The Elements of Evolution – Life-History Variables

Life-History Variables

“You have these two lineages, plants and animals, that are very different and yet they arrive at the same conclusion. That is what’s called convergent evolution, and the stunning result is that it’s being driven by the underlying physics.” ~ Italian physicist Amos Maritan

From an evolutionary perspective, all organisms face strategic survival trade-offs. Viruses typically carry only the genes essential to their needs. In contrast, bacteria must take into account their social environment, both among their own kind and others. Horizontal gene transfer facilitates adaptability, though this is employed selectively, and so is itself a life-history variable.

“Evolution constantly faces trade-offs between objectives.” ~ Israeli biologists Elad Noor & Ron Milo

Numerous life-history variables correlate to each other. Following constructal law, energy flows – from the origination of life onwards – favor certain sets of traits. Internal and envirotypic flows interact to produce scale-invariance. Interactively, life works at every scale toward an optimality.

Whereas size factors into selected sets of life-history variables, the relationships between some life-history variables remain constant irrespective of size. Several observations suggest that relative size distribution adheres to a universal power law: a quantitative ratio that is scale-invariant. The size of an organism matters for its metabolism, growth, survival risks, and advantages within its habitat.

How quickly an organism can evolve is size-dependent. The current global warming is having the greatest impact on large animals. Small animals can more easily adapt.

Both smaller plants and animals are capable of faster rates of molecular evolution. As such, rapid climate change provokes diminution as a hedge against risk.

Certain life-history variable sets are strongly influenced by the habitat in which an organism lives. Environment and ecology drive evolution adaptively.

“Specialization between animal and plant species tend to be a consequence of the available resources.” ~ German evolutionary ecologist Matthias Schleuning

The intensity of competition and the vibrancy of interspecies interactivity makes a decisive difference in life-history variables. Darwin and later evolutionary biologists long assumed that species-rich tropical communities inclined plants and animals toward specialization as compared to their temperate counterparts. Instead, competition amid tropical diversity tends toward less dietary specialization.

“High tropical plant diversity provides many different resources to animals in a low density. Whoever is not especially choosy is at an advantage, because then the next food source is not very far away, making foraging more efficient.” ~ German ecologist Jochen Fründ

Less-persnickety animals are advantageous to plants when it comes to pollination and seed dispersal, as a plant is less likely to suffer extirpation if a certain helpful animal agent goes extinct.

“Due to the generalised relationships and the greater diversity, more species can replace the functions of individual declining species.” ~ German ecologist Nico Blüthgen

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In many instances, the trade-offs that constitute life-history variables can be inscrutable. Zebrafish bred to be bold or shy exhibit changes in body shape and locomotion.

“Complex behaviors, like the behaviors we call ‘personality’ or ‘temperament,’ can be associated – genetically correlated – with other traits that one might think are independent of such behaviors, like body shape and swimming abilities. Traits that seem unrelated may not be unrelated.” ~ American zoologist Brian Langerhans

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.


“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.


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.


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.


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 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.


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.


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.


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.


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.


“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 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


“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.


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.

Intelligence as a Life-History Variable

As with all variables of life intelligence is an adaptation, suited for the lifestyle of the specific organism. For instance, research on birds demonstrated that populations (of the same species) which lived through harsher winters showed enhanced problem-solving ability. That withstanding, there are generalities which apply to all.

The mind is a symbolic pattern matcher. All cognition and learning are a play of concepts. A concept is a locus of cohesive conception in a context of related concepts. Hence, a concept is a mental tensor: all information is relational. Conceptual contexts are organized hierarchically, from general to specific, with associative links.

Microbes live the purest mental lives. Their world is of molecular interactions. They must distinguish between foodstuffs, toxins, and friend from foe. Microbial communications are molecular, and often include messages made by fabricating specific molecules.

Microbes also understand genetics at the molecular level. This aspect of microbial intellect is utterly alien to animals, who have no such ability.

Plants extend microbial cognitive acumen into the richest possible mental life. Plant molecular savvy extends into creating concoctions designed to enchant potential friends or foil foes in the most devious ways – beyond our ken in how such feats may be accomplished, let alone in how plants know so much about the intimacies of other life forms.

Along with the social categories of friends, competitors, and community, plants also have suppliers, symbionts, and pollinators to constantly consider and manage. The advent of angiosperms greatly enhanced the social possibilities and demands of daily life. Floral relations are considerably more complicated than those fauna face, and involve more subtle communications and interactions.

Facets of intelligence are life-history variables. The solutions to many challenges faced in life may be instinctual. This is true for every organism, which also has as precocious knowledge how to learn in specific realms and in certain ways. Learning is needed to adapt to situations that are habitat-specific, most notably food sources and their procurement; even as the basic algorithms for foraging may be innately encoded (e.g., Lévy walk).

 Eyesight to the Blind

In 1688 Irish philosopher William Molyneux, whose wife was blind, posed a question to English philosopher and physician John Locke: could someone born blind, used to categorizing shapes by feel, recognize those shapes if sight were restored? Locke answered “no,” figuring that the connection between the senses was learned. Dozens of philosophers since have considered the problem.

Experiments to solve Molyneux’s problem began in the early 18th century. For 2 centuries studies done were inadequate to resolve the issue. Finally, in 2011 the answer came – Locke was right: making sense of sensation is acquired, but the means to do so is inborn.

“Cross-modal learning is possible despite years of deprivation.” ~ American psychologist Richard Held et al

Molyneux’s problem illustrates a baseline between precocious knowledge and learning: whereas specifics such as shape must be learned, the ability to categorize in certain ways is innate. The mind is preformatted with methodology, whereupon experience may impress via learning.


Broadly, the shorter the life span or less parental care the more acumen tends to be inborn. This is especially true for precocial species which hatch and are on their own from birth.

Another consideration is that longevity is an evolutionary advance. Organisms with long, fluid, evolutionary lineages are short-lived, and their life solutions are generally baked in, if for no other reason than there has been abundant evolutionary time for the most-needed behavioral variations to have be encoded.

That withstanding, microbes pick up new tricks all the time. Literally. Horizontal gene transfer (HGT) affords instant knowledge uptake: canned wisdom, where plasmids act as the can. HGT provides a virtual planetary library to viruses and is a ready source of local self-help tips to microbes of every stripe.

In contrast, animals with longer lives, which are typically altricial, have much to learn, though they too are born with a hefty set of instincts. Still, as corvids illustrate, memory and flexibility in problem-solving pay survival dividends.

Intelligence comprises situational comprehension coupled to the ability to take advantage of that information. The source of intelligent behavior, whether innate or ad hoc, is entirely beside the point of its exhibition.

One is rationally hard-pressed to demean as inferior instinctual reflex to cogitation when hard-wired responses get the job done. Instinct is intelligence, at least when it works: success always being the metric of intelligence, however arrived at.

 Manipularity–Intelligence Theory

The savvy of life forms is a life-history variable. The manipularity–intelligence theory correlates innate shrewdness with manipularity: the ease by which an organism can manipulate its environment. Basically, organisms which have lesser ability to change their environment must have more “on the ball” to survive.

Microbes individually have little ability to alter their world. As such, they must rely upon their wiles: discernment at the molecular level, including being able to assess the utility of information resident in DNA. Microbes use sophisticated quorum sensing to assess the power they may have as a crowd. Pluricellularity has a power to craft fate which individual cells may only dream of.

Sessile plants survive by their wits. The head start of autotrophy – not needing to find food – belies the difficulties of managing the myriad of risk-based potentialities in a largely uncontrolled environment. A universe of decisions must constantly be made related to resource allocations and the probabilities of chance encounters, especially interspecifically (e.g., herbivory by savage animals). The unsurpassed intelligence of plants is amply illustrated by how they conduct their sociality with bacteria, fungi, other plants, and animals.

Problem-solving as a proxy for intelligence is, obviously, a partial indicator. Intelligence originates with perspicacity which gives rise to comprehension. At the base of intelligence is the mental ability to manage symbols (e.g., concepts) to survive.

Intellectual capacity in animals is exemplified by how frequently they resort to trial and error in problem-solving. With the limited manipulatability of beaks for hands, birds commonly think through a problem to its solution before beginning implementation. Corvids, for example, are known to act only after they have mentally figured the steps for solving a complex task. Methodological adjustment only happens when new relevant information is revealed during implementation.

Like birds, dolphins cannot easily manipulate their environment. They are compensated with generous aptitude for problem-solving, abetted by a convivial sociality which engenders cooperative effort.

In contrast, humans, who have unrivaled ability to control their environment, constantly resort to trial and error to solve problems. Human memory and ability to mentally map out solutions is feeble compared to rodents, who have much less facility for physical manipulation.

The relative stupidity of people is demonstrated by their invariable environmental destruction. Throughout their existence, humans have degraded their habitats. (Habitat destruction by species is rare, as it is an excellent formula for self-extinction, as we see with humans.)

Rare indeed have been the instances where humans have been able to sustain their populations without resort to technological advances, which only accelerate the sapping of natural resources. That humans have been unable to devise a social system to sustain themselves without essentially enslaving sizable percentages of their societies further evidences the relative mental weaknesses of this species.

Human overestimation of their own intelligence is laughable. Dimwits smugly think they know what is going on. In this, so-called geniuses are no different. Despite copious evidence to the contrary, the resolute conclusion of modern science is naïve realism and a simple-minded empirical matterism: that the experienced actuality of materiality is objective reality, and that what comes to mind has validity (notwithstanding having experienced countless instances of false thoughts and self-deception).

People think they are smarter than other animals merely because they cherish their beliefs, and the artifacts which abstractions have brought: technology. But intelligence is not the capacity to have woolly thoughts or to produce mass consumption and destruction. Intelligence is instead the ability to behave aptly. On this count humans have failed miserably.

Plant Life-History Variables

“The ideal plant design would depend strongly on what other plant species were doing.” ~ English botanist Michael Crawley

Plants have their own life-history variables, manifest by their tailoring for the specific habitats to which they are adapted. The more extreme biomes, such as deserts, rain forests, and the polar regions, result in a remarkable degree of evolutionary convergence.

Conversely, areas with temperate climate are typically home to a vast diversity of coexisting vascular plants, each with its own life history. There is marvelous diversity in the growth regimes of plants. Genotypic changes can be passed on in any part of a plant. This demonstrates the difficulty of readily categorizing a spectrum of floral life-history facets as can be done with animals.

“Plants vary not just in their pace of life, but also in their reproductive output and frequency of reproduction.” ~ English ecologist Dave Hodgson

Small plants generate seeds as often as the odds are that some of the next generation may survive. That said, many annuals adopt a mixed strategy for their seeds of intended destination and the precise conditions under which their seeds will attempt their start in life.

The calculus changes when competition toughens. To forge their spot in the Sun, trees delay investment in reproduction until they have gained enough girth to ensure their own survival over a protracted period. Only then do they devote resources to reproduction, but strictly when a plant thinks its offspring may fare well: during times of high productivity, in years of good weather for pollination, or when herbivores are scarce. Trees unable to obtain a place in the canopy usually die without leaving any progeny at all.

One life-history-variable commonality plants share with animals is relative size. Short plants live in the evolutionary fast lane. Short plants have up to 5 times the adaptive ability of tall ones. Growth rate makes the difference. Small plants grow faster.

“Taller plants have lower rates of molecular evolution.” ~ English evolutionary biologist Robert Lanfear et al


“Seed survival, dispersal and persistence in soil determine the composition and dynamics of plant communities.” ~ English botanist Louise Colville et al

For spermatophytes (gymnosperms and angiosperms), seeds start the engine of life. Tiny annual plants may produce 10 seeds within a few weeks before dormancy, while giant conifers produce huge seed crops over centuries. For a plant investing in the next generation, a trade-off exists between number and size.

Bigger seeds beget more competitive seedlings, but they cost more to produce, have a lower dispersal distance, and a higher risk of predation. (Exceptionally, coconuts managed worldwide dispersal despite huge seeds. Their seed strategy was uniquely far-sighted. But then, one’s perspective is different when life’s a beach.) Larger seeds tend to germinate quickly, but this growth spurt advantage leaves a young plant less likely to prosper in the face of intense herbivory.

Producing enormous numbers of seeds is common for plants that have grown in good conditions. A single poppy plant may produce a million seeds. Yet few find a suitable location for germination, and most will be eaten by a ravenous horde of herbivores.

The alternative life-history is for a seed is to be swaddled in protection; its mother trading raw numbers for better odds. So it is with the coconut. Only a smattering of coconuts are made by an individual tree. A strong fibrous cover and hard shell protects the embryo against most herbivores. The milk within is a life-giving care package, consumed when a suitable home is found. A coconut typically starts life when its nut washes ashore, but what signals the embryo to initiate germination is unknown.

As illustrated with coconuts, coating is a life-history variable for seeds. Animal seed eaters naturally prefer soft seeds, whose consumption spells an end to their germination potential. Soft seeds are more easily located via their cocktail of volatile compounds that give their presence away. By contrast, hard seeds, even if detected, may survive a 2nd dispersal event, outlasting digestion or the travails of hoarding by a fastidious seed eater.

“Most hard-seeded plants produce dimorphic seeds. In terms of reward, dimorphic seeds support the hypothesis that the primary evolutionary explanation for hard seeds is predator escape.” ~ Norwegian evolutionary ecologist Torbjørn Paulsen

Seed dispersal may be had by the wind, water, or animal agent. Presentation and packaging of seeds depends upon the path to propagation.

Environmental uncertainty leads plants to adopt a mixed strategy for seed dispersal. Annuals often create seeds in different packages: some designed for long distance travel while others drop nearby. Potential competition between siblings is instrumental in this decision-making. Plants in the sunflower family are exemplary: fruits of the outermost flowers lack parachutes, while the more numerous, smaller inner seeds are readily wind-dispersed via pappi.

For annuals, the advantages of fecundity and survival via dormancy are offset by size constraints and the need for fertile soil. The disadvantages are offset by a hedging strategy: seeds that germinate under different conditions.

Synchronous sprouting of an entire seed crop would risk total loss if environmental conditions deteriorated. Thus, individual plants give any single clutch of seeds a range of dormancy-breaking and germination thresholds.

For a seed coat to split, the embryo must imbibe (soak up water), causing the seed to swell and split. Only then can germination begin.

Seed embryos monitor their situation and may resolve upon a false start. A seed may imbibe, but still decide that conditions are insufficient to risk germination – so, the embryo waits.

There are 2 forms of seed dormancy: physiological and physical. Soft seeds rely solely on physiological dormancy: sprouting when sensing an opportune time. Hard-shell seeds also have physical dormancy. Hard-seededness helps protect against microbial attack, reduces external chemical signaling of presence, and extends seed longevity, at the cost of a plant producing a hard coat for each of its latent offspring.

“Many plant species rely on hoarding rodents for secondary seed dispersal. Hard seeds are surprisingly frequent in hot deserts, where granivory by small rodents is very intense.” ~ Torbjørn Paulsen et al

Hard seeds are used by only a few plant families. Legumes are one of them. At least 6 times, different legumes independently decided to pay extra for hard coating, which both reduces olfactory signaling and improves survivability when a seed is snatched.


More than 1,000 tree species may be in a small area of a tropical rainforest. While competition is inevitable, the trees must get along with their neighbors to some degree.

Wood density and leaf morphology are exemplary traits that influence a tree’s capacity to compete. There are trade-offs. Lighter wood trees can grow more quickly than those with dense wood, but the lightweights tend to die sooner and are poor competitors. Trees with dense tissues are more imposing on their neighbors.

While the density of forest diversity is not a fully understood dynamic, it is more than avoiding being different from the neighbors in resource use and life-history strategy. Certain traits are more advantageous at the various stages of forest succession regardless of whether they are different from nearby trees. In young forests, where trees are more spread out and there is little competition, fast-growing trees have an advantage. As a forest matures and the neighborhood becomes more crowded, slow growers do better in getting the resources they need: minerals, water, and light.

Larger leaves can be helpful in gathering light and thereby garnering energy but trees that can readily vary leaf size based upon prevailing conditions are more competitive.

Certain life-history selections preclude adaptations to other biomes. Grasses’ adjustment to relative aridity precludes their appearance in rain forests. Geophytes, with their underground storage organs, shun Arctic heathlands, as long-frozen soil is too much to bear.

Secondary metabolites are a plant life-history variable, both in compound selection and deployment. These are driven not only by ecology interactions with microbes, other plants, and animals, but also by the ready availability of ingredients and the economy of biosynthesis.

Unlike animals, being stuck in one spot for a lifetime creates a fundamentally different scenario by which life-history facets are favored. For plants, seasonal light, air (e.g., moisture, temperature), and substrate (e.g., soil) conditions are primary drivers.

Climate determines the seasonal growth patterns of annuals. The customary practice of blooming in late spring from seeds dormant through the winter is belied if summer is unwelcoming. In habitats with moist winters and dry summers, many annuals germinate in autumn, producing a rosette of overwintering leaves. Rapid growth starts early in the year, with flowering in early spring, and ripened seeds before summer scorching.

While winter annuals avoid the attentions of most invertebrate herbivores, they are especially vulnerable to vertebrates that forage throughout the winter. Young, green foliage at a time when many other species are largely leafless is irresistible.

As with animals, allometric relationships indicate constraints which determine which plant architectures are possible and those that are likely to be successful. Such trade-offs are as much ecological as they are internal.

A host of plant traits conform to the neighborhood in which plants live and how plants conduct their lives. Interspecific ecology is a major influence on plant life-history variables.

The life-history spectrum of perennials presents different strategies. Both trees and herbaceous perennials maintain tenure of their plot of land for many years. The most obvious lifestyle distinction between the two is their occupancy of the aerial environment. Trees maintain their presence year-round while the shoots of renascent herbs die back every growing season.

Unlike trees, herbs must reestablish each year their position in the canopy of plants competing for access to light. This is offset by lessened vulnerability to drought, cold, or pests during the inclement season, though herbs must have various defensive measures in place. Herbs being spared the considerable investment in wood is a mixed blessing.

Herbs are also more mobile than trees: able to spread radially via rhizomes, stolons or rooting shoots. This is something of a hedging stratagem for having a less commanding presence than trees.

Obtaining sufficient sunlight is the great challenge for shrubs and trees. Reproduction is delayed until a homestead can be established. Trees unable to obtain a place in the canopy die without progeny.

The race for a place is the sun creates options for various growth regimes, particularly branching patterns and shoot rates. Regardless of the chosen pattern, a ubiquitous mitigation is to cut losses by culling branches that do not prosper.

Conversely, trees tend to grow only as tall as necessary to compete and meet reproductive goals. Growth is a series of decision-laden stages.

Above all, both above ground and below plants are opportunistic. The overarching life-history strategy of plants is to optimize the possibilities for success in ways that have no analogues with animals.

Phenotypic and growth pattern variability in plants exceeds that of animals. This may seem odd, as plants are sessile. But heterotrophy is an overwhelming burden as a life-history variable, dictating motility. There is no need for ambulation if sustenance can be manufactured on the spot.

From a life-history perspective, autotrophy spells freedom. The flexibility afforded by photosynthesis has yielded tremendous diversity, as creative design is relatively unconstrained. Overall, plant life-history variables are selected from countless compromises, and so are not easily characterized.

Rate of Living as a Life-History Variable

“Live fast, die young….” ~ American novelist Willard Motley in the novel Knock on Any Door (1947)

Some plants last mere months, while others stand sentry for millennia. Fruit flies have a fleeting existence of 40 days, while an ocean quahog – a mollusk native to the North Atlantic – may live 400 years or more. One quahog – Ming the Clam (~1498–) – is over 500 years old. Despite such disparity, it has long been pondered whether there is an approximate constant among life-history variables: relative lifespan related to the rate of living.

Different flora and fauna live at different paces. For animals, basal metabolic rate – energy consumption at rest – has been used as a proxy. Heart rate has also been considered another rough measure.

German physiologist Max Rubner studied metabolism and energy physiology. In 1883 he introduced the surface hypothesis: that the metabolic rate of endotherms is roughly proportional to body surface area.

Subsequent work led to the broader concept of life’s pace dictating longevity. In 1908 Rubner proposed the rate-of-living hypothesis: that the faster an animal’s metabolism the shorter its lifespan. Rubner observed that the lifespan of large animals exceeds small ones and that larger animals had slower metabolisms.

In the mid-1920s American biologist Raymond Pearl expanded on Rubner’s work by studying the life histories of fruit flies and cantaloupe seeds. Pearl corroborated Rubner’s observation that slower metabolism increased lifespan.

“Across species a gram of tissue on average expends about the same amount of energy before it dies regardless of whether that tissue is located in a shrew, a cow, an elephant or a whale. This fact led to the notion that aging and lifespan are processes regulated by energy metabolism rates, and that elevating metabolism will be associated with premature mortality – the rate-of-living theory.” ~ Scottish biologist John Speakman

The rate-of-living hypothesis was later popularized as the heartbeat hypothesis: that every endotherm has a lifespan of a billion heartbeats. Hummingbirds and humans are commonly analogized, though hummingbirds last 1.26 billion heartbeats, while humans may make 2.45 billion.

One study noted that athletically fit people tend to a lower resting heart rate and are prone to a longer life than the unhealthy. This is, at best, tepid support for the heartbeat hypothesis, as it ignores many factors associated with physical fitness.

Anecdotal evidence upholds a relation between longevity and the pace of living. Life in the slow lane lets giant tortoises live 150 years. Even houseflies live longer if they take it easy.

While larger animals tend to live longer than smaller ones, there are consistent exceptions. Some species live far longer than expected based on their size. A crucial factor for this seems to be flight. Birds and bats tend to live longer than other animals of similar size. This is especially true of those active during the day (diurnal) or night (nocturnal). Those in action at dusk or dawn suffer somewhat from greater predation.

Max Kleiber’s allometric power law succored the notion that rate-of-living was a life-history variable. Various cellular mechanics were proposed as attributive to longevity for both plants and animals.

In 1954, free OH radicals in cells provoking reactive oxygen species (ROS) stress were cited by American biogerontologist Denham Harman as causing aging.

In 2003, Australian biologist A.J. Hulbert pointed to the fatty acid composition of cellular membranes as seminal in lifespan determination.

“Life requires membranes. The rate-of-living theory cannot alone explain all of the variation in longevity of animals. Many of the exceptions can be explained by knowledge of membrane fatty acid composition in each particular case.” ~ A.J. Hulbert

Exceptions which violate straightforward rate-of-living correlation suggest a complex relationship between energy expenditure and longevity.

The rate-of-living theory (RLT) faces 4 types of challenges. 1st, the predicted correlation between energy expenditure and lifespan does not hold when comparisons are made across taxons. A typical example is that birds have higher metabolic rate than mammals with the same body mass yet live much longer. 2nd, RLT also fails to explain why within a species, such as domestic dogs, the larger breeds with lower mass-specific metabolic rates usually have shorter lifespans. 3rd, a few lifespan extending interventions, such as diet restriction (DR) and genetic modification of growth hormone, generally do not alter, or only slightly reduce, mass-specific metabolic rate. Moreover, a few studies even showed that when metabolic rate is altered by DR, it is positively correlated with lifespan. The 4th challenge comes from experimental manipulations that increase metabolic rate but do not shorten lifespan. For example, long-term cold exposure largely increases energy expenditures in mice, rats, and voles, but has no effects on lifespan. Moreover, voluntary exercises increased food intake in female rats while increasing lifespan.

Oxidative metabolism can affect cellular damage and longevity in different ways in animals with different life histories and under different conditions. Qualitative data and the linearity between energy expenditure, cellular damage, and lifespan assumed in previous studies are not sufficient to understand the complexity of the relationships.

“The oxidative stress theory of aging (OST), another theory that links energy metabolism and longevity, suggests that the deleterious productions of oxidative metabolism (e.g., reactive oxygen species, ROS) cause various forms of molecular and cellular damage, and the accumulation of the damage is associated with the process of aging. Widely considered by many as a modern version of the RLT at the molecular and cellular level, this theory shares all the supports and challenges of the RLT, as well as a few of its own. New sources of supports include the evidence that (1) external oxidative insults shorten lifespan, (2) the level of oxidative damage to macromolecules increases with age, and (3) genetic interventions and diet restriction, while extending lifespan, reduce the oxidative damage. New challenges to OST mainly come from the studies in which adding antioxidants to diet or genetically altering the expression of antioxidant enzymes, which were assumed to change the oxidative damage, failed to affect longevity. In some cases these interventions even yielded results that opposed the theory’s predictions.” ~ Chinese zoologist Chen Hou & Indian molecular biologist Kaushalya Amunugama in 2015

Hou and Amunugama proposed that rate-of-living is more complicated than mere energy expenditure, as cells have mechanisms to repair the effects of stress which incur aging – activities which take energy but extend life.

“Energy trade-offs and protective efficiency affect animal lifespan. Biosynthesis plays a role in oxidative damage accumulation and the process of aging. The detailed energy trade-offs between life-history traits and the efficiency of energy utilization are the keys to understanding the complex nature of the energy-longevity correlation.” ~ Chen Hou and Kaushalya Amunugama

“Energy used during growth is the key to understanding longevity.” ~ Chen Hou

It is unsurprising that the molecular biomechanics of energy use as they relate to lifespan are too complex to model, even as there appears to be some connection between metabolism and longevity. Then there is luck.

 George the Lobster

George was a 9 kg American lobster captured off the coast of Newfoundland in December 2008 and sold to City Crab and Seafood restaurant in New York City for $100. George arrived at the restaurant on New Year’s Eve. He spent 10 days in a tank, ogled by eaters.

The restaurant owner, Keith Valenti, agreed to let George go after being pressured by the animal rights group PETA. Recanting his initial refusal, Valenti declared freedom for George a “no brainer.”

Now a celebrity, George was driven in a lobster limo to Kennebunkport, Maine, and released back into the wild. Lobster fishing is prohibited in the waters off Kennebunkport.

At the time of his capture George was 104 years old.

 Full Lives

“The ability to perceive and react to a dynamic environment is a key behavioural and ecological trait.” ~ Irish zoologist Kevin Healy et al

Many of the interactions that shape behavior and ecology rely on the rate that an animal can process sensory information. Body size and metabolic rate constrain how an animal interacts with its environment.

Smaller animals interact more quickly while larger live longer at a more leisurely pace. The patience of a primate is correlated to its body size: smaller ones with higher metabolism lose patience more quickly. This measure is, of course, in absolute time. Relatively, based on rate of living, a smaller animal is just as patient.

“There is a whole world of detail out there that only some animals can perceive.” ~ Irish zoologist Andrew Jackson

The pace at which an animal lives and its perception of time is tied to its nominal lifespan. The trade-offs of life-history variables that determine rate-of-living and lifespan provide a rough constant: the existential experience of life is equally rich for all animals. The same may apply to plants and other organisms as well.

With a quicker mind at work, fast lives are as full as those that are lived more slowly; hence the full-life hypothesis, related to rate-of-living.

Aging & Mortality

“Many people, including scientists, think that aging is inevitable and occurs in all organisms on Earth as it does for humans: that every species becomes weaker with age and more likely to die. That is not the case.” ~ American evolutionary biologist Owen Jones

Aging and mortality (rates of death) among the tree of life are an extreme example of divergent evolution. Some organisms don’t age but succumb at widely varying rates. Others age and die in deviating ways. Some species with pronounced aging (that is, those with sharply rising mortality rates) live a long time, whereas others don’t.

Humans and some other mammals weaken as they age, with rising mortality. By contrast, some plants and many animals have a rather constant mortality throughout their lives, regardless of lifespan.

Many flora and some fauna, such as the desert tortoise, experience the highest mortality early in life, with a steadily declining rate thereafter. Many of these organisms suffer no loss of vitality with age as humans do.

“Species with very different life spans can display similar patterns of mortality, fertility and survivorship.” ~ German demographer Alexander Scheuerlein et al


Fertility also has great diversity. Women are fertile for a relatively short mid-section of their lives. A similar fertility period is seen in some other mammals, including chimps, orca, and chamois, and in sparrow hawks.

Some species become more fertile with age. This pattern is especially common in plants.

Others start life fertile, then quickly lose the ability to produce offspring. The soil-dwelling roundworm Caenorhabditis elegans is exemplary.

 Negligible Senescence

“The puzzle of longevity is not why we die so soon but rather why we live so long.” ~ English ecologist Jonathan Silvertown

Negligible senescence characterizes organisms which lack aging symptoms.

Microbes asexually reproduce via fission: a cell divides into 2. The split is not of identical halves. One half gets older because it is laden with defective cell material while the other half is kitted out with new workings. By this microbes produce offspring younger than the parent.

“Some organisms can exist for millennia. They are metabolically active but in stasis, with less energy than we thought possible of supporting life.” ~ American microbiologist Karen Lloyd

Fission yeast were discovered in East African millet beer in 1893. In a stressful environment, these yeast suffer the pangs of senescence. Contrastingly, when life is good, fission yeast do not age: mother and daughters are equally well equipped for immortality.

Hydra are small, simple predators of other aquatic invertebrates. They appear immortal.

Turritopsis nutricula is an exemplary hydrozoan which can revert adult cells back to their childhood vigor. In essence, T. nutricula can reincarnate itself.

Some fish, including varieties of lake sturgeon and rougheye rockfish, are thought to be negligibly senescent.

Though the ocean is a rough neighborhood, with morbid mortality rates, sea urchins, lobsters, and certain clams are not known to die of old age. At the least, they may live well over a hundred years.

Sometimes bandied as living forever, there are no reliable reports of turtles or tortoises having negligible senescence.

An endolith is an organism that lives a sheltered life, inside rock, coral, or animal shell. Most are autotrophic extremophiles. The roster of endoliths includes certain viruses, archaea, bacteria, fungi, lichen, amoebae, and lichen.

Microbes living 2 km beneath the ocean floor have been found that appear to be millions of years old. Their reproductive cycle is on the order of 10,000 years.

The larvae of carrion beetles can developmentally retrogress when starved. They then regrow back toward maturity when food is available. This cycle is repeatable.

Bristlecone pines are long-lived. Individuals in the White Mountains of eastern California are ~5,000 years old.

Some plants approximate immortality via clonal colonies. Among these are Neptune grass, creosote bushes, and aspen trees.

Neptune grass is a slow-growing seagrass endemic to the Mediterranean Sea. This flowering and fruiting seagrass forms expansive underwater meadows which are crucial to the habitat of other species. Under favorable environmental conditions, Neptune grass may live 200,000 years.

Creosote bushes have a hard time in their early years: the hot, parched desert takes its toll. But once established, creosote bush colonies can live over 10,000 years.

An individual aspen tree may live 40–150 years above ground but its root system is a long-lived colony which may last thousands of years. One colony in Utah is 80,000 years old.