Animal Life-History Variables
“In the game of life an animal stakes its offspring against a capricious environment. The game is won if offspring live to play another round. What is an appropriate tactical strategy for winning this game? How many offspring are needed? At what age should they be born? Should they be born in one large batch or spread out over a long lifespan? Should the offspring in a particular batch be few and tough or many and flimsy? Should parents lavish care on their offspring? Should parents lavish care on themselves to survive and breed again? Should the young grow up as a family, or should they be broadcast over the landscape at an early age to seek their fortunes independently?” ~ American paleobiologist & ecologist Henry Horn
A surfeit of strategic survival trade-offs exists for animals. A logical place to start is how life begins.
Oysters produce millions of offspring in a lifetime, providing no parental attention. Conversely, elephants produce very few offspring, each singly born, each the object of intense and prolonged parental care.
The statistical tails of the animal life-histories spectrum may be characterized by how prodigiously a female creates progeny: many or few. The adapted traits which sum to a reproductive survival strategy are numerous, with more biological features interrelated than may be immediately apparent.
Mouse lemurs are the smallest primate, gorillas the largest. An 80-gram adult female lemur may produce each year 1 or 2 litters of 2 to 3 offspring per litter. Lemurs become reproductive within a year.
A 93-kilogram adult female gorilla produces a single offspring every 4 or 5 years. Gorillas do not reach reproductive age for a decade.
Mathematically at least, a female mouse lemur can leave 10 million descendants by the time it takes a female gorilla
to procreate a single offspring. Mouse lemurs may live to 15 years; gorillas 50.
Mouse lemurs and gorillas are exemplary of fast and slow lives. Generally, small animals live fast while large animals have longevity.
Greenland sharks may live 500 years: the longest lifespan of any vertebrate. The largest of extant sharks, Greenland sharks swim leisurely, grow just 1 cm a year, and females (which are typically larger than males) may not reach sexual maturity until they are 156 years old.
While losing out on reproductive numbers, size has its compensations. Larger prey commonly have better antipredator defenses, while large carnivores may have a wider choice of prey.
Larger body size offers efficiencies. Thermoregulatory efficiency improves with size, as does the efficiency of basal energy needs.
On a per weight basis, larger animals require less food, and so can subsist on lower-quality fare. Mouse lemurs must feed on energy-rich insects, while gorillas can get by on energy-poor foliage. One might think that larger animals require more sustenance, but the gating factors are generally metabolic rate and various efficiencies, not size per se.
Size can be a limiting advantage. Because they have longer legs, bigger animals can typically run faster than smaller ones. This only works up to a point. In a family of animals, the fastest is midsized. Above a certain size, body mass requires a change of stride. Size imposes biomechanical constraints.
The maximum size of an animal is limited by population mortality. Because larger animals typically breed less frequently, if the mortality rate doubles, maximum size becomes 16 times smaller.
The adaptive response to increasing mortality is downsizing, which also occurs when environmental conditions are in flux. In the early 21st century many animals were having smaller offspring in reaction to rapid climate change.
Allometry
“Body size plays a crucial role affecting generation times, energy demands, and population sizes.” ~ American biologists Lauren Sallan & Andrew Galimberti
Allometry is an umbrella term for the relations between body size to shape, anatomy, physiology, and behavior. The term is also used to refer to the study of such relationships.
In insects, an incremental increase in body size can result in disproportionate enlargement of appendages: legs, antennae, and, in some beetles, horns.
Allometric relationships are often expressed as a power law. Allometric scaling deviates from the isometric, which is a 1-to-1 relationship.
“Allometric power laws are often strongly conserved across evolutionary time, invariant across taxa, and have long been hypothesized to reflect developmental constraints.” ~ Norwegian evolutionary biologist Geir Bolstad et al
In flies, small wings are typically rounder than large wings. Researchers bred fruit flies to violate that allometry. Once they stopped, the natural allometric relationship of wing roundness rapidly returned (in 15 generations).
Allometric scaling laws often express dynamics related to ontogeny (developmental growth). In an organism that grows as it matures, size increases but bodily shape remains similar; still there can be allometric changes during development. Lizards often exhibit such changes.
Based upon study of a variety of species, Swiss agricultural biologist Max Kleiber concluded in 1932 that animal metabolic rate is a 0.75 power of body weight. This allometric law is a consequence of the physics and geometry of animal circulatory systems. Allometric relations suggest that Nature follows laws for biology, just as in physics.
In the same species, smaller youngsters respire more per weight unit than larger oldsters because growth carries an overhead. By contrast, small adults of a species breath more per unit of weight than larger ones of another species because a larger proportion of their body weight is of structure rather than reserve. Structural mass has maintenance costs, whereas reserve mass does not.
To nourish itself, an animal needs energy. Employing that energy generates heat. An animal must rid itself of excess body heat. The obvious way is surface cooling.
As vertebrates get larger, they have relatively less surface area to dissipate heat. So, to be able to rid excess heat, metabolism must increase at a slower rate than body volume enlarges. According to Kleiber’s law, the relation between mass and metabolic rate is fixed. Something seems amiss. As it turns out, the missing element leads to another power law.
As vertebrates get bigger, the speed at which nutrients are carried through the body and heat is carried away increases to ensure heat disposal. The velocity of blood flow to an animal’s mass is a 1/12th (0.0833…) power.
“Animals need to adjust the flow of nutrients and heat as their mass changes to maintain the greatest possible energy efficiency. That is why animals need a pump – a heart – and trees do not.” ~ Italian hydrologist Andrea Rinaldo
Allometric scaling applies from the genome on out. In vertebrates, there is a correlation between the sizes of genomes and cells, especially red blood cells, and metabolic rate.
Having a petite genome affords smaller cells. Smaller cells support a larger surface-to-volume ratio, which makes for more efficient gas exchange.
Flying is energetically expensive. Birds and bats that take to the air must be as metabolically efficient as possible. Among birds, the strongest fliers have the smallest genomes, while flightless birds possess some of the largest.
Hummingbirds approach the theoretical limit for aerobic metabolism. Their genome weighs an average of 1.03 picograms (pg).
An average bird genome weighs 1.42 pg, reptiles 2.24 pg, and humans 3.5 pg. Some salamanders lug around 100 picogram genomes. Slimming down the avian genome began with the theropod dinosaurs from which birds descended.
Adaptive or environmental demands cause body sizes to diminish or enlarge. Body size is a resultant trait, not a driver. Size is determined by way of functional or developmental association, which ultimately relates to risk-based efficiency; with risk expressed as an extent of adaptability to environmental fluctuation. Depending upon ecological interaction, risk-based efficiency favors a certain size.
Allometry illustrates that life forms exist within physics-based mathematical constraints, via intricate component relationships.
Bat Echolocation
“Echolocation is a dynamic system that allows different species, regardless of their body size, to converge on optimal fields of view in response to habitat and task. All bats adapted their calls to achieve similar acoustic fields of view.” ~ Danish zoologist Lasse Jakobsen et al
Bats adjust their echolocation calls to go from surveying an area to pinpointing objects as they close in. How a bat does this depends upon its size, and especially depends upon its echolocation emitter: the mouth.
There is a strong relationship between bat body size and echolocation peak frequency, which is the maximum energy in echolocation calls. While large bats can produce focused beams at lower frequencies, smaller bats employ higher frequencies to achieve directional sonar beams.
The size of bat’s mouth determines its wavelength capabilities. Different bat species of equivalent size use sonar beams with selfsame beam shape and volume. This example of convergent evolution illustrates the functional nature of allometry.
Bat Shelter and Sociality
22 species of bat are known to make tents for shelter. Most build their tents from palm leaves and other sturdy plant parts. These structures often last for more than a year.
Other tent makers cannot access such durable materials available. They rely upon herbaceous plants, which make rather ramshackle rooms that last for less than 2 months.
Domestic comfort does not make for social cohesion; quite the contrary. Nomadic bats that must make a new home every few weeks are much more collaborative.
In contrast, group cohesion in those species with sturdy shelters is low. Whereas frequent builder bats stick together for more than a year, no species that stays in long-lasting lodgings does.
As the cohesive bat species are not closely related, the social stability that went with frequent shelter building evolved independently. Adversity promotes solidarity.
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Besides the distinction between fast and slow lives based on absolute body size, the lives of certain species may be fast or slow for a given body size. Habitats with instable food supplies engender fast lives with high reproductive output, as does a relatively high mortality rate.
Populations that inordinately die off adaptively trend to shorter gestation, smaller neonates (offspring), larger litters, and faster development (weaning and reaching maturity). Species populations with a high death rate tend to reproduce earlier.
Infanticide
Social dynamics can selectively influence infant development. Infanticide is exemplary.
“Infanticide is the result of an evolutionary arms race, where males compete with each other for reproduction and try to influence females in mating with them. In species where it happens more often, it can certainly influence the nature of the social relationships between males, as well as between males and females.” ~ Canadian primatologist Pascale Sicotte
Infanticide occurs in several mammal species, including lions, bears, rodents, and primates. Typically, an adult male kills infants sired by another so that he can mate with the mother and have her raise his offspring instead.
The evolutionary response to potential infanticide in colobus monkeys is for a mom to accelerate her offspring’s development by investing more energy in early growth. This only happens when a female lives in a group where infanticide appears a threat.
Monkey moms are forewarned by new males entering the group, and by their aggressive behaviors toward younglings. Efforts are most pronounced by mothers with the youngest offspring which are most at risk.
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Reproductive strategy is only one facet of a larger life-history scheme: survival. How life is lived is paramount in propagating quality for lives of the next generation.
Cicadas
Cicadas are large flying insects that live in temperate and tropical climates. There are about 3,000 species, living on all continents except Antarctica.
The cicada lifestyle is predominantly subterranean. They live as nymphs for most of their lives sucking root juice. Their strong front legs are built for digging.
In their 5th and final nymphal stage, cicadas tunnel to the surface and make their way to a nearby plant where they molt and emerge as adults.
Adults mate for few weeks to few months. Males form large choruses to loudly sing to receptive females. Cicadas have species-specific songs. Some sing at up to 120 decibels: about as much racket as any insect can muster.
Once mated, females cut slits into tree barks to deposit their eggs. Newly hatched nymphs drop to the ground, burrowing to find roots.
Cicadas are individually defenseless, making them an easy meal for birds, cicada killer wasps, praying mantises, and sometimes squirrels. In Australia, bass are keen to dine on cicadas that crash-land on the water.
The en masse cicada defense mechanism is predator satiation: being so concentrated in an area – up to 350 individuals per meter – that some survive despite many being eaten alive.
Most cicadas have a life cycle of 2 to 5 years. One North American genus – Magicicada – has a number of distinct broods that go through 13- or 17-year life cycles, depending upon region. Southern Magicicada emerge every 13 years. Their northern cousins have a 17-year cycle. These are the longest insect life cycles.
“The cicadas are driving the birds’ populations; they’re setting the birds on a trajectory that leads to significantly lower populations at the time of the next emergence.” ~ American ethologist Walter Koenig
The deluge of dead cicadas that litter the forest floor after mating temporarily boosts tree growth. At 10% nitrogen, cicada bodies are natural fertilizer. Early stage nymphs don’t tax trees sucking roots as much as when they get older.
The forest nutrient boost does not last. Bird populations hit a cyclical low when periodical cicadas emerge.
Nymphs somehow know when they should emerge. It may be that the trees upon which they feed signal the cycle, perhaps coupled with some cicada counting.
Some cicadas occasionally emerge 4 years off-schedule. These stragglers are easy prey that do not usually survive. In the off chance that they did, it would be a way of instant speciation.
Mammal Predation
Most small mammal predators are themselves preyed upon. Hence they evolved a suite of antipredator defenses that tend to sort them into 2 quite different lifestyles.
The solitary ones, such as skunks, are often aposematically colored and armed with noxious anal sprays. Subject to predation by other mammals, they tend to be nocturnal.
Gregarious species, such as mongeese, are socially vigilant. Such creatures tend to be diurnal, and at risk from birds of prey.
Mammal Defecation
“The smell of body waste attracts predators, which is dangerous for animals.” ~ American mechanical engineer Patricia Yang
Defecation is not a life-history variable for mammals. From mice to elephants, mammals excrete cylinders of waste like humans, and they all do so in about 12 seconds. This is consistent with the fact that all mammals take the same duration to empty their bladders.
Cylindrical feces are not squeezed through a nozzle (like a toothpaste tube).
“It’s more like a plug that just goes through a chute.” ~ Patricia Yang
The normal, low level pressure that mammals apply to push through a bowel movement is constant, unrelated to animal size. To achieve this consistency among species, the lubricating mucus in the colon varies in viscosity. Larger animals have longer feces and correspondingly longer rectums, so they produce thicker mucus which accelerates excretion: hence, feces flows a longer distance in the same time.
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In evolutionary time all possible phenotypic variables are in play. Phenotypic genetic malleability is limited only by physics.
“An autopoietic system is a homeostat: a device for holding a critical systemic variable within physiological limits. In the case of autopoietic homeostasis, the critical variable is the system’s own organization. It does not matter, it seems, whether every measurable property of that organizational structure changes utterly in the system’s process of continuing adaptation. It survives.” ~ English theorist Stafford Beer
Each habitat has its own difficulties: environmental demands and the trials of relations with one’s own species and others. These immediate requisites are common drivers of evolution. As a set of adaptations, tolerances to ecological changes also present challenges and opportunities to survival of a population.
Reptiles rose when drier times made amphibian life difficult. Mammals had a long wait under reptilian rule before their time came, a duration that might have been much longer but for a massive meteor strike. While supersized dinosaurs were wiped out, their smaller cousins, notably birds, survived; examples of evolutionary payback and payoff respectively.
Life-history variables invariably represent evolutionary trade-offs which murres and penguins exemplify.
Diving Birds
Several seabirds fly and swim. Being able to do both means neither is optimally efficient.
Cormorants dive into the ocean, propelling themselves underwater with webbed feet. Their dips into the sea to snag fish are brief. Even so, cormorants must expend enormous amounts of energy to fly.
Murres also dive to hunt, flapping their wings underwater to swim. Because murre wings are built for flight, they create drag underwater.
Murres’ small bodies are just light enough to let them take off out of the water. Keeping their weight down means they cool quickly. In contrast, penguins stay comfortable in frigid waters.
Murres are on an evolutionary knife edge. In being able to employ their wings for propulsion underwater, they are the most inefficient fliers. To be better swimmers, murres would have to be bigger, or trim their wingspan. Either would sacrifice flight.
Like many marine birds, penguins have a considerable commute between feeding and breeding grounds. Rather than fly, penguins swim.
Penguin life-history variables favored the water as a way of life, forgoing flight. Penguins today are a long way from their ancestors 70 MYA who could soar through the sky.
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Stable environments favor slower lives and lower reproduction levels, with competitive efficiency, where there gets to be some degree of competition between individuals of the same species, as a population tends to increase to the habitat’s carrying capacity.
Carrying capacity is the population size that a habitat might sustain. Given stable conditions, animal populations tend to grow toward carrying capacity.
Sexuality
As a life-history strategy, both sexuality and its orientation are options.
Asexuality was the original mode of reproduction and remains de rigueur for all prokaryotes. Many fungi and plants reproduce asexually. Genetic diversity is the facile drawback of asexual reproduction, overcome by genic transfer and pickup.
Sexual reproduction arose 1.2 BYA. Bacterial conjugation – DNA transfer between 2 consenting bacteria – has similar mechanics.
One hypothesis explaining the evolution of sexual reproduction is the ability of a population to more rapidly radiate in response to a changing habitat than parthenogenesis affords. Expansion speed could be advantageous in competition with other species for limited resources.
Simultaneous hermaphrodites have the advantage of sexual reproduction but don’t have separate sexes. Hermaphroditism yields sexual flexibility at the slight expense of expressing both sex organs. Besides grasses and some other plants, earthworms, certain snails and slugs, and some fish are simultaneous hermaphrodites.
A form of hermaphroditism in botany is monoecy: a plant with both staminate (pollen-producing male) and carpellate (ovule-producing female) parts. Monoecy is common in conifers, but only 7% of angiosperms are monoecious. Compared to long-lived conifers, flowering plants generally prefer greater genetic diversity as an edge in evolvability.
Sequential hermaphrodites change sex. A male that changes into a female practices protandry; clownfish are exemplary. With protogyny, a born female may change into a male. Wrasses and groupers practice protogyny after maturing. Sequential hermaphroditism allows populations to respond to ecological pressures by ensuring sufficient numbers of reproductive members in reasonable ratios.
Banana Slug Sex
The hermaphroditic banana slug savors sex. It has an outsized male member and a proportionately petite female organ.
In the act, slug penises becoming spirally wrapped; an entanglement not easily undone. If post-coital vigorous wiggling fails to separate the big bananas, one slug gnaws a penis off: either its own or its partner. This apophallation leaves one slug permanently memberless, though able to enjoy sex in the future as a female.
If partner sex is not possible, banana slugs practice safe sex and reproduce by self-fertilization.
Ram Horns and Longevity
“There is often considerable genetic variation underlying sexually selected traits.” ~ English evolutionary biologist Susan Johnston et al
Ample horns are a ram’s ticket to reproductive dominance. During breeding season males butt heads for access to females. Big horns triumph – at least for Soay sheep living on Hirta, an island off the coast of Scotland. Yet this has not launched an evolutionary escalation cycle. Some male sheep have short horns or none at all.
A single gene controls horn size, with various alleles that determine a male’s headset destiny. Those with the biggest horns father nearly twice as many lambs each year: 3 versus 1.6 for the less horny. But big-horned rams are less likely to last the harsh Hirta winter. Lesser-horned sheep live longer, and so breed for more years.
Lechwe Reproductive Strategy
Nile lechwe are an African antelope that live up to 12 years. Almost all adult females breed. As a female gets older, the odds of her producing a son increment.
Yearling first-time mothers bear sons 57% of the time. This rises to 67% by the time a female lechwe is 7 years old. The biomechanics of ratcheting the sex ratio is not known.
A female lechwe is 3 times more likely to die from bearing a son than a daughter. Sons are heavier than daughters, and so more taxing and risker to birth.
Sons are risky in other ways. Only dominant males have breeding rights. Hence, sons are genetic gambles, whereas daughters are an insurance policy for keeping a lineage alive.
Having played it safe while younger, older females risk getting more bang for their bucks.
Mating and Parental Care
“In mammals, males typically have shorter lives than females. This difference is due to behavioural traits which enhance competitive abilities, and hence male reproductive success, but impair survival.” ~ Austrian evolutionary biologist Helmut Schaschl et al
Animal mating and parental care practices are life-history variables with knock-on effects across a wide range of conspecific interactions. To hedge bets on survival via greater numbers, the earliest mammals produced large litters. The shift to fewer offspring occurred as the payoff for parental care rose.
Male competition for mating privilege is typical in animal species where a female is the primary caretaker of progeny. Besides sexual dimorphism being the norm, males pay the price in shorter lifespans.
Testes size is indicative of the trade-off in male primates between mating and parenting effort. Male chimpanzees are especially promiscuous. They generally do not provide parental care. Chimp testes are twice as big as humans. In contrast, male gorillas protect their young and sport small testes. Among humans, those with smaller testes are innately more interested in taking care of their children. But – irrespective of testes size – testosterone level drops as a father spends more time with his children.
Parenting is an outcome of social interactions between and within sexes, and so cannot itself evolve. But behaviors that provide information about parentage can evolve and affect mating and parental care.
The heuristic that parental care increases with evolutionary descent, size, and lifespan is full of exceptions. Hooded seal pups are nursed only ~4 days, whereas parental cichlid fish care may last months.
Precocial Versus Altricial
A key dichotomy exists in developmental strategy that affects life-history factors: altricial or precocial. Newborn of precocial species come into the world equipped to cope independently. By contrast, offspring of altricial species are dependent upon parental care to survive, as they are born or hatched in an immature form.
In altricial species embryonic development is relatively rapid. The neonatal brain will grow from its small size after birth. In contrast, development before birth is longer for precocial species and the neonatal brain larger. There is no consistent difference in adult brain size between altricial and precocial species.
Most mammals are small; less than 32 centimeters long.
Humans are large. An early hominid, Australopithecus afarensis, stood 1–1.7 meters, and weighed 60–65 kilos (female–male for both measurements). These proportions persisted for 2 million years, until Homo erectus, 1.5 MYA, with a 1.8-meter height and less sexual dimorphism.
As a group, mammals are relatively precocial. With their altriciality and an unusually big brain, primates are quite the exception.
Throughout descent, hominins lived slow lives. Modern humans extended earlier trends to enhanced altriciality.
Whereas many mammals can walk immediately after birth, it takes human infants up to a year to be able to ambulate on their own. A baby cannot even coordinate its sight with physical manipulation until it is 4 months old.
Climate & Body Size
Bodies adapt to their climate. For instance, humans rely heavily on sweating to keep cool. Surface area translates to ability to shed heat. Thinner and smaller translates to a higher ratio of surface area to body volume. The heavyset sweat while the gracile shiver.
In 1847 German biologist Christian Bergmann published Bergmann’s rule: in a geographically dispersed species, populations in warmer climates tend to be smaller than those in colder climes. Bergmann figured that body size in birds and mammals was an adaptation for heat dissipation. Smaller animals have a higher surface-area-to-volume ratio which facilitates heat loss through the skin. By contrast, heftier animals better retain bodily warmth.
30 years after Bergmann’s rule American zoologist Joel Allen published Allen’s rule: endotherms in colder climates tend to have shorter limbs or appendages than similar species more tropically resident. Human tribes who live near the poles, such as the Inuit, Aleut, and Sami, are heavier, with shorter limbs and broader torsos, than their equatorial counterparts.
Both Bergmann’s rule and Allen’s rule go to the relative ratio of body mass to surface area. Neither applies to absolute size.
Hot Horse
65 MYA the Cenozoic era celebrated the radiation of mammals after dinosaur demise. The Paleocene–Eocene epoch boundary marked rapid climate fluctuation. The world was hot 55 MYA.
Sifrhippus was the first horse: a Wyoming mustang that ponied up 56 MYA – a minute pony, as Sifrhippus was the size of a house cat. Over thousands of years Sifrhippus’ size waxed and waned as the climate wavered in its warmth.
In the first 130,000 years as it got hotter the horse shrank some 30%: from an average 5.6 kg to a truly tiny 3.9 kg, the smallest horse ever. Sifrhippus’s size rebounded in the cooling trend that followed: gaining 75% over the next 45,000 years.
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A 2017 statistical survey discounted Bergmann’s rule.
“Past studies that confirmed Bergmann’s rule were mostly looking at just a few species at a time, over only small areas, or were data-limited. Bergmann’s rule is not general, and temperature is not a dominant driver of biogeographic variation in mass. For 87% of species, temperature explained less than 10% of variation in mass, and for 79% of species the correlation was not statistically significant.” ~ American wildlife ecologist Kristina Riemer et al
Throwing statistical cold water on Bergmann’s rule does not explain what has been and is now observed. Fossil evidence repeatedly shows animal body sizes going down when the environment hots up. This dynamic is now being seen in butterflies, fishes, salamanders, snakes, sheep, and rodents.
“It’s an incredibly widespread phenomenon.” ~ English evolutionary biologist Andrew Hirst, upon reviewing experiments on 169 animal species
The current explanation is that body size correlates with metabolism, not heat dissipation, as Bergmann hypothesized: a smaller, less energy-intensive body may be advantageous in a warmer habitat. Young animals in warmer environments tend to grow faster but mature sooner. The result is a smaller adult size.
Faster maturation and smaller size may be explained as an evolutionary hedge for getting enough to eat: a changing habitat may mean a less reliable food supply.
Regardless of the evolutionary impetus, Bergmann’s rule is not uniformly seen: some animals are not getting smaller with warming and climate change.
“There are just so many things that are changing at the same time that it’s difficult to predict how every single organism is going to respond.” ~ American environmental scientist Jennifer Sheridan
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Adaptations in life-history traits are expectable from climactic changes, as plants and animals respond to temperature ratchets and altered rain patterns. Beyond temperature and moisture are other effects in the wind, literally. One of the changes from human intervention in the planetary ecology has been in the winds over the Southern Ocean. Wind speeds have picked up in subAntarctic waters, even within the past 2 decades. Winds in the more tropical waters, such as in the West Indian Ocean, have yet to be affected.
Wandering Albatross Winds
The wandering albatross is one of the most wide-ranging pelagic seabirds, with a circumpolar range over the Southern Ocean. Females prefer the warmer waters whereas males thrill to the chill of the waters closer to Antarctica.
Life-history traits of the wandering albatross have changed because of more intense winds in recent decades. Daily travel distance has increased with a pickup in wind speed, as has foraging range. Females travel further and have headed poleward to take advantage of the change in the breeze.
As a consequence of changes in foraging trip duration breeding failure has dropped. Because males and females share incubation duties, reduced foraging time has resulted in shorter incubation shifts, and thus a lower probability of losing offspring.
The upshot to the confluence of dynamics is that wandering albatrosses are gaining girth. This increase in body mass has not been accompanied by an increase in body size: the birds are not getting bigger, just heavier. This is an adaptive response to windier conditions, to improve flight performance (via the aerodynamics of wing loading); a response made possible by improved breeding success, which is an offshoot from faster flight for foraging in windier conditions.
Brains
“If ever you don’t feed your brain, you die immediately.” ~ Australian evolutionary zoologist Vera Weisbecker
It takes a lot of metabolic energy to run a big brain. Mammals that hibernate have smaller brains.
“There’s just 3 orders of lemurs which hibernate, and these 3 have the smallest brains in primates. Hibernation really is a constraint for brain size.” ~ Swiss anthropologist Sandra Heldstab
The correlation between brain size and warmth explains why primates arose near the equator in Asia, and why hominids emerged in equatorial Africa.
Once a species has evolved a larger brain, it may colonize more seasonal biomes. Evidence suggests that successful invasive vertebrates – whether amphibian, reptile, bird, or mammal – tend to have relatively larger brains, although Australasia seems exceptional in this regard.
Sociality
Sociality is entwined as a life-history variable.
“Social living poses challenges for individual fitness because of the increased risk of disease transmission among conspecifics. Despite this challenge, sociality is an evolutionarily successful lifestyle, occurring in the most abundant and diverse group of organisms on Earth – the social insects.” ~ Columbian entomologist Margarita López-Uribe et al
Given the greater risk of disease among gregarious animals, one might think that sociality and a more robust immune system go hand in hand. Instead, the opposite is true, at least in insects.
The more social insects are, the weaker their immune responses tend to be. Eusocial insects that live in large colonies have the most languid immune systems. This puts a premium on hygiene. Being truly colonial makes clean living easier for individuals as the benefits of shared labor are enjoyed by the whole group.
“The behaviors we see in eusocial species – like grooming each other or bringing antifungal materials into nests or hives – play an important role in colony health.” ~ Margarita López-Uribe
Locusts
“Desert locusts transform between two extreme phases, solitary and gregarious, depending upon their local population density.” ~ Brazilian biologist Patrício Simões et al
Locusts are nominally solitary grasshoppers that abruptly switch lifestyle to swarm as a ravenous horde after food becomes abundant and population size soars. It is a Jekyll-and-Hyde transformation.
When in their solitary phase, locusts are unassuming: their brown-green bodies ideal camouflage to blend into the foliage as they saunter with a low, creeping gait. Solitary locusts generally avoid one another unless they are mating.
Lifestyle dramatically changes after it rains and enough food becomes available to spur population multiplication. Then, as their numbers skyrocket, food exhaustion and physical contact with other locusts from overcrowding triggers a cascade of metabolic, mental, and behavioral changes.
The solitary and gregarious forms are so distinct in physiology and aspect that they were once thought to be distinct species. Instead, the extravagant show of adaptability is an ecologically induced fervor.
Gregarious locusts are colorful, move faster, and are attracted to other locusts. In this phase locusts form oppressive swarms that can blacken the skies and decimate vast fields of vegetation.
As a solitary insect, a locust is quite picky about what it eats. In swarm mode, locusts have no aversion to the bitter tastes which plants present to avoid being eaten alive.
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Hominid evolution is generally characterized by these animals becoming more gracile. Humans are impressively physically weak compared to their primate cousins.
Concomitant with hominin descent came more cohesive sociality, which included greater regularity of cooperative behaviors, notably tribal defense (a necessity in the face of augmented group conflict). Technology also afforded the ability to survive in the face of a hostile environment as well as fortifying tribes with advancing weaponry.
Humans became more gracile and their bones weaker as they adopted agriculture and settled down to a more sedentary lifestyle. Ingenuity and loss of strength represents a complex life-history trade-off, especially with sociality stirred in.
