Le Lépilémur du sud de Madagascar : Écologie, alimentation et vie sociale

Abstract

This study represents a combination of observations on the biology of the nocturnal Sportive Lemur (Lepilemur mustelinus leucopus) living in the arid bush zone (characterised by the Family Didiereacae) of South Madagascar. Comparative data relating to nutrition, diurnal resting sites and population density in gallery forest of the same area are also presented. During its phase of nocturnal activity, this Lemur remains immobile for long periods of time, and the actual time spent in search of food is extremely limited. During daytime, it remains close to the orifice of its retreat, thus permitting constant surveillance (state of somnolence without deep sleep). The dietary regime, which is almost exclusively folivorous, represents an extreme case of specialisation within the Order Primates, as is demonstrated by quantitative analysis of the assimilable components of the diet. The digestive tract is specialised as an adaptation to this diet ; there are several peculiar features in the proportions between the different segments of the tract, in the histological structure of the mucosa and in the différenciation of the cæcum and the colon, which both possess villosities of a special type. A description is given of an organ situated at the ileo-colic junction, which is derived from a Peyer patch and which possibly plays an important part in the digestive sequence. In addition, Lepilemur exhibits reingestion of its faeces (cæcotrophy), thus presenting an apparently unique case among the Primates. This observation under natural conditions helps to explain how the animal is able to benefit from a small quantity of ingested matter which is extremely poor in directly assimilable matter. An attempt has been made to calculate the balance of energy-expenditure, and this reveals great economy in muscular expenditure relative to the total availability of energy. This ecological study was conducted during the critical period during which the availability of food (evaluated according to the mean area exploited by each individual animal) was passing a minimum. During this period, the ratio of available energy to energy consumed was 1,6 : 1 (a value quite close to those calculated by Smith, 1968 with respect to annual availability for a squirrel species in North America). Study of the social behaviour of Lepilemur was undertaken following capture, marking and release of all the individuals in a particular population. The individual territories whose limits were determined to within a matter of meters, are defended by males against other males or by females against other females. Each individual devotes the major part of the activity period to surveillance of adjacent individuals. Aggressive signals (postures and vocalisations) are predominantly associated with territorial defence. Even outside the period of reproductive activity, social relationships exist between males and females whose territories overlap. Social bonds were also identified between adult females and juvenile females ; territorial independence of the latter seems to develop progressively and relatively slowly. This type of social system is found in other nocturnal prosimians which have been studied, and it would appear to be a primitive character. An experiment conducted in the field permitted observation of rapid modifications in the territorial limits of males following the removal of one male. These modifications were associated with establishment of social links with the females whose territories coincided with that of the male eliminated in the experiment, even though this took place outside the period of reproductive activity.