Life histories and the evolution of cooperative breeding in mammals

Abstract

While the evolution of cooperative breeding systems (where non-breeding helpers participate in rearing young produced by dominant females) has been restricted to lineages with socially monogamous mating systems where coefficients of relatedness between group members are usually high, not all monogamous lineages have produced species with cooperative breeding systems, suggesting that other factors constrain the evolution of cooperative breeding. Previous studies have suggested that life-history parameters, including longevity, may constrain the evolution of cooperative breeding. Here, we show that transitions to cooperative breeding across the mammalian phylogeny have been restricted to lineages where females produce multiple offspring per birth. We find no support for effects of longevity or of other life-history parameters. We suggest that the evolution of cooperative breeding has been restricted to monogamous lineages where helpers have the potential to increase the reproductive output of breeders.

Figure 1.

(a) Litter size and (b) inter-birth intervals in cooperatively breeding and non-cooperatively breeding mammals. Boxplots depict the median species values as well as the quantiles (box: 25–75%; lines 0–100%) for (a) the number of offspring a female produces per birth, and (b) the length of the inter-birth interval in number of months. All cooperatively breeding mammals produce two or more offspring per birth. Cooperatively breeding mammals have, on average, inter-birth intervals that are half as long (six months) as compared with the remaining mammalian species (12 months).

Figure 2.

Evolutionary model of transitions. The phylogenetical analyses show that cooperative breeding and a high reproductive rate coevolved. Ancestral and extant species can be classified into one of the four trait combinations: monotocous or polytocous, and cooperative breeder or not. Transitions occur owing to changes in a single trait at a time: horizontal arrows reflect changes between mono- and polytocous species, whereas vertical arrows reflect the gain and loss of cooperative breeding. The line width of the arrows reflects the rate of transition. The most likely evolutionary model shows that transitions to cooperative breeding never occurred in monotocous species, supporting the prediction that polytocy is a necessary condition for the evolution of cooperative breeding. By contrast, it does not support the prediction that the presence of helpers necessarily leads to increases in reproductive rate, as the transition rate to polytocy in cooperative breeders is similar to the transition rate in species that are not cooperative.