Female-biased mortality in African penguins

Abstract

Sex-biased mortality can increase the risk of extinction of threatened populations. Numbers of the endangered African penguin Spheniscus demersus are decreasing rapidly and the smaller size of females, associated with their higher foraging effort during the breeding season, may place them at greater risk than males. Using records for the period 2004–2012 from a seabird rehabilitation centre in Cape Town, South Africa, we investigated the sex ratio of African penguins that died at the centre according to age class (adult, juvenile or chick) and cause of death (starvation/ parasitism/disease, injury/trauma or oiling). The large majority of adult and juvenile birds that died at the centre did so following starvation, parasitism or disease, rather than injury or oiling, and most of them (>60%) were females, whereas mortality of sexes was similar among chicks. Oiled birds received at the centre had high survival and release rates (93%); hence sex-biased mortality could not be determined for oiled birds. Female-biased mortality in African penguins could skew the adult sex ratio, which may further accelerate the rapid rate of decline of this species.