Sex, long life and the evolutionary transition to cooperative breeding in birds

Philip A Downing,Charlie K Cornwallis,Ashleigh S Griffin

doi:10.1098/rspb.2015.1663

Abstract

Long life is a typical feature of individuals living in cooperative societies. One explanation is that group living lowers mortality, which selects for longer life. Alternatively, long life may make the evolution of cooperation more likely by ensuring a long breeding tenure, making helping behaviour and queuing for breeding positions worthwhile. The benefit of queuing will, however, depend on whether individuals gain indirect fitness benefits while helping, which is determined by female promiscuity. Where promiscuity is high and therefore the indirect fitness benefits of helping are low, cooperation can still be favoured by an even longer life span. We present the results of comparative analyses designed to test the likelihood of a causal relationship between longevity and cooperative breeding by reconstructing ancestral states of cooperative breeding across birds, and by examining the effect of female promiscuity on the relationship between these two traits. We found that long life makes the evolution of cooperation more likely and that promiscuous cooperative species are exceptionally long lived. These results make sense of promiscuity in cooperative breeders and clarify the importance of life-history traits in the evolution of cooperative breeding, illustrating that cooperation can evolve via the combination of indirect and direct fitness benefits.

Highlights

Individuals help raise young produced by others in approximately one-tenth of all bird species [1,2]
Our analyses show that the evolution of cooperative breeding in birds is associated with a life-history strategy that is distinct from that of non-cooperative breeders
Our results suggest that cooperative breeding is more likely to evolve in long-lived species and that high levels of promiscuity are only evolutionarily stable in cooperative societies when individuals are sufficiently long lived to obtain future direct fitness benefits

Summary

Introduction

Individuals help raise young produced by others in approximately one-tenth of all bird species [1,2]. Four transitions were possible: (i) a gain of cooperation (a non-cooperative ancestor with a cooperative descendant), (ii) a loss of cooperation (a cooperative ancestor with a non-cooperative descendant), (iii) no change (a cooperative ancestor with a cooperative descendant) and (iv) no change (a non-cooperative ancestor with a non-cooperative descendant) These nodal classifications were fitted as the explanatory variable (four-level fixed factor) in a BPMM with survival as the response (using a binomial distribution with a logit link function) and a phylogenetic covariance matrix linked to ancestral nodes included as a random effect. This model estimates survival at each of the internal nodes in the phylogeny and tests whether there are differences in survival among the four transition categories. We provide annotated R code in the electronic supplementary material

Results

Discussion

Findings

33. Healy K et al 2014 Ecology and mode-of-life