Sex-ratio selection in species with helpers-at-the-nest.

Among many cooperatively breeding birds and mammals, grown offspring remain with their parents and aid them in rearing successive broods/litters of young. Through such actions, helpers can be said to 'repay' part of the cost of their production. When a strong sexual asymmetry exists in the preponderance of philopatry and helping behavior, selection should favor females that skew the sex ratio of their offspring in favor of the helping sex. Fisher's sex-ratio model is expanded to incorporate the concept of a sex-specific helper payback. The result is an imbalance in the evolutionarily stable strategy sex ratio at the termination of parental care; the magnitude of the imbalance is proportional to the product of P, the fraction of the helper sex that actually acts as helpers, and alpha , the average magnitude of the helper contribution measured relative to that of either parent. This formulation leads to the testable hypothesis that the male-biased adult sex ratio found in most species of cooperatively breeding birds may be a result rather than a cause of the helping behavior. -Authors