The social organization of most mammals is characterised by female philopatry and male dispersal. This thesis examines the social organization of sperm whales, Physeter macrocephalus, using a combination of field observations and molecular genetic techniques. A major conclusion is that widely dispersing males sometimes may move between oceans, whereas this is probably less common in females.
Analyses of re-sightings of recognisable female and immature sperm whales in the Galápagos islands indicated that they live in stable social groups of about 13 whales, which may represent family units, and that these units associated with each other for periods of days. There was a high turnover by movements of groups in and out of the study area, and the groups seemed to be part of a larger population which may be geographically localised.
These observations, together with previous knowledge that males disperse from their natal groups to higher latitudes, while females and juveniles are limited to low latitudes, led to the hypothesis that breeding dispersal between oceans may be more common among males than among females. If so, one might expect less differentiation in the bi-parental nuclear genome than in the maternally inherited mitochondrial DNA (mtDNA) on a global scale. This hypothesis was examined by genetic analyses of samples from the North Atlantic, North Pacific and Southern Hemisphere, representing a substantial part of the global range of the species.
Global variation in mtDNA control region sequences revealed an unusually low diversity, and an evolutionarily recent common ancestry of less than 100,000 years, perhaps even less than 25,000 years. This implies a young global population structure of sperm whales. There was highly significant heterogeneity in mtDNA haplotype frequencies between oceans, indicating that female dispersal between oceans has been limited during the period since mtDNA ancestry, allowing some global differentiation to develop. In contrast, analysis of variation in allele frequencies at nine nuclear microsatellite loci did not indicate significant genetic heterogeneity between oceans. Taken together, these patterns suggest that movements between oceans have been more prevalent among males than among females. This is also consistent with observations of females being the philopatric sex and having a more limited latitudinal distribution than males. Consequently, the typical mammalian dispersal pattern may have operated on a global scale in sperm whales.
Analysis of genetic differentiation between potential social groups of females within oceanic areas indicated significant heterogeneity in both mtDNA and microsatellites, and the amount of differentiation in mtDNA was estimated to be an order of magnitude higher than that between oceans. These results thus seem to support the notion of groups of females at least partly being composed of matrilineally related individuals, which may be of importance for an understanding of the evolution of sociality in this species.
Stockholm: Department of Zoology, Stockholm University , 1998. , 36 p.