Temperature Adaptation. Time Course of Adaptation
Special terms apply to temperature. Poikilotherms vary in body temperature (TB) with environmental temperature (TA); homeotherms maintain constant TB in varying TA. Heterotherms are intermediate with body temperature varying temporally (e.g., diurnally or seasonally) or spatially in different regions of the body as between skin and liver. Ectothermy and endothermy refer to source of heat—external as by sunlight or internal as by metabolism. Therefore, ectothermy and endothermy are not synonymous with poikilothermy and homeothermy.
Temperature regulation is maintained by a variety of mechanisms, which vary in relation to body size and habitat (Fig. 3). One mode of regulation is behavioral (i.e., seeking an optimal external temperature); another is circulatory (i.e., metabolic responses, insulation, blood flow, and metabolic rate).
FIGURE 3. Schematic representation of changes in metabolism CM) and body temperature (TB) of homeothermic animals at different ambient temperatures (TA). TNZ is thermoneutral zone of minimum metabolism. TC is critical temperature below which M increases; M also rises above TNZ. Curves for cold (C) and warm (W) acclimated animals at different ТA. Body temperature is maintained by vasomotor and metabolic reactions in midregion; regulation fails at both high and low extremes. (From Prosser, Adaptational Biology, 1986.)
Constancy of internal state is homeostasis (e.g., constancy of body temperature, blood sugar, arterial PO2, blood sodium concentration). Homeostasis is characteristic not only of blood but also of cellular composition. For example, intracellular potassium concentration is extremely constant in variable blood potassium.
Constancy of energy production is homeokinesis. Many animals with variable body temperature (conformity) compensate energetically for changes in TB. Homeokinesis or constant energy production can occur in absence of homeostasis. Both homeostasis and homeokinesis are adaptive.
Time Course of Adaptation. Direct Responses. Adaptations can occur during each of three time courses. The first course is direct response or immediate reactions to environmental change. Morphological responses can be changes in fur or feathers associated with warmth or cold. Poikilo-osmotic crabs and molluscs change serum concentrations of organic solutes more than those of inorganic ions
Acclimatory Changes. The second period is that of days or weeks. Acclimation occurs when single environmental parameters change, as in the laboratory. Acclimatization refers to changes with multiple factor alterations such as seasonal or geographical. A laboratory rat acclimates differently to cold than does a wild rat to winter conditions.
Acclimation in poikilotherms is compensation for temperature such that some constancy of functional capacity is ultimately attained. Energy-yielding reactions increase in the cold and decrease in the warm in compensation for change in temperature (Fig. 4). Several patterns of acclimation have been identified (Fig. 5).
FIGURE 4. Representation of a reaction rate of a poikilothermic animal measured at three temperatures: T1 T2, and T3. Thin line represents direct effect of temperature change. Arrows indicate compensation (rise in rate or fall in rate) during acclimation to low or high temperatures
FIGURE 5. Patterns of rate functions of poikilotherms during temperature acclimation. T2 is intermediate temperature; T1 is lower, and T3 is higher temperature. Rates shown after acclimation; pattern 4 shows no acclimation, only Q10 effect; pattern 2 shows perfect acclimation with rate the same at each temperature after acclimation; pattern 3 shows partial acclimation; pattern 1 shows overcompensation; pattern 5 shows inverse or negactive acclimation
The pattern of acclimation varies with measured rate function, with tissue, and with species. There may be no compensation (pattern 4), complete compensation (pattern 2), or, most common, partial compensation (pattern 3). A few examples of over compensation (pattern 1) and of inverse or paradoxical acclimation (pattern 5) are known, especially for hydrolytic reactions. Cellular mechanisms of acclimatory compensation are increases or decreases in enzyme activities, changes in amounts of specific proteins or of total cellular proteins, and changes in proportion of saturated and unsaturated fatty acids in membrane lipids. Acclimatory changes occur only within genetically determined limits.
Long-Term Selection of Genetic Changes. The third period for adaptation extends over long times—generations—by selection of genetic mutants and establishment of local races and varieties. In geographic dines and circles, the differences between terminal populations are highly adaptive and may result in differences at the species level.
Examples of molecular differences that are genetic include changes in abundance of specific allo- zymes as distinguished by kinetic properties. Metabolic enzymes of deep-sea fish do not function at pressure of one atmosphere. Selection of secondary and tertiary structure results in adaptation. The Qio, or temperature dependence, of homeothermic enzymes is steeper than that of poikilotherms. Many examples of selection of morphological characters are well known—use of insulating fat in cetaceans and of hair in ungulates.
Interesting differences in adaptation to heat in mammals relate to body size. Figure 6 diagrams the heat exchanges and thermal responses of mammals of the size of dog or human. Large animals such as elephants and camels in a warm climate store heat subcutaneously with resulting constancy of temperature of deep organs; small mammals such as rodents with large surface-volume ratios rely more on vasomotor changes. Heat gain or loss varies with circulation in exposed structures, as in ears of rabbits. The fat of lower legs and feet of reindeer has lower melting points than internal body fat.
Date added: 2023-05-09; views: 336;