Adaptive suppression of subordinate reproduction in cooperative mammals

In many animals that breed cooperatively, one dominant female in each group effectively monopolises reproduction and other females help to rear her offspring. However, in most animal societies, subordinate females sometimes breed and, in a few species, most mature females breed regularly. Understanding the reasons for these contrasts and the behavioural and physiological mechanisms controlling the distribution of reproduction among females and the degree of reproductive skew is of central importance in explaining the evolution of cooperation, the reproductive strategies of individuals and the population dynamics of cooperative breeders. However, although variation in reproductive skew has been the focus of considerable research in behavioural ecology for more than twenty years, it has not yet been possible to account for the distribution of subordinate breeding either between or within species. Previous attempts to explain the frequency of breeding by subordinates have either assumed that subordinate females breed where dominant females grant them reproductive concessions to ensure that they remain in the group and assist in rearing their offspring or that subordinates breed where dominants are incapable of preventing them from conceiving or rearing young. An alternative possibility, which has received relatively little attention, is that suppressing or evicting subordinate females has substantial costs to the fitness of dominants (partly because subordinate females are commonly their daughters or sisters and partly because the breeding success of dominants depends on the number of resident helpers) and that differences in the frequency of subordinate breeding are a consequence of variation in the net benefits of suppressing subordinates to dominants. While some correlational evidence suggests that this may be the case, no studies have yet measured the costs of subordinate breeding to dominant females or have determined whether variation in the net benefits of suppressing subordinates affects the frequency with which subordinates breed or are evicted from the group by dominants. By far the best way to investigate the ultimate and proximate mechanisms controlling reproductive skew is to manipulate the frequency of breeding by subordinates and dominants and to determine how this affects the breeding success of dominants and their treatment of subordinates. Drawing on an existing study population of Kalahari meerkats Suricta suricatta, where all individuals are recognisable and detailed records of individual differences in growth and breeding success are available for multiple groups over fifteen years, we shall use a combination of quantitative comparisons and experiments manipulating the frequency of reproduction by subordinates and dominants, to (1) measure the costs of subordinate breeding to dominants (2) determine whether variation in the net benefits of suppressing subordinates to dominants affects the extent to which they tolerate subordinates (3) determine whether dominants monitor the growth of subordinates and selectively evict likely challengers from the group (4) determine whether dominants attempt to influence the identity of their likely successors (5) develop generalised game-theory models that can be used to predict the extent to which dominants tolerate subordinates, the degree of reproductive skew and the size and structure of groups in other social vertebrates.