Group size and demography

In gregarious animals, group size should reflect a maximization of the difference between costs and benefits of group living. In primates, upper limits to group size are thought to reflect primarily the costs of within-group competition for food. Blue monkeys prefer ripe fruits, and aggressively compete over them, suggesting that contest competition does in fact occur. Group fissions also occur, with females initiating the process. Nonetheless, rates of aggression are very low, and fission is not associated with a change in rates. We have now documented, however, that these rates are generally lower in smaller groups. We have also found that the probability of conception for females is a function of group size, but the relationship is non-linear: conception probability is lower both in the smallest and largest groups, and highest in groups of intermediate size. Overall, the emerging picture suggests some scramble competition and weak contest competition, with an optimal group size intermediate among those we have studied. This is consistent with the limited way in which group size is regulated in this species, i.e. via group fission, which may require animals to 'overshoot' the optimal size in order ultimately to reduce it (the fission products must be large enough, if not optimally large, to survive).

For males living in a one-male group, group size should relate to reproductive opportunities. Relatively little research has investigated the role of group size in affecting male reproduction in species living in one-male groups. Resident blue monkeys males fail to sire about 40% of the offspring in their groups, and the number of females that are simultaneously sexually active when an offspring is conceived reduces their chance of siring that infant. These results suggest that males are likely to lose paternity more often in larger groups. Still, our studies of paternity suggest that they are likely to outcompete bachelor males unless their groups are small.

We continue to examine group size effects on direct measures of female fitness (interbirth interval when the first infant survives, infant survival, rate of producing surviving infants, age at first birth), and measures of feeding and social behavior.