The Elements of Evolution (51-1-10) Climate & Body Size

 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.