The gregarious lemurs of Madagascar show a lack of convergence with anthropoid primates in several social, demographic, morphological and ecological features. They lack sexual dimorphism in canine and body size, and live in groups with equal adult sex ratios that can vary in composition from pairs to larger units. In most species, females dominate males, and have brief and often synchronized estruses. Finally, lemurs are often active during both day and night (i.e. cathemeral). Three hypotheses for this lack of convergence are discussed. The traditional explanation is that Madagascar's unique ecological pressures have forced females to exclude males from access to limiting resources, selecting for the other unique features as means to maintain female dominance over males. This idea is not logically consistent and is also poorly supported empirically. The second hypothesis claims that cathemerality imposes unique constraints on lemur social organization. The social behavior of cathemeral taxa is largely consistent with this idea. However, the social organization of the diurnal species is not. The third hypothesis claims that recent ecological changes, in particular the demise of large diurnal raptors, have produced a mismatch between current activity periods and adaptations to activity period. This idea is supported by a review of morphological adaptations to light conditions among lemurs, and, more generally, by a comparative analysis of cathemerality among tropical forest mammals. We conclude that the social systems of non-nocturnal lemurs are best considered as groups formed by species adapted to live in pairs. However, we cannot conclusively exclude the possibility that cathemeral activity is an old and stable activity pattern among lemurs. We indicate which data would decide the issue, and discuss the implications for views of social evolution of lemurs and other animals.