Variation in langur social organization in relation to the socioecological model, human habitat alteration, and phylogenetic constraints

The socioecological model is to date the best evolutionary model to explain variation in primate behaviour. Some species or populations, however, deviate from the predictions. These deviations may be due to a disequilibrium between evolutionary causes and behavioural adaptations, caused by recent human changes of the environment. The relationship between human habitat alteration and primate social behaviour is reviewed and investigated for langurs. Provisioning affects the spatial distribution of food. In these areas, but also in areas with natural monopolizable food sources, female dominance relationships are linear, but not nepotistic. This does not fit into the evolutionary model. Provisioning also affects the temporal distribution of food. This reduces the seasonality of mating, increases female monopolizability and gives rise to more one-male groups than in undisturbed areas. A human reduction of the number of predators leads to fewer males per group. It also results in female philopatry. Infanticide risk was higher in disturbed than in undisturbed areas. This was not caused by the lower proportion of one-male groups in disturbed areas, but by the lack of female dispersal. Thus, female behaviour was affected by human habitat disturbance in a way that does not fit the socioecological model. However, the extension of habitat disturbance does not explain all results. We could say that the model is refuted. Phylogenetic constraints, however, do explain the behaviour of despotic Hanuman langurs. Such phylogenetic constraints complement the socioecological model. To understand the evolutionary history of a current set of features, these two approaches should be studied simultaneously.