Environment and time as constraints on the biogeographical distribution of gibbons.

We develop a time budget model for the hylobatid family with the aim of assessing the extent to which their contemporary and historical biogeographic distributions might be explained by ecological constraints. The model uses local climate to predict time budgets, and from this the limiting size of social group that animals could manage at a given location. The model predicts maximum group sizes that vary between 3 and 15 within the taxon's current distribution, indicating that the combination of their dietary and locomotor styles with the kinds of habitats they inhabit radically constrain group size. Beyond the edges of their current distribution, sustainable group size rapidly tends to zero, although if they had been able to bypass some of these areas, they would have found very suitable habitats in southern India and beyond the Wallace Line. While travel time would be a major constraint on group size at larger group sizes, as it is in great apes, the main factor limiting the gibbon's current distribution is the time they need to spend resting that is imposed on them by the environment. The model also indicates that gibbons would not now be able to survive in regions of central and southeastern China where they are known to have occurred within historical times, perhaps because historical climate change following the Little Ice Age of the C18th made these regions uninhabitable for them. Finally, our results indicate that gibbons have the ecological capacity to live in larger groups than they do, making it unlikely that their adoption of monogamy reflects purely ecological constraints.