Maternal care in the subantarctic fur seals on Amsterdam Island
Résumé
This paper reports the first study of maternal input and care from birth to weaning in a fur seal with a long pup-rearing period: the subantarctic fur seal Arctocephalus tropicalis breeding on the temperate Amsterdam Island, Indian Ocean. The protracted weaning period provided the opportunity for examination of maternal care in relation to seasonal changes in the requirements of the mother–pup pair and environmental conditions. During the reproductive season 1995–1996, maternal care was investigated in terms of provisioning (maternal attendance) pattern while diving effort was investigated using time depth recorders in summer and winter. Maternal input was calculated in terms of the absolute rate of pup mass gain and, ultimately, pup growth rate and pup body mass at weaning. Lactating subantarctic fur seals perform one of the longest attendance cycles described in fur seals, spending on average 11–23 d at sea from summer to winter. The time mothers spend ashore suckling their pup is also long (;4 d) but remains constant throughout the year. Throughout the year, maternal input should be described as follows: mothers spending a long time at sea store a large amount of body reserves that provide them a good body condition. Consequently, they spend a long time ashore to transfer their body reserves to their pups. However, mothers spending short attendance periods increase the mass transfer efficiency, probably by decreasing their metabolic overhead. In summer, maternal care was mostly controlled by pup traits: maternal absences appeared to be controlled by pup fasting ability, while maternal input was controlled by pup ingestion ability, i.e., pup body size and the time the pup was suckling. In fall, pups were no longer limited in milk ingestion, and maternal input was mostly controlled by maternal traits (e.g., body length and experience). In winter, maternal input decreased as the pup became older despite an increase in maternal diving effort. We propose that, in winter, maternal requirements increase, probably in response to increasing costs of gestation and because of a decrease in food resource availability. Pups whose mother performed short and regular foraging trips grew faster and were heavier at weaning than other pups. This is discussed in term of pup fasting endurance and maternal experience. Finally, we found a window of foraging trip durations that maximizes the net rate of energy acquisition of the pup, suggesting that in subantarctic fur seals there may not exist one optimal maternal attendance pattern, but a range of patterns promoting the same maternal fitness.