Heterothermy in lemurs – flexibility is key
All dwarf lemurs (Cheirogaleus) of Madagascar are obligate hibernators in their natural environments (Petter, 1962, 1978; Petter et al., 1977; Hladik et al., 1980; Petter-Rousseaux et al., 1980; Wright and Martin, 1995; Dausmann, 2008, 2014). Hibernation is an actively suppressed metabolic state of endothermic species to overcome predictable, usually seasonal, metabolic energy and water crises. It is typified by dramatically reduced bodily functions and expressed by lowered metabolic rates and core body temperature (Tb), prolonged pauses between breaths, and lowered heart rates (Carey et al., 2003; Geiser, 2004; Heldmaier et al., 2004). This strategy is especially advantageous for small species, due to the allometric relationship between metabolism and body size (higher mass-specific metabolic rates and greater heat losses over their relatively larger surface areas; Heldmaier et al., 2004; White and Seymour, 2005). The energy needed to sustain vital functions during hibernation is stored as fat deposits or food caches at the end of the active season. Although the ultimate aim – decreasing energy and water demands drastically by using hibernation to survive the harsh conditions of the Malagasy winter – is the same for all Cheirogaleus species, proximate modi operandi differ notably between species inhabiting different habitats.
Biomes in Madagascar can roughly be divided between western dry forests and eastern rainforests (Dufils, 2003). Degree of seasonality (i.e., changes in occurrence and intensity of rainfall, and variation in ambient temperature, Ta) and resource predictability differ markedly between these two regions due to a combination of geographic history (mountains as physical barriers running north to south along central Madagascar), wind systems, and oceanic currents (Donque, 1972; Dufils, 2003; Jury, 2003). In addition, altitudinal gradients also influence environmental conditions and are important in explaining geographic distributions of certain taxa (Raxworthy and Nussbaum, 1994). These differences in environmental conditions not only shape Malagasy taxonomy, but also influence species’ physiological processes.
In lemurs, heterothermy has so far only been confirmed in the Cheirogaleidae, a family of rather small, omnivorous, night-active species. Within this family, individuals have proven flexible in regard to their use and patterns of heterothermy, even within populations inhabiting the same forest areas, or within individuals between different years, depending on environmental seasonality (mainly changes in temperature and precipitation), unpredictability of environmental factors, and individual body condition (Dausmann, 2008, 2014).