Reptiles are ectothermic: maintaining body temperature through environmental means. Conversely, birds and mammal independently evolved endothermy: internally maintaining a metabolically favorable body temperature.
Normal body temperature for endotherms varies by group but ranges 38–41 ºC in birds and 25–38 ºC in mammals. Feathers, fur, and body fat help conserve heat. Bodily organs produce heat, as do active muscles, either voluntarily or by shivering.
Heat is shed via evaporation – either sweating or panting – and by convection and radiation from the body’s surface. A small animal has a larger surface-to-volume ratio than a larger one. Hence, body size and configuration are life-history variables correlated with an animal’s natural biome.
Adapting to habitats which have a different ambient temperature range often involves bodily changes to better maintain internal temperature. Seabirds, for instance, are generally stouter than those that live over land, as are birds that migrate long distances compared to those that stay close to home.
Birds and mammals are the only animals to universally practice endothermy, and even some of them save energy and lower their temperature by going into a state of torpor. Bats may do this during the day. When disturbed, they shiver briefly to restore body heat before flying off.
Some hummingbirds become torpid at night. Their body temperature falls with air temperature until 20 ºC. If the external temperature falls further, hummingbirds maintain themselves at 20 ºC.
Body temperature may fall to as low as 6 ºC in mammals that hibernate, such as bears and hedgehogs. The body temperature of the Arctic ground squirrel may drop as low as minus 2.9 ºC. Its blood stays liquid and its tissues are protected by the biological equivalent of antifreeze.
The brains of hibernating animals are kept warmer than their bodies, even as the cold takes its toll on nerve cells near the surface. 80–90% of the energy used during hibernation goes to keeping the brain alive.
Endothermy necessitates a metabolism that runs several times that of ectotherms. The trade-off involves what an animal is relying upon the environment for: heat (ectothermy) or abundant food (endothermy).
In being economical in every way, from digestion to movement, sloths have a well-earned name. 3-toed sloths do not even maintain a constant body temperature: allowing a nearly 5 °C swing in their everyday lives.
Sloths are on the reptile end of being a mammal. ~ English zoologist Rebecca Cliffe
At the expense of greater energy consumption, endotherms can grow and develop much faster than ectotherms. Shorter adolescence allows the development of parental care without taking too much of the parent’s life span and so curtailing multiple broods.
If ectothermic crocodiles were to care for their young until adulthood, they would be able to reproduce only every 9 years or so. Most birds and mammals have 1 or more broods per year.
Not only can endotherms offer better parental care, they can provide some degree of warmth within their nests. In temperate latitudes this extends the breeding season.
There are exceptional ectotherms who achieve this by special adaptations. The diamond python keeps its eggs suitably warm by coiling around them.
There are some ectothermic animals that provide parental care, but only short-term. Ectothermic parental care does not extend until offspring are well on their way to being full grown, as is common with birds and mammals.
Endothermy affords cognitive stability in the wildly fluctuating temperatures of terrestrial habitats. This improves response time to danger, or for hunting, independent of the weather.
Overall, endothermy improves survival prospects and affords shorter generation time. Together these constitute an intrinsic potential for faster population growth, which offers an evolutionary edge.
Mesotherms rely on metabolic heat to raise their body temperatures above ambient temperature, but do not defend a thermal set point as do endotherms. ~ American zoologist Robert Eagle et al
As mesotherms, dinosaurs split the difference: able to internally generate body heat, but not to the degree of maintaining a thermal homeostasis like endotherms. Great white sharks are one the few living animals that are mesothermic.
Relative to reptile ectotherms, dinosaur mesothermy afforded increased speed and agility. Dinosaurs were faster predators, or faster to flee danger, than the reptiles that dominated the early Mesozoic. This performance advantage was instrumental in the rise of dinosaurs.
Dinosaur thermophysiology was not uniform. Different lineages had distinct levels of bodily warmth.
During the Triassic there were numerous large predators that preyed upon smaller reptiles and early mammals. One way to reduce predatory risk was to become nocturnal, or at least crepuscular. Although there are now specialized nocturnal predators (e.g., owls), most minute mammals retain their nocturnality, or at least a strong preference for being in the shadows.
Because large ectothermic predators can retain body heat longer when the ambient temperature drops, endothermy is a great advantage for small animals; hence the evolution of endothermy in mammals, which started their ascent when they were modestly sized.