Helping behaviour in the Arabian babbler: was Wright's dismissal of the unselected hypothesis warranted?

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Cooperatively breeding birds are characterised by the presence of non-breeding individuals who help in the care of offspring (Brown 1987). These helpers contribute to the rearing of young primarily by provisioning offspring at the nest. A number of hypotheses have been proposed to explain the evolution of this seemingly altruistic behaviour (see Emlen et al. 1991). Most of these explanations see helping behaviour evolving and being maintained as a direct product of natural selection, and thus focus on the potential adaptive advantage(s) helping confers to non-breeders. Alternatively, the provisioning of young in the nest by non-breeding helpers could be an unselected by-product of the intense selection on birds in general for parental feeding of offspring (Jamieson 1989, 1991). According to this hypothesis, the delayed dispersal of young in cooperatively breeding birds means that non-breeders encounter begging nestlings and respond to this strong stimulus without there being previous or additional selection on helpers to do so. Although the provisioning of young by helpers might be associated with perceived fitness benefits such as the feeding of close kin, that does not mean that helping has been favoured by selection. Therefore, simply showing that helping may have fitness benefits or costs does not differentiate between the hypotheses that provisioning by helpers is maintained directly by natural selection or is maintained indirectly by selection for parental care. What one needs to demonstrate is that the provisioning response of helpers has been modified or fine-tuned in such a way that would not be expected if it were simply parental behaviour being expressed in another context. Although the unselected hypothesis prompted several counter responses (e.g. Emlen et al. 1991; Ligon and Stacey 1991), one outcome of the debate has been to direct more attention to the underlying mechanisms that control provisioning behaviour