Ecological constraints, life history traits and the evolution of cooperative breeding

The ecological constraints hypothesis is widely accepted as an explanation for the evolution of delayed dispersal in cooperatively breeding birds. Int raspecific studies offer the strongest support. Observational studies have demonstrated a positive association between the severity of ecological cons traints and the prevalence of cooperation, and experimental studies in whic h constraints on independent breeding were relaxed resulted in helpers movi ng to adopt the vacant breeding opportunities. However, this hypothesis has proved less successful in explaining why cooperative breeding has evolved in some species or lineages but not in others; Comparative studies have fai led to identify ecological factors that differ consistently between coopera tive and noncooperative species. The life history hypothesis, which emphasi zes the role of life history traits in the evolution of cooperative breedin g, coffers a solution to this difficulty. A recent analysis showed that low adult mortality and low dispersal predisposed certain lineages to show coo perative behaviour, given the right ecological conditions. This represents an important advance, not least by offering an explanation for the patchy p hylogenetic distribution of cooperative breeding. We discuss the complement ary nature of these two hypotheses and suggest that rather than regarding l ife history traits as predisposing and ecological factors as facilitating c ooperation, they are more likely to act in concert. While acknowledging tha t different cooperative systems may be a consequence of different selective pressures, we suggest that to identify the key differences between-coopera tive and noncooperative species, a broad constraints hypothesis that incorp orates ecological and life history traits in a single measure of 'turnover of breeding opportunities' may provide the most promising avenue for future comparative studies. (C) 2000 The Association for the Study of Animal Beha viour.