Factors affecting maternal expenditure in southern elephant seals during lactation
Southern elephant seals Mirounga leonina provide a unique opportunity for examination of parental investment because postpartum pup growth is fueled exclusively by energy from stored reserves in fasting mothers, and the seals are extremely sexually dimorphic as adults. We examined the influence of pup sex, maternal size, and other factors on the variation in postpartum maternal mass change and pup growth. Elephant seals (178 mothers and 445 pups) were weighed during four breeding periods at South Georgia Island. Maternal mass change during lactation increased markedly with the mass of the mother at parturition. Postpartum maternal mass accounted for 75% of the variation in mass loss and 62% of the variation of pup mass at weaning. Size of the pup at birth explained <4% of this variation, and the sex of the pup explained virtually none (<0.1%). The duration of lactation was positively correlated with the pstpartumo mass of mothers, but negatively correlated with the rate of maternal mass loss when corrected for the effect of maternal postpartum mass. Mothers giving birth late in the season had shorter lactation periods than those that gave birth early but seemed to compensate for this by increasing the rate of mass transfer. Average transfer efficiency (pup mass gain/maternal mass loss) was 46±0.5%. Mothers lost, on average, 35% of their postpartum mass during lactation and 40% during the whole breeding period. Females whose postpartum mass increased between seasons increased their expenditure on their pups; females whose postpartum mass decreased, decreased their expenditure. These data from mothers with single pups do not clarify whether differences in investment were controlled by mothers or their offspring. However on three occasions, study females raised two pups in a season. Despite the increased demand, these females did not increase their expenditure, suggesting that levels of investment are maternally controlled. These results show that levels of expenditure in southern elephant seals appear to be determined largely by a single variable: female mass at parturition.