Which vertebrates are homeothermic

McMahon, T. Hochachka, P. Strategies of Biochemical Adaptation Saunders, Philadelphia, Dawson, T. Whittow, G. Book Google Scholar. Bakker, R. Springer, Berlin, Theory 1 , 51—80 Lilligraven, J. Taxon 11 , — Kielan-Jaworowska, A. Nature , 97 Article Google Scholar. Eisenberg, J. Kleiber, M. The Fire of Life. Hildwein, C.

CAS Google Scholar. Shkolnik, A. The Physiologist , 15 Taylor, C. Pedley, T. Academic, London, Download references. Crompton, C. You can also search for this author in PubMed Google Scholar. Reprints and Permissions. Evolution of homeothermy in mammals. Nature , — Subsequently, they can eat any size plant or animal. Many reptiles must swallow their prey whole, which limits them to hunting smaller game. Like birds, mammals are endothermic , or warm blooded. They are able to maintain a relatively constant body temperature regardless of external environmental conditions mainly by using internal physiological mechanisms.

In other words , they are homeothermic , or stable in core body temperature, as a result of endothermy. All of the living species of insects, fish, reptiles, and amphibians are ectothermic , or cold blooded. They keep their body temperature in a normal range mainly by avoiding exposure to environmental temperature extremes. For instance, reptiles usually remain in shaded areas on hot days to prevent fatal overheating. On cold nights, their lowered body temperature can cause them to become sluggish and inactive.

In contrast, endothermic animals are able to remain active at night and often in the winter when the air temperatures are especially cold.

They can also move about in the heat of very warm days. This ability most likely provided an advantage for the early small mammals in surviving alongside dinosaurs and other large reptiles , which apparently were mostly ectothermic.

The downside of endothermy is the need to consume far more calories relative to body size in order to maintain a constant core body temperature. Small mammals, such as moles with their rapid metabolism rates, must eat insects or other high calorie foods every half hour or so in order to stay alive. By comparison, cold blooded rattlesnakes usually eat only once every weeks and have been known to go without food for as long as two years.

Aiding in mammal body temperature control is their insulating hair and sweat glands. Sweating helps to dissipate heat by evaporative cooling. Compared to most other land mammals, humans are relatively hairless, but they have far more sweat glands. Mammals have four chambered hearts like birds , complex nervous systems, and large brains relative to the size of their bodies.

This broad range of useful features has made mammals highly adaptive and successful. They first appeared about ,, years ago , early in the age of dinosaurs , and replaced reptiles as the dominant class of land animals after 65,, years ago. As the rapidly changing environment at that time led to the mass extinction of most large reptiles, it left vast evolutionary possibilities which mammals took advantage of by rapidly diversifying through adaptive radiation.

Important to mammalian success is their reproductive system. Their bodies took the amniote egg revolution of reptiles and birds one step further. Relatively large mammals rely more on their control of surface temperature to thermoregulate and they are unable to lower core temperature as quickly. These fundamental differences in thermoregulation between small and large mammals should be taken into consideration when extrapolating hypothermic responses from rodents to humans.

This is an abstract of a proposed presentation and does not necessarily reflect EPA policy. Jump to main content. Contact Us. Description: Homeothermic organisms birds and mammals have evolved autonomic and behavioral thermoeffectors to maintain a relatively constant core temperature over a wide range of environmental temperatures. Show Additional Record Data. Hide Additional Record Data.