Flexible patterns in energy savings: heterothermy in primates

Abstract

Heterothermy is an energy‐saving strategy usually employed in response to environmental bottlenecks, which is common in almost all mammalian orders. Within the order primates, heterothermy has been physiologically confirmed only in the family Cheirogaleidae (Cheirogaleus, Microcebus, Allocebus, Mirza) of the Malagasy lemurs, and the southern lesser bushbaby (Galago moholi) of the family Galagonidae. These closely related species employ a spectrum of daily torpor, prolonged torpor and obligate hibernation under tropical, but nevertheless seasonal and energetically demanding conditions. There is a remarkable physiological flexibility in regard to their thermoregulatory adaptations not only between species of the same genera within one habitat, but also between populations of the same species, within populations and even within the same individual, permitting immediate responses in seasonal and unpredictable environments, and possibly aiding these species to master challenges of globally changing climatic conditions. Whereas heterothermy is a flexible, but regular seasonal response in the Cheirogaleidae, it is only used as a last emergency strategy in G. moholi. In the other primate species, there is either no or only anecdotal evidence of only minor reductions in metabolic rate, presumably rather reflecting pronounced circadian cycles in body temperature, or local heterothermy. Maybe because of their relatively high degree of sociality and larger body sizes, behavioural and ecological adaptations to seasonality are selected for in other primates.

Body temperature and thermal environment in a generalized arboreal anthropoid, wild mantled howling monkeys (Alouatta palliata)

“…The mechanism for this cooling is unknown. The observation that primates in a dry tropical forest potentially experience both cool and warm thermal challenges suggests that thermoregulatory pressures may currently be underestimated for many tropical and midlatitude primates (Dausmann, 2014). Howlers possess eccrine sweat glands on their palms and soles, glabrous portion of the tail, and throughout the hairy skin of the body (Montagna, 1972;Perkins, 1975), yet we have never observed visible sweating by howlers.…”

Torpor and hypothermia: reversed hysteresis of metabolic rate and body temperature

“…A second diagnostic feature of torpor, then, is that animals are capable of rewarming actively at any time during a bout of torpor (37), whereas hypothermic animals are unable to do so without an external source of heat. Although some torpid animals may use passive warming to reduce the energy cost of active heating during arousal (8,13,40), which is the most energetically expensive phase of a torpor bout, they are physiologically capable of spontaneous, endogenously regulated arousal in the absence of an external heat source, thus distinguishing themselves physiologically from hypothermic animals.…”

A phenology of the evolution of endothermy in birds and mammals

“…These animals abandon endothermy completely, for nine consecutive months, and do not, unlike any other hibernator (although see Dausmann, 2014), arouse periodically to a T b within the mammalian range of normothermic T b s. To date, the lack of interbout arousals has been observed in tropical mammals only and probably involves a threshold torpor T b below which ischaemic imbalances occur (sensu Carey, Andrews & Martin, 2003).…”