In the laboratory rock elephant shrews (Elephantulus myurus; mean body mass 56.6 g) displayed the lowest torpor T yet recorded (ca. 5°C) in a placental daily heterotherm. It was unknown whether these low Ts were characteristic of daily heterothermy in free-ranging animals. It was also unclear how cost effective these low T s were since considerable energy is required to arouse from low Ts on a daily basis. We continuously measured body temperature once every hour for 85 days in 13 free-ranging E. myurus from May to August 2001 (winter) in Weenen Game Reserve, KwaZulu-Natal, South Africa. We recorded a total of 412 torpor bouts. Free-ranging E. myurus had a high propensity for torpor with females displaying higher torpor frequency than males. The lowest T recorded was 7.5°C at T=2.7°C and the minimum torpor T was strongly correlated with ambient temperature. Torpor arousal was tightly coupled with ambient temperature cycles. Low torpor T at low T s was therefore cost-effective because the animals offset the high cost of arousal through exogenous passive heating. Laboratory studies under constant ambient temperatures may therefore underestimate the energetic benefits of torpor in free-ranging small mammals that inhabit regions where seasonality is moderate.

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.

Under laboratory conditions, rock elephant shrews, Elephantulus myurus, use daily torpor under both short and long photoperiod acclimation. However, use of heterothermy often differs under field and laboratory conditions. We investigated the use of torpor in free-ranging elephant shrews from May 2001 to May 2002. The elephant shrews were capable of daily torpor throughout the year, with torpor most prevalent during winter. We recorded two torpor bouts during early summer (November). We recorded a total of 467 torpor bouts during the year. The mean torpor minimum body temperature (Tbmin) for the whole year was 15.3 degrees +/-4.4 degrees C, and the mean bout length was 8.6+/-3.5 h. These values were in the range expected for daily heterotherms. However, there was some marginal overlap with hibernation characteristics; a few torpor bouts were longer than 24 h in duration, and Tbmin decreased below 10 degrees C. Torpor was highly correlated with low ambient temperature and photoperiod. Torpor was also correlated with invertebrate abundance after controlling for photoperiod effects. During the year in which this study was conducted, the rainfall was 14% below long-term average. Historical rainfall records show that summer rainfall during strong El Nino years is up to 40% below the long-term average. During these drought years, the frequency of summer torpor may be higher, highlighting the need for long-term physiological data in free-ranging animals.

BackgroundHibernation 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 FindingsOne 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/SignificanceOur 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.