Demography and Floating Males in a Population of Cheirogaleus Medius

Abstract

According to recent field studies, the Fat-tailed Dwarf Lemur (Cheirogaleus medius) lives in permanent pairs together with the subadult offspring from the previous breeding season, within defended territories. Obligate paternal care was suggested to be responsible for the evolution of pair-living in this species. Aims of this study were to describe the population dynamics in C. medius and to draw attention to the occurrence of floating males as possible alternative to monogamy in these nocturnal lemurs. A mark-recapture study in the Kirindy Forest/CFPF western Madagascar, was combined with observations and radio-tracking of 34 individuals during the rainy seasons from 1995 to 1998. The study area was saturated with territories during the whole study period. Nevertheless, demographic variables such as population density, sex ratio, and age structure varied strongly between years. In 1996 and 1998 population density was highest (280 and 360 individuals/km(2)) and up to 21\% of the complete study population were subadults, while in 1995 and 1997 only about 4\% of the captured animals were not adult and population density was relatively low Consistent with this hi-annual fluctuation, no infants were recorded in 1998. In two of the four study periods, the sex ratio of adults was male biased. These surplus males (= floaters) were not associated with a given female and differed significantly from males with pair-bonds (= territorial males) concerning movement patterns and sleeping habits. The large home ranges of these floaters overlapped mutually as well as with the territories of territorial pairs. Apart from testis size, that was significantly smaller in floaters during the mating season, morphometric data were not different between male of those two categories. Floaters were between two and at least four years old and sexually mature. Disappearance rate was not different between floaters and territorial males. The results suggest that floaters represent a pool of potential breeders, that may be prevented from pair-living by skewed sex ratios, high population densities and a subsequent lack of vacant territories. But since they have the chance to sneak copulations with females living in pairs, the possibility that floaters represent an alternative mating strategy in Cheirogaleus medius, cannot be ruled out. Future genetic studies about paternity and reproductive success of floaters and pair-bonded males, combined with demographic data, will show whether or not floaters represent an evolutionary stable strategy.