Distinctive increase in offspring size in sea otters: evolutionary changes in and beyond the trade-off against offspring number

Offspring size and number, key fitness components, are expected to evolve under a trade-off between them. In mammals, the evolution of larger offspring size may be promoted in aquatic environments, where the selective advantages of large body size are more pronounced than in terrestrial environments. A transition from a fully terrestrial to an aquatic lifestyle has occurred within the family Mustelidae (Mammalia: Carnivora). Sea otters (Enhydra lutris) are unique in this family for giving birth in water, with offspring living almost exclusively in the aquatic environment. Controlling for phylogeny and adult body mass, we identified a trade-off between neonate mass and litter size (i.e., offspring number per litter) across species in the Mustelidae. Using models of trait evolution on the phylogeny of this family, we revealed lineage-specific directional selection in sea otters favoring larger neonate mass and smaller litter size relative to adult mass. These results support the unique evolution of larger offspring size in sea otters, resulting in a decreased litter size through the trade-off. Furthermore, we revealed lineage-specific directional selection in sea otters favoring larger total litter mass relative to adult mass, as well as larger neonate mass after excluding the effect of the trade-off against litter size. Lineage-specific directional selection in sea otters favoring larger neonate mass and litter mass cannot be solely attributed to the prolonged gestation period. This study suggests that sea otters have undergone distinctive evolutionary changes, investing more resources in offspring size than can be allocated merely by reducing offspring number.