The capacity for nonshivering thermogenesis (NST) plays an important role during arousal from torpid states. Recent data on heterotherms inhabiting warmer regions, however, suggest that passive rewarming reduces the need of metabolic heat production during arousal significantly, leading to the question: to what extent do subtropical or tropical heterotherms depend on NST? The African lesser bushbaby, 'Galago moholi', enters torpid states as an emergency response only, but otherwise stays normothermic throughout the cold and dry winter season. In addition, this species shows unusual rewarming difficulties during arousal from torpor on cold days. We therefore examined the seasonal adjustments of the capacity for NST of naturally acclimatized G. moholi by stimulation with noradrenaline (NA) injection. Dissection of two adult female bushbabies revealed that 'G. moholi' possesses brown adipose tissue, and NA treatment (0.5 mg kg-1, s.c.) induced a significant elevation in oxygen consumption compared with control (saline) injection. However, the increase in oxygen consumption following injection of NA was not significantly different between winter and summer. Our results show that the ability to produce heat via NST seems to be available throughout the year and that 'G. moholi' is able to change NST capacity within a very short time frame in response to cold spells. Together with results from studies on other (Afro-)tropical heterotherms, which also indicate low or even absent seasonal difference in NST capacity, this raises the question of whether the definition of NST needs to be refined for (Afro-)tropical mammals.

Hibernation and daily torpor are energy- and water-saving adaptations employed to survive unfavourable periods mostly in temperate and arctic environments, but also in tropical and arid climates. Heterothermy has been found in a number of mammalian orders, but within the primates so far it seems to be restricted to one family of Malagasy lemurs. As currently there is no evidence of heterothermy of a primate outside of Madagascar, the aim of our study was to investigate whether small primates from mainland Africa are indeed always homeothermic despite pronounced seasonal changes in weather and food availability. Methodology/Principal Findings: One of the nearest relatives of Malagasy lemurs, the African lesser bushbaby, 'Galago moholi', which inhabits a highly seasonal habitat with a hot wet-season and a cold dry-season with lower food abundance, was investigated to determine whether it is capable of heterothermy. We measured skin temperature of free-ranging individuals throughout the cool dry season using temperature-sensitive collars as well as metabolic rate in captured individuals. Torpor was employed by 15% of 20 animals. Only one of these animals displayed heterothermy in response to natural availability of food and water, whereas the other animals became torpid without access to food and water. Conclusions/Significance: Our results show that 'G. moholi' are physiologically capable of employing torpor. However they do not use it as a routine behaviour, but only under adverse conditions. This reluctance is presumably a result of conflicting selective pressures for energy savings versus other ecological and evolutionary forces, such as reproduction or territory defence. Our results support the view that heterothermy in primates evolved before the division of African and Malagasy Strepsirhini, with the possible implication that more primate species than previously thought might still have the potential to call upon this possibility, if the situation necessitates it.

Behavioral and physiological adaptations are common and successful strategies used by small endothermic species to adjust to unfavorable seasons. Physiological adaptations, such as heterothermy, e.g., torpor, are usually thought to be more effective energy-saving strategies than behavioral adjustments. The African lesser bushbaby, 'Galago moholi', is physiologically capable of torpor but expresses heterothermy only under conditions of extreme energy limitation, suggesting that it has evolved alternative strategies to compensate energetic bottlenecks. We hypothesized that 'Galago moholi' survives the unfavorable winter period, without-or only rarely-employing torpid phases to save energy, by using behavioral thermoregulation. We compared the ecology and behavior of 'Galago moholi' in summer and winter by telemetric tracking and examined food availability by determination of arthropod and gum availability. We found a significant increase in huddling behavior and a significant increase in the use of enclosed and insulated sleeping sites during winter, as well as a reduction in nightly activity. 'Galago moholi' hunted for insects significantly less in winter than in summer, and increased gum intake in winter, when gum showed an increase in energy content. The availability of high-quality food, albeit in low quantities, presumably enables 'Galago moholi' to stay normothermic throughout the cold, dry period and to focus on reproduction activities. We propose that 'Galago moholi' favors ecological and behavioral adjustments over torpor because these are sufficient to meet energy requirements of this species, and their advantages (flexibility, unrestricted activity, and reproduction) outweigh the energetic benefits of heterothermy.

The expression of heterothermy in the African lesser bushbaby, 'Galago moholi', seems to be strikingly different to most other heterotherms: 'G. moholi' uses its ability to enter torpor only rarely and torpor is only used by a small fraction of the population. The aim of this study was, therefore, to summarize the parameters of torpor use in 'G. moholi' to conclude the general patterns and discuss them in comparison to other heterotherms to elucidate possible causes and constraints that underlie these differences in deployment of heterothermy. Our study was carried out on wild animals using temperature loggers and open-flow respirometry for measurements of body temperature and metabolic rate, respectively. 'G. moholi' uses torpor only as a last resort and not as a routine, seasonal behavior. Nevertheless, we found that the general physiological patterns of torpor, e.g., torpor bout duration or entry and arousal times from torpor, were mainly consistent with those described for other nocturnal daily heterotherms. The greatest difference found was the unusual low rewarming rates during arousal from torpor, probably due to already depleted internal energy stores and thus inability to mobilize sufficient energy for endogenous heating. We therefore conclude that while general physiological parameters of heterothermy seem to have remained conserved in heterotherms, the underlying causes which elicit this physiological response, and thus the extent of expression and timing of heterothermy, have evolved very differently in different groups, depending on body mass and the specific habitat and lifestyle of the species.