ONTOGENY OF DIFFERENTIAL MIGRATION AND SEXUAL SEGREGATION IN NORTHERNELEPHANT SEALS

Adult northern elephant seals (Mirounga angustirostris) are substantia lly sexually dimorphic in size and in migratory behavior. Both sexes m igrate over vast areas of the North Pacific Ocean during biannual migr ations between rookeries in California waters and northern foraging gr ounds, but adult males segregate from adult females during each of the se double migrations. I used satellite-linked radiotelemetry to docume nt the ontogeny of differential migration and sexual segregation as a means to better define the ultimate causal mechanisms of these pattern s and to evaluate the potential influence of this sexually dimorphic t rait 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 g reater 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 elep hant seals is more precisely related to differential metabolic needs o f males during the period of sexual maturation and accelerated growth rather than to sexual differences in gross energy requirements of adul ts. Segregation appears to confer to pubescent males survival benefits , which more than compensate for the higher mortality that generally i s associated with rapid growth during puberty in other taxa with male- biased, sexual dimorphism. The most parsimonious explanation for the e volution of sexual dimorphism in size in northern elephant seals still appears to be sexual selection (intrasexual competition) acting throu gh differential mating success of males. Moreover, differential, rapid growth during puberty and deferred maturity of males versus females a re proximate mechanisms promoting larger size of males as adults. I su ggest, however, that natural selection has led to sexually dimorphic m igratory patterns and segregation of foraging elephant seals and that the segregation may partially maintain and, indeed, enhance sexual dim orphism in size.