Heterothermy in Afrotropical Mammals and Birds: A Review

Abstract

Recent years have seen a rapid increase in the number of Afrotropical endotherms known to avoid mismatches between energy supply and demand by using daily torpor and/or hibernation. Among mammals, heterothermy has been reported in 40 species in six orders, namely Macroscelidea, Afrosoricida, Rodentia, Eulipotyphla, Primates and Chiroptera. These species span a range in body mass of 7-770 g, with minimum heterothermic body temperatures ranging from 1-27°C and bout length varying from 1 h to 70 days. Daily torpor is the most common form of heterothermy, with true hibernation being observed in only seven species, Graphiurus murinus, Graphiurus ocularis, Atelerix frontalis, Cheirogaleus medius, Cheirogaleus major, Microcebus murinus and Microcebus griseorufus. The traditional distinction between daily torpor and hibernation is blurred in some species, with free-ranging individuals exhibiting bouts of > 24 h and body temperatures < 16 °C, but none of the classical behaviours associated with hibernation. Several species bask in the sun during rewarming. Among birds, heterothermy has been reported in 16 species in seven orders, and is more pronounced in phylogenetically older taxa. Both in mammals and birds, patterns of heterothermy can vary dramatically among species occurring at a particular site, and even among individuals of a single species. For instance, patterns of heterothermy among cheirogalid primates in western Madagascar vary from daily torpor to uninterrupted hibernation for up to seven months. Other examples of variation among closely-related species involve small owls, elephant shrews and vespertilionid bats. There may also be variation in terms of the ecological correlates of torpor within a species, as is the case in the Freckled Nightjar Caprimulgus tristigma.

Seasonal changes in thermogenesis of a free-ranging afrotherian small mammal, the Western rock elephant shrew (Elephantulus rupestris)

“…Even though heterothermy is a physiological response which is usually associated with cold conditions that coincide with short photoperiod, decreased food availability and food quality, summer torpor is not an unusual behaviour in mammals (reviewed by McKechnie and Mzilikazi 2011). It has been suggested that summer torpor could be advantageous during uncharacteristically dry summers or at low food availability.…”

It Takes Time to Be Cool: On the Relationship between Hyperthermia and Body Cooling in a Migrating Seaduck

“…Although, birds are known to regulate body temperature to a high degree and within very narrow margins, there is building evidence that substantial deviations from normothermia naturally occur (Guillemette et al, 2016). Indeed, hypothermic events such as torpor is evident in many families of afrotropical birds while there are numerous studies reporting cases of controlled hypothermia as a strategy to conserve energy in temperate and arctic environments as well (McKechnie and Mzilikazi, 2011; Lewden et al, 2014). While T b may be lowered as an energy-saving mechanism in many bird species, the result of the high-energy demands imposed by flight, with flapping flight often being described as the costliest locomotion mode among vertebrates (Butler and Woakes, 1990; Butler, 2016), is associated with the highest levels of known heat production among endotherms (Clarke and Rothery, 2008).…”

Full Issue

“…A growing body of evidence suggests that many birds from a diverse group of unrelated families may not maintain constant body temperature set points as originally assumed, and may instead demonstrate heterothermic flexibility in response to increased energetic costs of thermoregulation (McKechnie and Lovegrove 2002, McKechnie and Mzilikazi 2011, Brigham et. al.…”