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 ev..., , # Data from:Distinctive increase in offspring size in sea otters: evolutionary changes in and beyond the trade-off against offspring number

**Short summary of the study**

Controlling for phylogeny and adult body mass, we identified the trade-off between neonate mass and litter size across the family Mustelidae. Using models of trait evolution on the phylogeny, we revealed lineage-specific directional selection in sea otters favoring larger neonate mass and smaller litter size relative to adult mass, which deviates from the evolutionary patterns observed for each trait across the Mustelidae. 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 even 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.
Our...,