Helpers and reproductive behavior in birds and mammals

Cooperative breeding is a breeding system in which some adults, known as “helpers”, forgo independent breeding and remain as subordinates within a group, helping to raise the offspring of others. The structure of such systems, which occur in 3%-10% of birds and mammals as well as a smaller proportion of fishes, varies greatly, from simple pairs with “helpers-at-the-nest” to true communal breeding in which multiple individuals of the same sex share reproductive status within a group. That some individuals spend part or all of their lives altruistically helping others to reproduce contradicts the concept of “selfish” natural selection and provides an intriguing evolutionary paradox. When and why such apparently altruistic behavior occurs has, therefore, been the focus of much study. Two key steps appear to be involved. First is delayed dispersal by offspring, which is generally fostered by constraints on gaining an independent breeding opportunity combined with the potential benefits of remaining at home. Second is helping behavior by individuals that have delayed dispersal, which frequently confers “indirect” fitness benefits by virtue of helpers improving the reproduction and survival of related individuals, but in some cases may include “direct” fitness benefits in the form of gaining breeding experience, enhanced survival or future breeding opportunities. Cooperative behavior can have wide-ranging consequences, not only on the biology of the cooperative species, but also on those species with which they interact.