| Zusammenfassung: | The African lesser bushbaby, G. moholi, is a small, nocturnal primate that lives in dry woodlands of Southern Africa, with a diet mainly restricted to arthropods and exudates from certain species of Acacia trees. Its small size, as well as unfavourable conditions in its habitat during the austral winter suggests that this species shows remarkable adjustments (e.g. nutritional, metabolic, and behav... The African lesser bushbaby, G. moholi, is a small, nocturnal primate that lives in dry woodlands of Southern Africa, with a diet mainly restricted to arthropods and exudates from certain species of Acacia trees. Its small size, as well as unfavourable conditions in its habitat during the austral winter suggests that this species shows remarkable adjustments (e.g. nutritional, metabolic, and behavioural) to cope with its highly seasonal habitat. As these adaptations remain elusive so far, this project addressed how G. moholi manages to cope with the pronounced seasonality of its habitat.
The study was conducted between March 2009 and August 2011 in the Nylsvley Nature Reserve in South Africa and aimed for a comprehensive overview of behavioural and physiological mechanisms of G. moholi for coping with food scarcity and low ambient temperatures. The methodology of this study has been diverse. In particular, body temperature and oxygen consumption measurements were used to identify general metabolic and possible heterothermic responses to the cold, dry period and non-shivering thermogenesis (NST) was determined to get further information about physiological adaptations. In addition, behavioural observations and tracking of animals were conducted with the aid of radio telemetry to gain information about (additional) behavioural strategies. Data on food availability (arthropods and gum of Acacia karroo) and ambient conditions (precipitation, ambient temperature) were obtained in parallel.
The results of this study showed that G. moholi does have the ability to employ torpor and therefore present the first confirmation of heterothermy in a primate outside Madagascar. Torpor was observed in all three study years, but only during the cold, dry season from April to the end of August. Only 27 % of individuals examined during the cold, dry period underwent daily torpor: six individuals after food and water deprivation in the laboratory and three individuals showed repeated torpid states within a close time frame under free-ranging conditions. This obvious reluctance of G. moholi to undergo torpor suggests that, in contrast to its close relatives the Malagasy lemurs, torpor in G. moholi is utilized in response to especially adverse conditions and not as a routine strategy to save energy during unfavourable seasons.
The torpor parameters of G. moholi (torpor bout length, entry and arousal times) did not vary markedly from the parameters described for torpor bouts of other daily heterotherms. Rewarming rates during arousal from torpor bouts, however, differed significantly. The average peak rewarming rates were found to be almost 50 % lower than expected for G. moholi on the basis of body mass, a condition probably due to depleted energy reserves of individuals. Animals obviously entered torpor when they were food stressed (e.g. food restriction in the laboratory) and torpor resulted in life-threatening situations when energy reserves of the individuals were low and ambient temperature was not high enough to exploit effective passive rewarming. The low heating rates of G. moholi during arousal from torpor could also be due to a lack of NST capacity. This mechanism allows small mammals to produce heat during long term cold-exposure and to quickly rewarm from torpid states. However, the determination of seasonal NST capacity revealed that G. moholi possesses the capacity for NST.
The occasional use of heterothermy by G. moholi seems puzzling as hibernation and torpor are generally seen as beneficial strategies to save energy and water during unfavourable periods. However, even if heterothermy may enhance fitness via energy savings it also results in selective costs. Several earlier studies revealed that reproduction is, at least in rodents, seriously impaired by the use of torpor or hibernation. This trade-off between reproduction and torpor use might also be the case for G. moholi, which has a long gestation period and two mating periods a year.
Behavioural observations and food analyses have revealed that availability of low quantities of gum with increased energy content presumably enables galagos to stay normothermic throughout the winter and to focus on reproduction activities: Instead of utilizing torpor, G. moholi copes with the low winter temperatures through behavioural adaptations. This study found a significant increase in huddling behaviour and a significant increase in the use of enclosed and insulated sleeping sites during winter, as well as a reduction in nightly activity. Furthermore, G. moholi showed a seasonal shift of feeding behaviour, and increased gum intake in winter, when gums showed a rise in energy content.
In summary, the results of this study suggest that G. moholi favours ecological and behavioural adjustments over torpor, because these suffice to meet energy requirements of this species, and the advantages of normothermy outweigh the energetic benefits of heterothermy. |
