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.