Question

In spatial heterothermy/mesothermy, (a) core body temperatures are maintained above ambient temperatures and (b) the temperature of the rest of the body varies with ambient temperature. Explain why each part (a and b) of this strategy could confer evolutionary fitness (increase reproductive success.)

Answer 1

Heterothermy describes variations in body temperature along both spatial and temporal scales. For example, animal body temperature is usually warmest at the core but may be much lower in the extremities. The extremities are usually allowed to cool in homeotherms, while the body core temperature can be conserved by warming the blood returning from the extremities through counter-current exchange. A relatively constant and high body temperature is widely regarded as a key characteristic of mammals and birds, the lineages that have evolved endothermic-homeothermy. Maintaining body temperature within a narrow range is costly in terms of water and energy use . In situations of energy deficit or energy unpredictability, many small mammals exhibit daily or multi-day torpor (hibernation), characterized by pronounced reductions in body temperature, water loss and energy expenditure. Such heterothermy is widely considered a key response for survival in changing environments. Maintaining a body temperature below ambient temperature requires the evaporation of water, while maintaining a body temperature above ambient temperature requires the generation of heat. Limited water or energy supply leads to a widening of the inter-threshold zone of thermoregulation, and thus a decrease in thermoregulatory precision. Vasomotor changes in the periphery can induce regional heterothermy in the peripheral shell of a mammal without major changes in core temperature , while more pronounced heterothermy is manifest as an increase in the daily amplitude of the core body temperature rhythm, without body temperature decreasing to the very low levels considered indicative of torpor . The lower core temperatures in the inactive phase of the daily rhythm appear to be regulated in birds and mammals . During exposure to very hot days, the body temperature of Arabian oryx (Oryx leucoryx) fluctuated by more than twice as much over 24 h when individuals had no drinking water and little food, compared to when they had access to water and food.

While extreme environmental conditions can exacerbate the heterothermy caused by water or energy deficit, extreme conditions are not required. Limiting the feed intake of sheep or rats at 22°C to 70% of their daily energy requirement resulted in more than a doubling of the daily amplitude of body temperature , with substantial inter-individual variability. Similar high inter-individual variation in body temperature amplitude was seen when monkeys faced food scarcity in winter. Such variability may reflect more resources being channelled towards thermoregulation in individuals with better access to resources (e.g. as a result of higher social rank), or in individuals with lower energetic demands.

In situations of water or energy deficit, many mammals sacrifice the precision of body temperature regulation. Water and energy are required for physiological processes other than thermoregulation, for example growth, osmoregulation and reproduction. Whether the reduction in the precision of body temperature regulation is reflected in detriments to other physiological systems is not known. We predict that the 24 h amplitude of body temperature will be inversely correlated with the performance of individuals, and ultimately with fitness. While the use of torpor and hibernation has been related to fitness outcomes in terms of reproductive output and winter survival and the evolution of a slow life history.