Little birds, warm place; big bears, cold place? Until now, biologists have speculated that it would be easier to maintain body temperatures in fluctuating weather if one has a large body. This was based on thermoregulation studies done by Laurence Irving and Per Scholander regarding Arctic birds and animals in the 1950s. Under their model, it was thought that warm-blooded mammals and birds need to balance their heat production by metabolism with their loss of heat to the outside environment.

But in a recent study, researchers from the University of New Mexico examined the geographic ranges of 6,356 bird species and 2,648 mammal species--and they concluded that mammals and birds of virtually every size live essentially everywhere, according to a release. 

How are the animals managing this?

The study gathered physiological data that shows that the birds and mammals adjust their metabolic rate and "thermal conductance." After all that, body size changes only play a small part. To give an example, a fox has a higher basal metabolic rate than a sloth. Thick fur can ensure more insulation and better thermal conductance; similarly, big ears or long legs can help slough off heat.

"We were interested in understanding ways other than body size that species can adapt their physiology and morphology in order to deal with environmental temperatures," lead author Trevor Fristoe, now a postdoctoral research associate at Washington University in St. Louis, said in the release.

The scientists looked at environmental temperature and physiological data for 178 mammal and 211 bird species, learning that in order to adapt to variations in geography that brought about varying temperatures, the creatures made changes in their thermal conductance and metabolic heat production, said the release.

The research findings were published recently in the journal Proceedings of the National Academy of Sciences.

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