Search

Homeotherms are substantially more capable of resisting cold than heat, if the normal rectal temperature is taken as the point of departure. With the possible exception of man, the number and effectiveness of the mechanisms protecting against loss of heat are also far greater. In contrast to the poikilotherms, whose temperature resistance can be essentially characterized by their lethal temperatures, a number of critical temperature ranges can be distinguished for homeotherms: 1. the external temperature at which the capacity for temperature regulation is exceeded and the core temperature changes; 2. the core temperature at which the regulatory centers are paralyzed and 3. the lethal temperature. Special conditions are also found, in which local temperature damage takes place while overall temperature regulation is maintained. Accordingly, the tolerance shown by homeotherms for extreme temperatures is determined by the interaction of a large number of factors.

In addition to the fast regulatory processes taking place within seconds, minutes or hours, which were dealt with earlier, homeothermic organisms are able to carry out long-term adjustments to changing climatic conditions. These processes taking place over periods of days, months, perhaps years, are called long-term adaptations.

Spermatogenesis, fertilizing capacity, and motility of the sperm of homeothermic species are strongly temperature dependent. Chicken sperm survive temperatures of -15° C and can still move at 6° C, but they are irreversibly damaged by heating to 50° C for more than two minutes. They use oxygen most rapidly at 40.8° C, and are damaged least by temperatures of 20° C (Winberg, 1941). Fertilized rabbit ova remain capable of development for up to 150 h at 10° (Chang, 1947). Polge et al. (1949) demonstrated that spermatozoa can be cooled to -79° C and thawed again without losing their capacity for fertilization. Similarly, ovarian tissue can be re-implanted successfully after several days of cooling to -79° C (p. 669).

Homeothermy is part of the ability of organisms to maintain homeostasis or stability (Ashby, 1960, 1966). Internal body temperature is regulated in higher vertebrates with a certain degree of accuracy. This means that internal temperature is compared in some way with a reference, and when a difference occurs an appropriate response is made tending to reduce that difference.

Some warm-blooded animals do not maintain homeothermy to the furthest limits, but instead undergo hibernation periodically. In this condition their body temperature can fall to 0–4° C, permitting them to survive for months without feeding. This condition differs from hypothermia in that, at the lower limit of tolerance, regulation begins again, and even at ambient temperatures below 0° C hibernation homeothermy (“second homeothermy” — Wyss, 1932) is maintained. Waking and active rewarming is possible at any time.

In this book (p. 422) it was emphasized that temperature is only one of many factors upon which the geographic distribution of organisms depends. This is especially true of warm-blooded animals, whose homeothermy has made them independent, to a certain degree, of external temperature influences. The very fact that with negligible exceptions homeotherms live in the open air means that they must be able to cope with greater temperature fluctuations than must aqueous animals. A direct limitation in the distribution of warm-blooded animals as a result of temperature is usually found only in extreme ranges. Thus, most authors in this field have emphasized that the temperature factor in the distribution of homeotherms should not be overestimated (e.g. Marcus, 1933; Niethammer, 1952). In addition, warm-blooded animals, especially birds, are able to evade seasonal fluctuations in temperature because of their mobility.

Production of heat from exothermic chemical reactions, especially in the skeletal muscles and certain internal organs, is one of the variable quantities by which homeothermic organisms can regulate their core temperatures. At high external temperatures and during heavy physical labor the formation of heat becomes more and more of a handicap, since performance of mechanical work by the muscles is always connected with an unavoidably high production of heat, amounting to two to ten times their mechanical energy, depending on the efficiency (Lehmann, 1953). Even in the absence of external work or cold exposure a relatively large amount of heat is produced, which, when measured under certain standard conditions (rest, calm, neutral external temperature, psychological relaxation), is designated as basal metabolism. Its determination in animals is often difficult due to uncertainty that the condition of “rest” is satisfied. Basal metabolism is appreciably higher than the level needed to maintain body temperature in a warm environment. Consequently it is not determined by a requirement for heat (cf. “surface area law”, p. 531).

The achievement of “thermodynamic freedom” (Burton and Edholm, 1955) by homeothermic (constant-temperature or warm-blooded) animals, who can maintain a practically constant internal temperature regardless of changes in external tem­perature, is undoubtedly the most significant advance in regard to the manifold relations between temperature and living beings. In contrast to the poikilotherms, whose vital processes depend entirely on external temperature conditions, homeotherms — birds and mammals — live at practically the same level of intensity during the entire year, and in all climatic regions; thus, they have a high level of reactivity, and can make use of their strengths and abilities whenever they desire. Their high internal temperature prevents them from freezing, which is the fate of poikilotherms under cold conditions unless they can retire to deep waters, to frost-free layers of the soil, or to warmer climatic zones, or unless they can survive in desiccated, encapsulated form. The geographic distribution of homeotherms is thus not so limited as that of poikilothermic forms of life, especially plants, which cannot change locality in order to avoid unfavorable temperatures. In regard to the direct influence of temperature, warm-blooded life is possible practically everywhere on earth; in regard to other factors, however, there are naturally a number of practical limitations, among them the problem of nutrition, which is also dependent upon temperature.