Homeothermic animals

Warm-blooded animals (homeotherms) maintain body temperatures within close tolerances. Yet, dnring altered states, body temperature may drop by a significant fraction. Are these animals still classified as warm-blooded when this happens Why ... 

The ability of some organisms to control the pH and temperature of their cells and tissues represents a major biological development. Homeothermic animals (e.g. mammals) maintain a constant temperature of about 37 °C as this corresponds to the temperature of optimum activity of most enzymes. Poikilothermic or so-called cold-blooded animals (e.g. reptiles) have to sun themselves for sometime every morning in order to raise their body temperature in order to optimize enzyme activity within their cells. 

Metabolism - a final factor in need of comparative studies is the metabolism of xenobiotics. One obvious difference between mammalian and fish species is that their bodies usually function at temperatures at least 10°C different. This fact undoubtedly explains some differences in metabolic rate but even when in vitro incubations are run at optimal temperatures there is a 10 - 100 fold higher rate of mammalian vs. fish metabolism (14, 15). In other words, the level of the xenobiotic-metabolizing capacity, especially for oxidative pathways, of the poikilothermic animals is considerably lower than that of the homeothermic species. Elsewhere in this volume Dr. Bend has focused on this aspect of the handling of xenobiotics by fish (16). 

In addition to these challenging physical constraints, the demands for food that an endothermic homeotherm faces are substantially greater than those for an ectotherm of the same body mass. At the same body temperature, on a unit mass basis, similar-sized endotherms and ectotherms differ in oxygen consumption on average by at least four- to fivefold (see Brand et al., 1991). It is obvious that many ectother-mic species may not have access to enough food, at all times, to make it possible, even in principle, for them to become endothermic homeotherms. An ecosystem in which all animals are endothermic homeotherms is probably impossible. 

Endothermic homeotherms are characterized by high mass-specific rates of oxygen consumption relative to similar-sized ectotherms. This difference can be shown by examining the allometric scaling relationship between rate of oxygen consumption (M) and body mass (IF). For ectothermic and endothermic animals, whole animal oxygen consumption rate is proportional to total body mass raised to approximately the 0.75 power ... 

Calder III, W.A. (1981), Scaling of physiological processes in homeothermic animals. Annual Review of Physiology, 43, 301-322. 

The oxidation of fuels converts approximately 25% of the potential energy present in the foods ingested by humans to ATP. This relative inefficiency of the human engine leads to the production of heat as a consequence of fuel utilization. This inefficiency, in part, allows homeothermic animals to maintain a constant body temperature in spite of rapidly changing environmental conditions. The acute response to cold exposure is shivering, which is probably secondary to increased activity of the sympathetic nervous system in response to this stressful stimulus. 

Both mammals and birds are homeotherms, which means that they attempt to keep their body temperature constant. Animals produce heat continuously and, if they are to maintain a constant body temperature, must lose heat to their surroimd-ings. The two main routes of heat loss are the so-called sensible losses by radiation, conduction and convection from their body surface, and evaporative losses of water from the body surface and lungs (2.52 MJ/kg water). The rate at which heat is lost is dependent in the first instance on the difference in temperature between the animal and its surroimdings for farm animals, the rectal temperature, which is slightly lower than the deep body temperature, lies in the range 36-43 C.The rate of heat loss is also influenced by animal characteristics, such as insulation provided by the tissues and coat, and by environmental characteristics, such as air velocity, relative humidity and solar radiation. In effect, the rate of heat loss is determined by a complex interaction of factors contributed by both the animal and its environment. 

On cooling the enviromnent of the cell membrane to just below the normal ambient temperature, the head groups become arranged in a more ordered hexagonal manner and the hydrocarbon chains become more straight. The temperature of the liquid crystal phase to gel phase transition depends upon the environment of the organism concerned. For example, homeothermic animals control their own body temperatures and so the cells are... 

Animals subjected to environmental stresses make physiological adjustments within their range of capability. In homeotherms, the thyroid influences thermoregulation and cell metabolism therefore, a measure of thyroid activity... 

Home, R. A., Almeida, J. R, Day, A. F., Yu, N. T. (1971). Macromolecule hydration and effect of solutes on eloud point of aqueous solutions of polyvinyl methyl ether - a possible model for protein denaturation and temperature control in homeothermic animals. Journal of Colloid and Interface Science, 35,77. 

Sometimes there is confusion due to different use of terms. Thermodynamics calls a process exothermic when heat is given off by the system and endothermic when it consumes heat. In biology, warm-blooded (homeothermic) animals like mammals and birds that produce heat to keep their body temperature constant are called endotherms and their behaviour endothermic, in contrast to the thermodynamical direction. Their counterparts with changing body temperatures are ecto-therms (not exotherms) or cold-blooded poikilothermic) animals. Nevertheless, all of them produce heat as a by-product of their metabolism so that they can be monitored by (exothermic) calorimetry. 

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