Reproductive partitioning among polyandrous alpha and beta pale chanting-goshawk males

Abstract

This study investigates the reproductive and parental roles of polyandrous male pale chanting-goshawks, Melierax canorus, and speculatively reviews the fitness outcomes of different skew and relatedness scenarios. The study was conducted over five years in the Little Karoo, South Africa. Although, polyandrous males participated equally in building nests, provisioning prey and incubating, in the fertility window the dominant alpha males copulated 31–5 days before the females laid, whereas subordinate beta males only copulated 5–3 days before laying. If this copulation timing by alpha males was indicative of a high reproductive skew, alpha males breeding as full sibs could skew paternity in their favour (ratio 68:32) and produce 0.69 offspring equivalents. Under this scenario, they compensated beta males with indirect fitness benefits by allowing them to produce 0.54 offspring equivalents, equal to monogamous males. Alternatively, if beta males controlled reproduction while breeding with non-relatives under a high skew scenario, they would have to restrain themselves to avoid eviction and produce 0.28 offspring equivalents to allow the fitness of alpha males at least to equal that of monogamous males. I suggest that alpha males and their females altered their reproductive roles to accommodate beta males, thereby increasing their inclusive fitness, whereas beta males tolerated subordination to acquire reproductive skills that non-breeder males do not have access to.