Cooperative breeding and monogamy in mammalian societies

Abstract

Comparative studies of social insects and birds show that the evolution of cooperative and eusocial breeding systems has been confined to species where females mate completely or almost exclusively with a single male, indicating that high levels of average kinship between group members are necessary for the evolution of reproductive altruism. In this paper, we show that in mammals, the evolution of cooperative breeding has been restricted to socially monogamous species which currently represent 5 per cent of all mammalian species. Since extra-pair paternity is relatively uncommon in socially monogamous and cooperatively breeding mammals, our analyses support the suggestion that high levels of average kinship between group members have played an important role in the evolution of cooperative breeding in non-human mammals, as well as in birds and insects.

Figure 1.

Mating systems in cooperative breeders (dark bars) and other social species (light bars). (a,b) In all cooperatively breeding mammals, a single female and a single male are the parents of the majority of all recruits. (c) Breeding males typically maintain their position for several breeding seasons. The values for cooperative breeders are significantly larger than those of other social species in all three comparisons (all p < 0.01). Horizontal bars represent the median, boxes the 75% quartile, and vertical lines indicate the minimum and maximum values.

Figure 2.

Transition rates for the most likely phylogenetic reconstruction. The most likely model assumes a dependent, sequential evolution between monogamy and cooperative breeding. The values depict the transition rates between the four respective combined states; as transitions are sequential, only one trait changes at a time. No transition to cooperative breeding occurred if an ancestral species was not monogamous. In addition, transition rates away from the intermediary states (monogamy but no cooperative breeding, no monogamy but cooperative breeding) are always higher than the transition rates towards them, indicating that monogamy and cooperative breeding are unstable traits. The given values reflect the probability that a specific transition occurs, and depend also on the frequency with which certain states are present. Values are based on a model assuming equal branch lengths, all dependent models though estimate that there have been no transitions from non-monogamous, non-cooperative ancestors to cooperative breeding.