BackgroundEvolutionary theories that account for the unusual socio-ecological traits and life history features of group-living prosimians, compared with other primates, predict behavioral and physiological mechanisms to conserve energy. Low energy output and possible fattening mechanisms are expected, as either an adaptive response to drastic seasonal fluctuations of food supplies in Madagascar, or persisting traits from previously nocturnal hypometabolic ancestors. Free ranging ring-tailed lemurs (Lemur catta) and brown lemurs (Eulemur sp.) of southern Madagascar have different socio-ecological characteristics which allow a test of these theories: Both gregarious primates have a phytophagous diet but different circadian activity rhythms, degree of arboreality, social systems, and slightly different body size.Methodology and ResultsDaily total energy expenditure and body composition were measured in the field with the doubly labeled water procedure. High body fat content was observed at the end of the rainy season, which supports the notion that individuals need to attain a sufficient physical condition prior to the long dry season. However, ring-tailed lemurs exhibited lower water flux rates and energy expenditure than brown lemurs after controlling for body mass differences. The difference was interpreted to reflect higher efficiency for coping with seasonally low quality foods and water scarcity. Daily energy expenditure of both species was much less than the field metabolic rates predicted by various scaling relationships found across mammals.DiscussionWe argue that low energy output in these species is mainly accounted for by low basal metabolic rate and reflects adaptation to harsh, unpredictable environments. The absence of observed sex differences in body weight, fat content, and daily energy expenditure converge with earlier investigations of physical activity levels in ring-tailed lemurs to suggest the absence of a relationship between energy constraints and the evolution of female dominance over males among lemurs. Nevertheless, additional seasonal data are required to provide a definitive conclusion.

The reproductive cycle of several lemur species is synchronized by photoperiodic changes and it is attuned to the seasonal fluctuations in food supply. Nevertheless, irregular periods of food shortage occur in Madagascar, which can result in a negative energy balance and reduced fitness, especially in frugivorous species. Here, we tested whether ring-tailed lemurs (Lemur catta) and brown lemurs (Eulemur rufifrons × collaris) use fat stores to maintain their energy balance in a gallery forest of Madagascar (Berenty Reserve). We assessed the energy intake from weight-based estimates of food intake (mouthful count) and macronutrient contents in the diet during dry and wet months. The metabolizable energy ingested by these frugivorous/folivorous species was high during the late wet season, but it was insufficient to meet their energy requirements during the dry season. These preliminary data suggest that these lemurs must store sufficient amount of fat mass during the period of food abundance to meet their maintenance requirements and to cope with the energetic burden of the mating season. Estimates based on models assuming no fiber digestibility during the dry season are consistent with the prediction that brown lemurs should supplement their daytime diet with nighttime feeding in addition to the use of body fat stores.

No abstract