Allometric growth and morphology of Steller sea lion fetuses

In precocial species, fetuses often show adaptation to their particular environments. We investigated growth and morphol- ogy of 11 fetuses of Steller sea lion (Eumetopias jubatus), a highly precocial animal. The specimens were collected around Hokkaido, Japan in 2006-2008. Body length was found to increase gradually with gestational period, while body weight rap- idly increased after the middle period (81-160 days) of gestation. Positive allometric growth indicating tachyauxesis was found only in the four limbs. This noteworthy limb development may indicate adaptation to postnatal movements on shore and in the water. The fur, which developed fully by birth, confirmed the importance of thermoregulation. MRI images suggested a pro- gressive status of ossification, leading us to surmise that bone development occurs during the early (<80 days) and middle periods of gestation. We presume that shapes of internal organs are determined for the most part during early gestation, although the period will vary from one organ to another.