Ontogeny of Differential Migration and Sexual Segregation in Northern Elephant Seals

Abstract

Adult northern elephant seals ( Mirounga angustirostris ) are substantially sexually dimorphic in size and in migratory behavior. Both sexes migrate over vast areas of the North Pacific Ocean during biannual migrations between rookeries in California waters and northern foraging grounds, but adult males segregate from adult females during each of these double migrations. I used satellite-linked radiotelemetry to document the ontogeny of differential migration and sexual segregation as a means to better define the ultimate causal mechanisms of these patterns and to evaluate the potential influence of this sexually dimorphic trait on the magnitude of size dimorphism between sexes in the northern elephant seal. The initial direction of migration (northwesterly from rookeries) was established during the 1st year of life in both males and females. Differential migration and sexual segregation appears to develop during puberty, when growth rates of males are substantially greater than those of females, and those patterns are well established by the time males are 4.5–5 years old. This outcome suggests that the development and proximate cause of sexual segregation of northern elephant seals is more precisely related to differential metabolic needs of males during the period of sexual maturation and accelerated growth rather than to sexual differences in gross energy requirements of adults. Segregation appears to confer to pubescent males survival benefits, which more than compensate for the higher mortality that generally is associated with rapid growth during puberty in other taxa with male-biased, sexual dimorphism. The most parsimonious explanation for the evolution of sexual dimorphism in size in northern elephant seals still appears to be sexual selection (intrasexual competition) acting through differential mating success of males. Moreover, differential, rapid growth during puberty and deferred maturity of males versus females are proximate mechanisms promoting larger size of males as adults. I suggest, however, that natural selection has led to sexually dimorphic migratory patterns and segregation of foraging elephant seals and that the segregation may partially maintain and, indeed, enhance sexual dimorphism in size.