In chapter 8 (pages 183-6) we discussed the idea that living in groups has costs as well as benefits, and that if the group is too big the costs come to outweigh the benefits. But are these costs big enough to actually have an effect on reproductive success? A recent study of leaf monkeys Trachypithecus phayrei suggests that they can be (Borries et al. 2008). The researchers studied three monkey groups, a small one, a medium one, and a large one, in the same habitat in Thailand. In the largest group, infant development to maturity was significantly slowed, and the inter-birth interval was longer, which means that females were taking longer to recover before being ready to have another offspring. The mechanism is probably the competition for food that being in a large group generates. Why, you might ask, do monkeys not just leave the large group? Perhaps they will, but striking out to live alone is probably not an attractive option either. Instead, it is more likely that once the group reaches approximately twice its optimal size, it will fission into two. This means however that there may be a period during which it is larger than optimal but life within it is still a bit better than living alone.

Borries, C. et al. (2008). Costs of group size: lower developmental and reproductive rates in larger groups of leaf monkeys. Behavioral Ecology 19: 1186-91.