Contents Updated: Thursday, August 05, 1999
In the 19th century, paleontologists thought dinosaurs were related to warm blooded birds and so they too were thought to have been warm blooded. From the 1920s until the 1960s, paleontologists thought dinosaurs to have been related to the cold blooded reptiles. Now, the idea of warm blooded dinosaurs is again fashionable though there are dissenters. What though do warm blooded and cold blooded mean?
They are shorthand terms for certain aspects of animal physiology:
Body Temperature Variation.
Body temperature can either vary or be essentially constant. Keeping a constant body temperature independently of the ambient temperature and usually above it, is called homoiothermy, but in the US where people normally add syllables to words here they have sensibly cut one and have shortened the word to homeothermy making it an analogous word to homeopathy. Maintaining a widely variant body temperature, usually dependent on changes in ambient temperature, is called poikilothermy.
Body Temperature Regulation.
Animals which regulate their temperature by using internal energy are endotherms and those which regulate their temperature by using energy from their environment are ectotherms. Both regulate their body temperature but they do it in different ways. Endotherms control bloodflow to the skin, contract muscles or burn carbohydrates and fats to get or preserve their body heat. Much of this heat regulation is automatic. Ectotherms, being alive, certainly produce internal heat but not in any substantial or controlled way and do not have the regulating mechanisms of warm blooded animals. Ectotherms apparently have to deliberately seek warmth or coolness in the environment to control their temperature. The distinction between endotherms and ectotherms is however not absolutely clear cut. Endotherms sometimes have to seek warmth or coolness in extreme conditions and we can guess that the apparently deliberate movement of ectotherms will often be instinctive rather than consciously deliberate.
Resting Metabolism.
Metabolism refers to the level of internal activity that takes place in an animal, such as cellular respiration or generating energy. Resting metabolism refers to the level of internal activity when an animal is at rest. An animal whose metabolism remains at a high level, producing calories and maintaining bodily activity at a high pace, is called tachymetabolic. An animal whose metabolism slows to a low level of activity when resting is called bradymetabolic.
Generally, animals referred to as warm-blooded are homeothermic, endothermic and tachymetabolic, while animals referred to as cold-blooded are poikilothermic , ectothermic and bradymetabolic. As we noted, however, these distinctions are not always clear cut.
Mammals are classified as warm blooded, yet mammals can exhibit the characteristics of cold blooded animals such as ectothermy and bradymetabolism. Some mammals are warm blooded at certain times in the year and cold blooded at others. Hibernating mammals are cold blooded. The temperature of an hibernating marmot can fall to just a couple of degrees Celsius above freezing point with subsequent recovery. Warm blooded mammals can also be induced to behave as if cold blooded under anaesthesia, and can revert to cold bloodedness through injury to the spinal cord.
Conversely insects are considered to be cold-blooded, yet insects can show characteristics of warm-blooded animals, such as endothermy.
So care should be taken when using the terms warm blooded and cold blooded. They are not absolutes and many animals have mixed characteristics. We have no reason to believe that Dinosaurs were any more regular in their heat regulatory regimes than other animals. They also will have had a mix of heat regulatory and metabolic strategies.
These notes are partly abstracted from the website, Dinosauria On-line, to which readers wanting more detail are referred. It has a wealth of dinosaur information for dinosaur amateurs and enthusiasts alike, aims to give the reader a broad exposure to dinosaur science and provides a forum for topical discussion. Discover the links between Archaeopteryx and modern birds and find out why the dinosaur DNA find claim may be a mistake. There is a link to the Dinosaur Electronic Mailing List and real enthusiasts can order a replica oviraptor egg for their mantelpiece. Send e-mail to Jeff Poling.
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