BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

19.3. Thermoregulation

19.3.5. Heat Gain - Ectothermy and Endothermy

All animals obtain heat from two sources: 1) directly from the external environment, and 2) through heat-releasing Chemical Reactions within their Cells.

The extent to which animals can generate and retain heat depends on the physiological mechanisms characteristic of their phylogenetic group. All invertebrates, fish, amphibians, and reptiles lack the physiological mechanisms required to maintain body Temperature within narrow limits, although this is often compensated for by behavioral adaptations. Such animals are termed poikilotherms (from the Greek poikilos meaning varied and thérmē meaning heat). Because they primarily rely on ambient heat to raise their body temperature, another term is also used: ectotherms (from the Greek ektos meaning outside). These animals were formerly referred to as cold-blooded, but this term is inaccurate and can be misleading.

Birds and mammals are capable of maintaining a nearly constant body temperature regardless of environmental conditions, thanks to specialized physiological mechanisms; hence, they are called homeotherms (from the Greek hómoios meaning similar) or, less precisely, warm-blooded animals. Their temperature is relatively independent of external heat sources and is sustained by a High Metabolic Rate combined with thermal insulation mechanisms. Because these animals rely on internal sources of heat, they are also referred to as endotherms (from the Greek endon meaning within). The body temperature of certain poikilotherms can occasionally exceed that of homeotherms; therefore, to avoid confusion, the terms "endotherm" and "ectotherm" will be used hereafter. Fig. 19.6 illustrates the ability of various vertebrates to regulate their body temperature in response to environmental conditions.

Class="center">

Fig. 19.6. Relationship between the internal body temperature (measured rectally) of selected vertebrates and the ambient temperature to which they were exposed for 2 hours. All curves except the bottom one correspond to mammals: the top two represent placental mammals (eutherians), the middle two represent egg-laying mammals (monotremes), and the lowest curve represents a reptile.