Researchers investigated the relationship between temperature and size of specific body parts of birds. The findings have broad application to all animals on Earth. The research will help scientists better understand how species may evolve due to human-caused climate change.
A team of researchers examined 7,000 species of non-migratory birds, which make up about two-thirds of all avian species, to study changes in body part size in relation to global surface temperature. Birds are warm-blooded creatures that generally follow certain biological rules for body part size. However, there are deviations from the rules, and scientists wanted to identify the underlying causes of the exceptions.
In 1847, German biologist Karl Bergmann noted a pattern of large animals found near the poles. The tendency for larger animals to occupy extreme latitudes is now called Bergmann’s rule, because smaller bodies are more efficient at shedding heat, while larger bodies can retain it. That’s why the polar bear is 2½ times larger than the sun bear that lives closer to the equator.
American zoologist Joel Asaph Allen proposed an ecological trend specifically associated with shorter, thicker limbs in warmer climates. Smaller limbs, ears, and beaks reduce the surface area through which heat can escape from the body. This is why Arctic hares have short fur and legs, and desert-dwelling jackrabbits have long ears and long legs to dissipate heat more easily.
Researchers have found that birds generally follow Bergman’s Law and Allen’s Law. For example, owls near the polar regions are much larger than owls near the equator or the tropics. However, there are competitive advantages that species may lose if they follow the rules, and that’s where nature steps in and creates exceptions. Smaller birds may not be able to hunt the same food as their ancestors, and changes in bill shape or size may alter mating calls, or prevent birds from foraging efficiently.
A paper describing the findings Published in Nature communication. First author of the paper, Justin Baldwin says, “When your hunting tactics depend on the size and shape of your beak, that structure is the last thing you want to change. Consequently, changing body size instead is not a problem.” Researchers have found that flycatchers, which have smaller bills that retain heat in the polar regions, are better suited to shed heat in the tropics. Because the birds are already small, they cannot grow any tinier to trap less heat in the first place, to compensate by using larger beaks.