DIFFERENTIAL EXPENDITURE OF MATERNAL RESOURCES IN ANTARCTIC FUR SEALS, ARCTOCEPHALUS-GAZELLA, AT HEARD-ISLAND, SOUTHERN INDIAN-OCEAN

Maternal expenditure in lactating Antarctic fur seals (Arctocephalus gazella) was studied at Heard Island in the 1987 to 1988 summer/autumn. The mean birth mass, growth rate, and mass at 60 days of sons were significantly greater than those of daughters. Maternal foraging trips lasted on average 5.9 days, and attendance bouts lasted 1.5 days. Over the course of this study, foraging trip duration increased from 5.0 to 7.0 days, and attendance duration declined from 2.0 to 1.5 days. Pups lost 3.2% of their body mass/day while their mothers foraged at sea, but gained mass rapidly during periods of maternal attendance. Sons gained significantly more body mass (1.9 kg) compared with daughters (1.3 kg) during maternal attendance, suggesting that sons consume more milk. Sex differences in mass gain were unrelated to pup age or body mass. During 2-day maternal attendance bouts, sons gained most of their mass (71%) during the first day, and daughters increased mass at almost the same rate each day. The increase in mass by sons during maternal attendance was significantly positively related to both the duration of their mothers' preceding and subsequent foraging trips. In contrast, mass gained by daughters was positively related to the duration of their mothers' attendance. Mass at 60 days age was negatively related to birth date in sons, and positively related to birth mass in daughters. These data indicate that (1) greater maternal resources are expended on sons than on daughters, (2) sons receive greater maternal resources because they are male, and not because of their greater birth mass and body size, (3) different factors appear to be important in determining high postnatal growth in sons and daughters, and (4) demand for resources by sons can influence maternal behavior and ultimately the level of resources received.