Phenotypic assortative mating and within-pair sexual dimorphism and its influence on breeding success and offspring quality in Magellanic penguins

Abstract

We examined within-pair sexual dimorphism and phenotypic assortative mating in Magellanic penguins (Spheniscus magellanicus) breeding in six colonies located on the Patagonian coast (Argentina). All measured phenotypic traits except the number of pectoral spots differed between the sexes; bill depth and flipper length were the most and least dimorphic traits, respectively. We found assortative mating by bill depth and body mass. The similarity in body condition within pairs was close to significant. When we performed separate correlations for birds that bred successfully, i.e., raised one or two offsprings, and birds that did not attempt to breed or bred unsuccessfully, only the successful breeders showed assortative mating by body mass. In addition, we attempted to relate the body size of each member of the pair and the degree of sexual dimorphism within pairs to the breeding performance of individuals, which was measured as brood size, and body condition and immunocompetence of offspring. We found that pairs that were less dimorphic in flipper length raised more offspring. This effect was due to female flipper length per se and not to the relative difference in flipper length between members of the pair. Females with larger flippers had a higher probability of raising two chicks. No effects of body measurements or degree of sexual dimorphism on body condition or T-cell-mediated immune response of offspring were found. We discuss these results in the context of potential factors responsible for the maintenance of sexual size dimorphism in this species.