Data from: 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.