Maternal investment in otariid seals and walruses

Abstract

We examined neonatal size, growth rate during lactation, weaning mass, adult body mass, and the degree of polygyny among fur seals and sea lions (Otariidae) and walruses (Odobenidae). Observed patterns of maternal and sexual investment were compared with those reported for true seals (Phocidae) and for other mammals. There was a strong positive interspecific allometric relationship between mass at birth and maternal mass for otariid species (p < 0.001). The slope of this relationship was significantly different from that for phocid species (p < 0.001). All pinniped data were above the regression line for terrestrial mammals. Lactation was longer and growth rates were slower for otariid and odobenid species than for phocid species. The cyclic foraging pattern of lactating otariids allows for more flexibility in the duration of lactation than is possible in phocids. Such flexibility may serve as a buffer when food is scarce. The longer lactation period of otariids and walruses also permits offspring to develop swimming and foraging skills while still depending on their mothers for energy. The relationship of mass at weaning to maternal mass suggests a greater investment in each neonate in otariids and walruses than in phocids. The large body size and advanced state of development at weaning may be an adaptive response to predation pressure on otariids. Adult male otariids and walruses are larger than females and, in both families, the average birth mass, mass at weaning, and growth rate of male pups are greater than those of females. The ratio of body masses of male pups to female pups at birth and at weaning was not correlated with the degree of adult sexual dimorphism within species, despite a positive correlation between the ratio of body masses of adult males to adult females and the degree of polygyny across otariid species.