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 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.