Fostering behaviour and milk stealing in Antarctic fur seals
During the lactation period, female otariid seals alternate trips at sea to feed with visits ashore to nurse their pups. A female returning ashore must be able to recognize her own pup, and it is generally agreed that this is facilitated by auditory and olfactory cues. Instances of fostering behaviour (females nursing nonfilial pups) and milk stealing are reportedly rare among the otariids. In the austral summer of 1989, I observed eight and two instances of fostering behaviour and milk stealing, respectively, by Antarctic fur seals at Bird Island, South Georgia. The following summer, 26 cases of fostering behaviour and 71 cases of milk stealing were documented. In 1990, females appeared to have difficulty acquiring sufficient resources to feed their pups, so nutritional stress was probably responsible for the increase in milk stealing. The occurrence of fostering behaviour suggests that mothers were unable to recognize their own pups, although in the above cases the cause was not clear; neither human disturbance nor density appeared to be the primary factor. Maternal experience may have been a factor in 1990, as 10 of 14 females fostering pups were 5 years of age or less and had given birth to either their first or second pup.