Abstract

In cooperatively breeding species, individuals help to raise offspring that are not their own. We use two inclusive-fitness models to study the advantage of this kind of helpful behaviour in social groups with high reproductive skew. Our first model does not allow for competition among relatives to occur but our second model does. Specifically, our second model assumes a competitive hierarchy among nest-mates, with non-breeding helpers ranked higher than their newborn siblings. For each model, we obtain an expression for the change in inclusive fitness experienced by a helpful individual in a selfish population. The prediction suggested by each expression is confirmed with computer simulation. When model predictions are compared to one another, we find that helping emerges under a broader range of conditions in the second model. Although competition among kin occurs in our second model, we conclude that the life-history features associated with this competition also act to promote the evolutionary transition from solitary to cooperative breeding.

Highlights

  • Helpful behaviour takes many forms in vertebrates [1] but some of the most interesting instances come from species that breed cooperatively

  • Our second model assumes a competitive hierarchy among nest-mates, with non-breeding helpers ranked higher than their newborn siblings

  • When model predictions are compared to one another, we find that helping emerges under a broader range of conditions in the second model

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Summary

Introduction

Helpful behaviour takes many forms in vertebrates [1] but some of the most interesting instances come from species that breed cooperatively. Some authors have argued that ecological constraints (e.g. low probability of successful, independent breeding) mean that helping changes the direct fitness of the helper by only a small amount. Other authors have taken a different stance and argued that inheritance of a breeding territory (especially inheritance of a high-quality breeding territory) can translate into large, direct-fitness gains for those who provide help at their natal site. Pen & Weissing [11] used an inclusive-fitness model to demonstrate how the direct-fitness benefits of territory inheritance depend on life-history details in species. Their model, did not address the possibility that an individual may eventually compete with the offspring it helps to rear. We find that a broader range of ecological conditions support cooperative breeding in the second model, even though competition between relatives enters into consideration

Population dynamics
Advantage of delayed dispersal with helping
Individual-based simulation
Discussion

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