and by their calls.
In a meerkat group, some will stand sentry while others forage. Further, all adult group members participate in protecting the young, not just parents.
Sentries are especially vigorous when pups are present. Sentinel behavior is geared to protecting the most vulnerable.
Despite the concern, dangerous duties go to a junior meerkat. A dominant meerkat lets a younger adult be the first to cross a road or other open area.
Just as human societies sacrifice their young in war, the realization of perceived imminent threats is inflicted upon those least valued by dominant members of a meerkat social group.
Alloparenting is not optional. For all their altruism and cooperation, meerkats are a hierarchical society.
An alpha pair reserves the right to mate. A dominant female will often kill any errant pups or force a subordinate mother to abort.
Some young female adult meerkats will wet-nurse a dominant mother’s offspring to stay within the community. Females can lactate by will, without ever having been pregnant. (Allolactation occurs in numerous mammal species.)
The dominant couple may banish offending mothers. New meerkat groups are often formed by roving males pairing with evicted females.
Meerkats don’t practice incest. If an alpha female dies and is succeeded by her daughter, her father will not mate with her. In this case, the colony reproduces via alpha group females stray-mating with roving males from other groups. Under this dynamic, pregnant females tend to kill and eat any pups of other non-alpha females.
Meerkat societies are the human equivalent of totalitarianism.
Uncoercively, Florida and Mexican scrub jays have cooperative parenting, where there are helpers at each nest, usually close relatives of the breeding pair.
Cooperative parenting may be inspired by resource shortage: insufficient food to afford more widespread breeding, at the risk of a group or population starving itself.
An acorn woodpecker couple with fledglings has several helpers that do not breed. They are generally young adults related to the family who have yet to mate. Helpers assist by foraging for food for the family.
Such assistance more beneficial in years of abundance than those of scarcity. In hard times, helpers may be consuming food that could otherwise go to the fledglings. During such times, alloparenting can be counterproductive to reproductive success.
Cooperative parenting is most common in tropical and subtropical bird species that are colonial, or monogamous pairs that occupy territory year-round. Helper tasks may vary by species: from nest-building, incubation, and feeding hatchlings to property protection and predation defense.
Ani is a tropical cuckoo whose members live communally, in groups of up to 4 pairs. Females lay their eggs in one large nest and take turns incubating. The glitch in the ani’s breeding system is sometimes having too many eggs to fit in the nest. If this happens, a female ani will roll existing eggs out of the nest before laying her own. The dominant female typically lays last. Hence her eggs are keepers.
Alloparenting is not always of obvious need. Emotional attachment keeps some youngsters at home longer than biology necessitates. Yearling beavers, physically mature, typically stay with their parents for a season, helping to care for younger siblings. So too coyotes, gibbons, and numerous other mammals.
Carnivores more typically face such a dilemma of exigent conditions when their natural inclination is to more freely breed. Wild cat groups in harsh circumstances will cooperatively breed and practice alloparenting to help ensure the group’s long-term prospects.
Primates often assist with child-rearing. Childless females babysit for short periods, grooming and playing with younglings. Such childcare is not altogether altruistic. Childcare by Japanese and rhesus macaques helps females form alliances with mothers of superior rank. Babysitting is beneficial to young females for their own training. Irrespective of motive attribution, doing well by doing good benefits all involved.
The human tendency of generalized kindly feelings toward babies is less natural to other animals, who as often as not regard the young of others, and even sometimes their own, as food. Parents knowingly caring for the offspring of others is out of the ordinary, especially toward genetic strangers. But it does happen.
Some birds are so stimulated by the calls of chicks in another nest that they respond by feeding the hungry others. A walrus may adopt an orphaned calf. A female monkey may care for another’s baby. Several instances of chimp adoption have been recorded.
A pack of African wild dog males adopted 9 5-week-old pups when the pups’ mother died. The adult males went to the pups’ den each day, delivering food; caring for the younglings until they were able to join the pack on hunting trips. Food sharing is common among these dogs.
Female burrower bugs lay their nest of eggs near others. Mothers tend to developing eggs before they hatch, then feed their offspring nutlets from mint plants. Finding mint nutlets is a competitive business. If a young bug decides mom is not provisioning to satisfaction, it may join a better-fed brood.
Colonial, ground-nesting gulls occasionally adopted unrelated chicks. Chicks may abandon poor parents who fail to provide enough food and take refuge in the nest of a neighbor. That desperate tactic works less than 10% of the time, but a lucky few improve their prospects in life.
The situation is different with goslings. There are widespread adoptions in the 1st week of a gosling’s life. The chicks themselves decide to throw in their lot with parents of higher social rank.
The chick decides when to leave the family, and which family it wants to join. ~ Dutch ornithologist Jan Komdeur
A chick’s freedom to switch parents is abetted by the fact that geese find it hard to distinguish their own goslings before they are ~9 days old. This is an adaptive inability. Young goslings are easy pickings for Arctic foxes and gulls. The grief of caring parents from such loss is lessened by adoption. And larger families fare better than small ones.
While goslings may choose new parents, other younglings have little say. Faced with chronic predation, convict cichlids like their families large. So, cichlid parents kidnap little ones to act as a living shield for their own offspring.
Cichlid parents are always actively searching around, and if they see babies, and they don’t eat them, they will lure them into their brood. ~ American ichthyologist Brian Wisenden
Cichlid adoptees tend to be the smallest of the brood, and the least likely to survive. Predators choose the easy prey.
Among tessellated darter fish, males tend the young on their own. A prospective father sets up his territory: cleaning the underside of a rock, waiting for a female to lay her eggs there, and then fertilizing them.
At some point, the father moves on; whereupon a smaller, unrelated male typically takes over the territory – picking up where the other male left off: keeping the rock clean, aerating the eggs, and defending them. Females prefer a doting dad. So, despite his size, an adoptive dad may get lucky and fertilize some eggs of his own.
Male hamadryas baboons willingly adopt juvenile females. Such adoption is far from selfless, as it helps in accumulating a harem. But it is caring, and for a goal far removed in time.
Many socially monogamous avian species have extra-pair matings. This cuckoldry is taken a step further by a female laying her eggs in the nest of another: brood parasitism (BP).
Parents are tricked into rearing the young of the brood parasite. BP birds lay more eggs than otherwise, as they forgo the chores of incubation and feeding hatchlings.
The eggshells of brood parasites – whether cuckoos, cowbirds, or honeyguides – are up to 30% thicker than those of the birds they parasitize. This is a classic case of convergent evolution. The greater shell thickness provides insulation which accelerates incubation. BP chicks hatch before their host’s eggs do, giving them a head start, and a chance to easily dispatch incipient rivals.
The host’s eggs are commonly disposed. A BP bird may remove the host eggs as she lays hers or pierce the eggs and kill the embryos. Otherwise, parasitic hatchling may kill other nestlings, though some parasitic chicks grow up alongside host offspring.
There are 5 brood parasite bird families with various specialties in victims: various ducks, Old World cuckoos, viduids, African honeyguides, and all New World cowbirds.
Goldeneye ducks will put their eggs in other goldeneye duck nests. Such intraspecific brood parasitism is practiced by other duck and geese species as well. These foul fowl may also nest themselves.
Interspecific brood parasitism is more the norm: parasitizing other species. Most avian brood parasites are specialists in parasitizing a single, or small number, of closely related host species, but 4 of the 5 parasitic cowbirds are generalists, with a wide variety of hosts.
The brown-headed cowbird has 221 known hosts. Usually only 1 parasitic egg per host nest is laid, though cowbirds are not so discriminating: a few different females may lay in the same host nest.
The common cuckoo parasitizes at least 125 hosts, but individual females specialize in a single species. Inheritance regulating egg coloration is passed exclusively along the maternal line, allowing females to lay mimetic eggs in the nest of the species they specialize in.
Females imprint upon the habitat, particularly vegetation, to find host nests of the species which reared them, and subsequently parasitize nests of that species. Male common cuckoos will happily fertilize females of any host, hence maintaining genetic diversity among different maternal lines.
Nostalgia in Nature is not just sentimental. Habitat imprinting is employed by many insects, birds, and mammals, for both feeding and breeding.
Cuckoos in the neighborhood are bad news. Their presence indicates that interloper eggs will be deposited.
Naïve birds with no experience of brood parasitism know no better. BP eggs are easily accepted. But experienced moms react. When a cuckoo is spotted in the neighborhood, an alarm is raised, and the offender is mobbed to drive her off. Hence, young mothers-to-be cotton to the evil of cuckoos via social learning.
In predator-prey and host-parasite interactions, an individual’s ability to combat an opponent often improves with experience. Individuals can also assess threats from social information. Such recognition promotes the evolution of polymorphisms, allowing rare variants to evade detection. ~ English zoologists Rose Thorogood & Nicholas Davies
As an evolutionary adaptation, cuckoos counter mob attacks by imitating the color of a local bird that preys on the cuckoo’s host, such as a hawk. That puts off would-be mobbers, at least for a while.
There is no benefit in looking like a dangerous species your target is not familiar with. ~ ornithologist Thanh-Lan Gluckman
As victimized birds learn to see through a mimetic disguise, it becomes less effective. To counter this, cuckoos evolved plumage polymorphism: individual cuckoos have different colorations.
Social learning by brood parasite victims and plumage polymorphism by cuckoos are typical of ecological adaptations that propel evolution. Evolutionary arms races between parasites and their hosts are common.
BP victims strive to identify their own eggs and weed out fakes. This can be difficult, as cuckoos typically produce eggs that look like the host they specialize in. This parasitic adaptation is furthered by the tricks that memory can play.
A host female rejects eggs based upon an internal mental template of what her eggs should look like. BP birds take advantage of memory fallibility by repeatedly putting their eggs into the same nests. As the proportion of foreign eggs increases, a host female has a harder time discerning differences between what her eggs look like and those of imposters.
Repeated parasitism interacts with egg mimicry to exploit cognitive and sensory limitations in host defences. ~ English zoologist Martin Stevens et al
Hosts fight back against cuckoo eggs mimicry by evolving recognizable patterns on their own eggs, similar to how currencies have distinctive watermarks to deter counterfeiters. This is another evolutionary arms race between a cuckoo and its intended victims.
Some host species evolve egg patterns that are repeated within a single clutch. Other species lay eggs with patterns that differ dramatically from female to female in a population. Still other host species produce egg patterns with high visual complexity. Each stratagem increases the likelihood that a host will be able to identify and reject a foreign egg.
Some species use 2 of these strategies, but none uses all 3. A signature like this would be too complex to be easily recognised. ~ English zoologist Rebecca Kilner
Australian blue wrens, a fairywren, are plagued by cuckoos that look like hawks, which prey on them. A cuckoo lays its eggs when a fairy wren mother flees her nest in fear.
Despite being cowed for BP egg-laying, blue wrens have a trick to feed their young and not the brood parasite. A mother blue wren sings to her eggs a password that hatchlings use as a begging call.
The closer the begging call is to the call mom sang during incubation, the more food a hatchling receives. The BP baby does not learn this password and so goes hungry.
While eggs may have passing resemblance, many BP birds look little like their nest mates, or what their nest mates may have been. The obvious question is why a host mother expends her energy on a fraud.
The Mafia hypothesis posits thuggery by BP birds. Brood parasite birds monitor their displacements. The nests of host birds that reject parasitic eggs are depredated, nestlings maimed and killed. Hosts are forced to relocate, giving the BP bird another chance.
Statistically, a host is better off accepting parasitic eggs, as the probability remains of rearing at least some chicks of her own. It appears that adaptive pressure favors aggressive brood parasites and compliant hosts.
Brood parasitism is not always an unmitigated tragedy for its host.
The great spotted cuckoo is a nonevicting brood parasite, specializing in corvids, mainly magpies and carrion crows. These cuckoos greatly reduce magpie reproductive success.
This does not happen with carrion crows, whose larger offspring are often raised alongside the parasite. Unlike magpies, crow parents do not evict alien eggs, or mob parasitic adults in proximity of the nest. Such acquiescence means that 68% of carrion crow nests are parasitized.
Cuckoo chicks put off predators with a noxious secretion that they release when harassed. This deterrence lessens predation of crow nests. Hence, while parasitized nests result in fewer crow offspring, reproductive success is enhanced. In this instance, brood parasitism may be a net benefit.
Brood parasitism is not limited to birds. Cuckoo bees are brood parasites that lay their eggs in the nest cells of other bees. Likewise cuckoo wasps, as well as other wasp species in various families that have adopted parasitic brooding.
Though life forms may possess a sense of fairness, it is not how Nature plays the game of life. Deception is a prevalent ploy to raise the entertainment value of the exhibition.
Another facet of parenting is teaching young to survive. Though many animals tutor offspring in various skills, the most common imperative for training is in carnivores teaching their young to hunt. Spotted hyena pups are weaned at a year, but it is months later before they are adept hunters. Cubs of cheetah, tigers, and polar bears are taught hunting skills for 2 years before fending for themselves.
Prolactin is a hormonal protein best known for its role in promoting mammal lactation. Prolactin has a long evolutionary employment, having been conserved in genomes for up to 800 million years. Prolactin helps fish, the oldest vertebrates, with water and salt balance.
Prolactin is an important immune system regulator as well playing a vital role in the cell cycle. Prolactin affects migratory behavior in fish and birds.
In fish, prolactin plays a role in attaining sexual maturity and in reproductive cycling. In mammals, prolactin provides sexual gratification: dampening dopamine, which causes sexual arousal.
After mammalian childbirth, previous inhibitions on prolactin – by estrogen and progesterone during pregnancy – are released. Prolactin stimulates milk production.
While prolactin plays a critical role in maternal care, it also stimulates paternal care in fish, amphibians, and birds. Male parental care in cichlids, the North American bluegill, and threespine stickleback, owes in part to pronounced prolactin levels.
In birds, prolactin levels rise during egg-laying, and peak during incubation. Levels decline when eggs hatch. Other stimuli – vocal and visual – prompt feeding hatchlings.
Prolactin is an avian paternity hormone: increasing in male birds during incubation. Nonbreeding birds that assist at the nest are primed by high levels of prolactin. Scrub jays are exemplary.
Chickens and several water birds are precocial, including ducks, geese, rails, and waders (shorebirds). Prolactin makes mom mind her ducklings, but for only a few weeks. As prolactin level drops, a hen’s bond with her brood lessens. By week 6, indifference is good behavior. A hen might even be antagonistic toward her ducklings without the influence of prolactin.
Dominant animals are worse off when subordinates in their group try to breed – explaining why they brutally suppress others much of the time. ~ English zoologist Matthew Bell
Infanticide – killing and often eating infants, either one’s own or others nearby – is practiced by microscopic rotifers, insects, fish, amphibians, snakes, rodents, colonial birds and several birds of prey, lions, and primates, including humans. Ovicide is the analogous destruction of eggs.
The root of infanticide is typically sexual conflict: a male taking a new partner after killing her offspring by another. As an effective evolutionary device, certain conditions must be met. It occurs in species where estrus is year-round; otherwise infanticide does not necessarily free up a female for breeding.
There’s no sense for a male to kill the offspring in the previous year, because he has to wait anyway. ~ English ethologist Dieter Lukas
Infanticide emerges in species with group living where females greatly outnumber males. This situation favors a male dominance hierarchy for mating rights.
In species where infanticide appears, females gradually mate with more males. This evolutionary adaptation confuses patrimony, and so lessens the incentive for baby killing.
Whenever promiscuity is high enough, it does not pay for males to commit infanticide. ~ Carel van Schaik
Conversely, infanticide also drives monogamy. This both makes patrimony clearer and provides for offspring protection by the male parent. Extra-pair coupling in females may still occur, but usually as a deceptive practice. Humans are exemplary.
Langurs are a social Old World monkey living in India. Each group is sexually dominated by a single male. In a violent overthrow of the alpha male, infants of the vanquished are killed, though only for a short time after the takeover.
The same dynamic applies to a pride of lions when a new male takes over. The domineering newcomer will try to kill any cubs 9 months or younger. As in other species, females defend their cubs viciously.
An intriguing exception to male infanticide occurs in Japanese anemone fish. If a male parent looking after a clutch of eggs is killed or deposed by a new male, the newcomer assumes paternal duties with as much vigor as his predecessor. Genetically this makes no sense. But the female will refuse the new male if he is not a decent foster-parent. Only when he starts caring for the young does she stop beating upon him. The male’s short-term altruism is rewarded with his own clutch next go-round. Such a system can only work when females are atypically aggressive.
Male mice have biorhythms of infanticide. After fertilizing a female, a male kills any pups it comes across for 3 weeks. Then the prolactin kicks in as they become caring for their own offspring. This last for 2 months, whereupon they return to their infanticidal ways. By no coincidence, female gestation is 3 weeks, and it takes 2 months for pups to be weaned and on their way. Thus, this is well-timed infanticide via sexual conflict.
Males can be infant killers even incidentally, or against their own genetic interests. Polar bear cubs must avoid adult males, even dad, who regard the little ones as prey.
Male eared seals (otariids), which includes sea lions and fur seals, form mating harems, with territories on breeding beaches. A pup that strays may be killed by the next-door bull for crossing into his dominion.
Another harem-polygynous species, the colonial black-tailed prairie dog, practices infanticide as part of invasion by males, and as a marauding behavior by females. The point for females is to reduce competition with other females for food and future offspring.
Such resource competition also occurs in meerkats. A female may kill the offspring of her mother, sister, or daughter. Infanticidal raids from neighboring groups also occurs.
Mothers of many ground squirrels, and especially rats, may cannibalize young if extremely stressed. Other mothers may kill younglings when not so mentally rent.
In species where paternal care is the norm, females compete with others by killing their offspring. Wattled jacanas, a tropical wading bird with a polyandrous mating system, practice this infanticide. Wattled jacana males exclusively brood while females defend territory.
A similar dynamic happens in giant water bugs, where males take care of egg masses. Females that cannot find a mate often stab the eggs of a brooding male. This ovicide is rewarded by the female being fertilized by the male, who is in the mood when he has nothing to brood.
Some nonbreeding colonial birds, such as gulls, kill and eat nestlings. Overcrowding and attendant resource competition is typically the inspiration.
Burying beetles have complex parental care practices. Both parents provide diligent nurturing; though, as in birds and humans, females take the lead.
Burying beetles find a carcass, usually a small bird or mouse; a prize worth fighting for. And they do.
Mating pairs pair off as rivals: female against female, male against male. The larger contestants typically triumph.
The winning pair gets the spoiled spoils, which is buried (hence the namesake) to remove it from further competition.
The burying beetle couple then busy themselves building a nursery: dig a hole under the corpse into which the kill collapses; curve the corpse into a circle; remove all the hair; coat the body with antibacterial and antifungal secretions from their saliva; eat enough of the corpse to create a cavity. The preparation process may take 8 hours.
A couple of days later, the female lays her eggs into the cavity. A parent – usually the female – watches over the eggs.
The eggs hatch into larvae, which feed upon the corpse.
Although the larvae can feed themselves, both parents practice progressive provisioning: digesting flesh and regurgitating liquid food for the larvae to feed on. Parental secretions may also benefit the youngsters.
Such hubbub helps grow the grubs. The adult beetles feed and protect the larvae for several days, until they are ready to move into the soil and pupate.
A few larvae get greedy, pestering their parents for more than their share. These younglings are cannibalized for their temerity, engendering more honest communication about apportioning food.
Older male beetles make more diligent parents than younger ones, particularly when paternity is uncertain. More breeding opportunities lay ahead for beetles in their prime. An elder beetle may be tending his last brood, and so takes extra care.
It certainly provides some incentive for females to want to mate with older males. ~ English evolutionary biologist Megen Head
If the carcass is large enough, several beetle pairs will cooperate to bury and breed a communal brood. Conversely, if pickings are slim, parents cull their young early on, to match the number of larvae to the size of the supply carcass, carefully estimating enough food to go around to pupation. Burying beetles are both caring parents and killer calculators.
Acorn woodpecker females nest together or risk egg tossing by rivals. Even early laid eggs in a nest are subject to ovicide. Eventually, the entire group lays on the same day, whereupon cooperation breaks out and the females collectively incubate eggs.
Likewise, colonial birds can be egg tossers as a strategy of clutch coordination. Ovicide occurs until all birds in a common nest are ready for brooding. Hence, early egg layers do not dominate reproduction.
Many adult snakes consider smaller snakes a snack, even their own offspring from eggs laid weeks earlier. The cannibalistic king cobra guards her eggs during incubation but leaves them just before they hatch, thus avoiding her own temptation.
Sometimes infanticide is a disguised form of selective culling, as a brood fitness test. When young bass hatch from a spawn, dad is on guard: circling and herding them together, providing protection from would-be predators. After a few days, most of the brood swim away. Paternal behavior flips like a switch. The stragglers are treated as any other small prey: eaten when in the way.
Siblicide – younglings killing each other – is a lethal hazard for herons, boobies, and several raptors. The youngest chick may succumb to the aggressive attentions of older nest mates.
In parasitic wasps, the strongest larva kills its sibling rivals; survival of the fittest grub. The only known instance of siblicide in mammals is the spotted hyena.
Parental investment is a life-history variable, varying based upon the biology of the species: precocial or altricial. The nature of parental care, maternal, paternal, or otherwise, further factors in as adaptive to individual and group survival. In birds with altricial young, laying eggs and incubating them is a meager investment compared to nestlings’ demands after hatching.
Conception begins the investment in mammals. Growing fetuses take energy. After birth, care commonly includes protection and may include keeping youngsters warm; but the major labor is typically feeding.
Unlike other flies, female tsetse flies produce only 1 egg at a time, which it retains internally. The larva hatches in its mother’s uterus. She nurses it with a milky liquid having the same enzyme important in mammal lactation.
Mother’s milk evolved with arthropods. Some jumping spiders lactate and feed their offspring milk. So too cockroaches and burying beetles.
Certain fish and birds also lactate milk to feed younglings, as seen in great white sharks, male emperor penguins, and flamingos of both sexes.
Monotremes – platypus and spiny anteater being extant examples – lay eggs, but then supply milk to hatchlings; bridging the gap between reptiles and mammals in reproduction and maternal care.
Marsupial and mammal mothers drain themselves by supplying milk. Marsupials have a sophisticated milk supply and delivery system. The composition and quantity of marsupial milk offered in lactation paces the stage of growth. Different teats in the same animal can deliver distinct milk.
Mother’s milk is customized to its offspring. Monkeys and ungulates make richer milk for sons, but more milk for daughters. A monkey mother delivers more calcium to her daughters, who need it for their faster-growing bones.
Mammal milk varies by lifestyle. Offspring that stay close and suckle on demand do so from a dilute mother’s milk. Ungulates and primates are exemplary. This is also true of the world’s largest rodent: the capybara, with precocial offspring who can eat solids within a few days of birth but continue to suckle until 4 months old.
In contrast, hooded seal pups suck 60%+ fat-content milk for only 4 days before weaning. After that, a pup must be on the up and up.
Cunning is an essential survival skill that is passed on by predator and social animal parents. The smallest placental carnivore – the least weasel – is weaned at 5 weeks, but hunts with mom until 10 weeks old. Bringing down a rabbit that may be 10 times one’s own weight takes some deft maneuvering.
Hyenas are the only practically precocial mammalian carnivore, but they are raised in dens like altricial babies. In several social species, such as pigs and crested porcupines, young are reared in a group.
Parental care in all primates is protracted. Marmoset juveniles are independent at 5 months, coincident with the timing of the next litter.
Higher primates have childcare for years. Baby baboons and macaques are physically and emotional dependent upon mom for 2 years. An infant chimp is nursed and carried by mother 4 years, but still dependent for another 2.
Maternal bonding is the norm in mammals. For an infant, separation can be agony. Contact with mom brings contentment, whether it is primate cuddling its child or a rodent carrying its youngling by the scruff of the neck.
From humans to mice, mammalian infants become calm and relaxed when they are carried by their mother. This infant response reduces the maternal burden of carrying and is beneficial for both the mother and the infant. ~ Japanese social behaviorist Kumi Kuroda
Baby mandrills are born with open eyes and a coat of hair. They cling tightly to their mother’s belly as she walks or climbs. Infant primates hold onto mom for dear life as she makes her way through the trees or otherwise moves through the forest.
A female mandrill stays with her mother into adulthood. A male leaves when it reaches sexual maturity.
It is common that one gender of juvenile primate leaves a troop while the other stays upon reaching adulthood, depending on mating practice and social dominance hierarchy of the species. Regardless, a child is not forgotten by its mother.
Maternal bonding is ubiquitous in social mammals, notably elephants, dolphins, primates. Gaining parenting skill is but one facet of what is commonly a richly rewarding emotional experience.
Parental bonds often continue throughout the lives of parent and offspring, at least in memory if not in everyday life. In practically all animals with extensive post-natal care, quality of parenting is passed to the next generation by example, for good or ill.
In all animals where postnatal care occurs, there comes a time when offspring dependency must end. In most bird and mammal species, this is a source of conflict as negotiation; a juvenile’s first lesson presaging others to come, as social existence invariably involves many such negotiations.
In birds, a full-fledged nestling will beggar its parents with the usual tactics of piercing cry, gaping mouth, and shuddering wings before acknowledging being ignored and pecking a meal for itself.
No mammal suckling volunteers itself off the teat. It must be weaned by motherly refusal.
Evolution favors strategies that optimally trade off offspring survival and reproduction opportunities for fertile females. It is not so simple as maximizing reproduction opportunities. There is a point where the parental cost of continued care outweighs the benefit to offspring.
Sea Otter Moms
Living in cold coastal waters with thick fur but no blubber, a high metabolic rate keeps sea otters warm. Keeping warm requires that they eat about a quarter of their body weight in seafood every day.
Rearing a pup doubles a mother’s energy demand. A sea otter mom may be so depleted physiologically by the time her pup is weaned that she is unable to survive the stress of a minor wound or infection. Some sea otter mothers suffer the risk. Others abandon their pups when they feel they can give no more.
In some gregarious mammals, terminating dependency is not simply an investment decision by the parents. Past the first step of weaning, younglings of numerous social species yearn for some degree of independence.
Menopause is reproductive senescence before natural death. The females of several species stop breeding for quite some time before they die, as do some males.
Menopause is somewhat common among invertebrates, fish, birds, and mammals. Female guppies, platyfish, Japanese quail, budgies, opossums, orca, pilot whales, some monkeys (tamarins, macaques, baboons), and primates (chimps, humans) are known to experience menopause. Platyfish and Japanese quail males also exhibit midlife menopause.
American evolutionary biologist George Williams posited the grandmother hypothesis for menopause in 1957. The idea was that an older female stops reproducing to assist in taking care of a succeeding generation, and so help ensure survival of her genetic lineage.
The grandmother hypothesis fits some species to a limited degree. There is often some leavening from experience in rearing offspring.
More broadly, menopause may enhance group survival where it is practiced. Fish populations that suffer high predation tend to mature, reproduce, and die earlier than those in less stressful environments. Those females that survive are fitter. There is a safety benefit in having experienced friendly fish about that may alert others of danger.
Menopause highlights that procreation has a limited place in the life’s panorama; that the experience of living is itself reason to be.