What Causes Body Size Variation in the Snow Petrel Pagodroma nivea?

Abstract

The Snow Petrel Pagodroma nivea is an Antarctic seabird that shows considerable geographic variation and sexual dimorphism in size, males being on average 10% larger than females. The origin and significance of body size variation in this species remain controversial. Two types P. n. and P. n. minor are said to hybridize at a few localities (Jouventin and Viot 1985). Here, we examine body size variation, mating system, and possible hybridization between the two types of Snow Petrel using 470 individuals from two different breeding sites (Casey and Ad6lieland) where they are suspected to hybridize. We then use measurements of individuals from 12 different populations to test the hypothesis that sexual selection is the factor that has led to sexual dimorphism in that species. At Casey two different populations breed sympatrically, whereas only one population showing great variation in body size was found at Ad6lieland. Mating was assortative by type but hybridization between P. n. and P. n. minor occurred at both sites. Hybridization appears to be more recent at Casey than at Ad6lieland, and results from Casey indicate that hybrids mostly result from pairs consisting of male major and female minor. The considerable body size variation observed at Casey and Ad6lieland probably reflects hybridization of the two types. The amount of sexual dimorphism in size correlates with body size in the 12 Snow Petrel populations studied; sexual dimorphism was greater among populations of the larger type than among populations of the smaller type. These results support the sexual selection hypothesis and suggest that a variety of processes may contribute to the observed patterns of geographic variation in body size in the Snow Petrel. Other hypotheses that seek to explain the evolution of sexual dimorphism are considered and discussed for the Snow Petrel.