Climate change will impose new constraints on the distribution of species through desertification. Small-scale endemists common in biodiversity hotspots such as Madagascar are especially threatened. Among them are the gray-brown mouse lemurs (Microcebus griseorufus), which occupy the driest habitats in Madagascar of all Microcebus spp. We studied impacts of aridity on this species to identify critical factors for distributional limits. Accordingly, we compared populations of 2 adjacent habitats that differ in their humidity levels. We found that the more humid habitat provided more high-quality food and maintained a higher population density of Microcebus griseorufus, with individuals in better condition compared to the drier habitat. At the end of the wet, but not in the dry season, Microcebus griseorufus adjusted its home range size to local food plant density, which indicates that individuals optimize food intake in the wet season to prepare for the dry season. We found a negative exponential relationship between food plant density and home range size, which suggests an upper limit for the size of home ranges. According to this relationship, individuals from the drier habitat could not compensate for reduced food availability by enlarging their home range beyond this threshold. Thus, in case of declining food availability during the wet season due to a generally drier climate, individuals will not be able to extend their home ranges to include more food resources, and hence to accumulate enough fat reserves for the dry season. In consequence, they will have to migrate toward more mesic refugia. Migration, however, requires habitat connectivity, which is scarce in Madagascar's largely Int J Primatol (2011) 32:901-913 anthropogenic and heavily fragmented landscape. Our data suggest that upper limits in home range sizes can limit a species' ability to adapt to increasing aridity.

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Species can either maintain a certain social organization in different habitats or show different social organizations in similar habitats. The reasons underlying this variability are not always clear but might have consequences for population dynamics, especially under changing environmental conditions. Among mammals, the primate genus Microcebus lives in small groups of closely related females, derived from female philopatry and dispersed males, as illustrated by the well-studied Microcebus murinus. Here, we studied the genetic structure of a population of the congeneric Microcebus griseorufus, inhabiting three adjacent habitats with different resource availabilities. In order to learn more about the plasticity of the species’ social organization under these different conditions, we analyzed the spatial arrangement of mitochondrial haplotypes of 122 individuals. The study revealed high haplotype diversity and a pronounced difference in spatial distribution between the sexes. Females exhibited spatial aggregation of haplotypes, suggesting a system of female philopatry and matrilines, similar to M. murinus. Male haplotypes were dispersed, and males were more likely to carry rare haplotypes, indicating higher dispersal activity. These findings hint towards the unity of the social organization across the genus Microcebus, suggesting a phylogenetic origin of the social organization. Yet, with decreasing resources, the clustering of female haplotypes declined and approached a random distribution in the marginal habitat, with cluster sizes correlating with resource availability as predicted by the socioecological model. Our study supports the notion that social organization is shaped by both phylogenetic origin and ecological conditions, at least in these small primates. Significance statement Impacts of habitat degradation are mostly described in terms of changes in population densities in relation to the reduction of resources. This neglects the possible effects of altered social organizations due to declining resources or population densities. Using a genetic sampling of three subpopulations of mouse lemurs in Madagascar along a gradient of food availability up to the limit of the species’ ecological tolerance, we show that their social organization consisting of spatial clusters of closely related females and overdispersed males converges towards random spatial distributions of both sexes with declining food availability.