Behavioral Heterochrony and the Evolution of Birds' Helping at the Nest: An Unselected Consequences of Communal Breeding?

Abstract

Helping behavior in communal or cooperative breeders is presumed by most researchers to have evolved as a direct result of natural selection. Hence, helping in different communal-species contexts is explained in terms of its adaptive or functional significance for the helper. The hypothesis that the expression of provisioning behavior by nonbreeding auxiliaries is an unselected consequence of the evolution of communal breeding is presented as an alternative to the functional hypotheses. First, I argue that the functional significance of nonbreeders' feeding offspring is irrelevant with respect to the evolutionary origin of the behavior because provisioning offspring is a constraint of many present-day species of birds, including those with helpers at the nest. I present a model that depicts provisioning by nonbreeding auxiliaries as an expression of a heterochronic change in ontogeny brought about as a result of a shift in life-history pattern (i.e., reduced breeding dispersal and parental tolerance). Because the cost to nonbreeding birds of exhibiting provisioning may be low relative to the benefits of (normal) parental provisioning, selection pressure on "helping" may be negligible. However, further evidence suggests that even if the costs were high, evolving the ability to discriminate between the contexts of the stimuli evoking the provision response would be difficult. Furthermore, even though the occurrence of communal breeding correlates almost perfectly with the occurrence of helping, estimates of inclusive fitness cannot be used to argue that there has been (separate) selection for provisioning by helpers. This problem arises because researchers have failed to distinguish between the causes and effects of natural selection. The "unselected hypothesis" is considered to have greater explanatory power than the various functional hypotheses because it provides a single explanation for provisioning by kin and non-kin in communal breeders and for the infrequent incidents of provisioning by individuals other than parents in non-communal species.