Socio-ecology: how the trees dictate the monkey's lives

Feeding babiesSocio-ecological theory predicts that female reproductive success, i.e. fertility and infant survival, is determined mainly by competition over access to safety and food resources. If food resources permit monopolization by one or a few individuals in the group and are worth defending, theory predicts strict dominance hierarchies to evolve and higher-ranking females to gain better access to food and higher reproductive success. In order to test this theory botanical plots and phenology trails were established with the help of Mr. Aim Chunchain, recent MSc from Kasetsart Forestry Faculty. Monthly quantification of the abundance of different phytophases since 2007 revealed that macaque food abundance is very difficult to predict. There is no clear relationship between food abundance and rainfall except that food is scarce sometime between June and August when rains are strong but even this pattern disappears in some years (unpubl. data). But food does affect fertility as is evident from the above example of the predictability of one year versus two year inter-birth intervals as well as from the fact that in the particularly good year 2010 with high food abundance even through June, July, and August an unusually high proportion of 13 out of 15 females conceived. Food does also shape competition as evident from the fact that subordinate females need to spend more time feeding every day in order to reach a similar energy intake as dominants do at the cost of their social time (Heesen et al. 2010, 2011 posters). Data also have been collected on behavior in food patches during feeding and will be analyzed for the effects of food resource characteristics on aggression and cooperation. A necessary further step will be analyses of stress and health because subordinates may very well be able to compensate feeding competition and attain energy intakes like dominants but may pay a price in terms of increased stress that has repercussions on health.

It is well established that the close social bonds (or friendships) that female baboons form influence their longevity, fertility and infant survival. The Assamese macaque study at PKWS promises to provide the missing link between sociality and reproductive success by assessing how rank and social bonds affect access to food resources and energy gain. These results are eagerly awaited by the scientific community. It is also worth noting that ecology is so powerful that variation in food abundance makes the number of birth/year vary between 5 and 13 which has important consequences for male reproductive competition.