ABSTRACT

Theories on the evolution of insect eusociality have developed in some isolation from theories on the evolution of sociality in other organisms. Facultatively eusocial groups are made up of adults who cooperatively rear young that are not direct descendants (alloparental care), with one individual in the group doing most of the breeding. This reproductive suppression is maintained through dominance or other asymmetries. Defined in this way, many species of birds and mammals show facultative eusociality (cooperative breeding). Explanations for the evolution of eusociality are somewhat different from those used to explain cooperative breeding. In particular, relatively little attention has been paid to the Ecological Constraints Model.

Cooperatively breeding vertebrates are thought to be living at the maximum possible population size for the available habitat (‘saturated’), with intense competition among conspecifics for breeding opportunities. Among those studying helpers–at–the–nest (birds) or den (mammals), it is generally agreed that delayed breeding occurs when the gain (in inclusive fitness) from helping is greater than the gain from independent breeding. This typically occurs when there is some constraint which either prevents some individuals from attaining breeding status, such as a shortage of territories, or which raises the costs of independent breeding to prohibitive levels, such as when one pair cannot bring in adequate food for their young or when more than two individuals are required to defend resources or ward off predators.