The prenatal molt and its ecological significance in hooded and harbor seals

Hooded and harbor seals are unique among true seals (family Phocidae) in that the lanugo (fetal pelage) is typically shed in utero. Shedding had occurred in all newborn hooded seals examined off the Labrador coast (n = 62) and in 94% of newborn harbor seals on Sable Island, Nova Scotia (n = 110), although in one-fifth of the latter the molt was incomplete. In hooded seals the shed hair in the amniotic sac was organized into tightly woven disks, whereas in harbor seals the shed hair formed a loose mat with no evident structure. We examined hair masses in the intestines of pups of both species but could find no evidence that the intestines were involved in hair disk formation. We argue that fetal shedding, like prenatal blubber deposition, is an adaptation enabling newborn pups to enter cold water without adverse consequences. This ability allows the use of pupping substrates that are unstable or regularly inundated with water.