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 i s fueled exclusively by energy from stored reserves in fasting mothers , and the seals are extremely sexually dimorphic as adults. We examine d the influence of pup sex, maternal size, and other factors on the va riation in postpartum maternal mass change and pup growth. Elephant se als (178 mothers and 445 pups) were weighed during four breeding perio ds at South Georgia Island. Maternal mass change during lactation incr eased 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 expl ained <4% of this variation, and the sex of the pup explained virtuall y none (<0.1%). The duration of lactation was positively correlated wi th the pstpartumo mass of mothers, but negatively correlated with the rate of maternal mass loss when corrected for the effect of maternal p ostpartum mass. Mothers giving birth late in the season had shorter la ctation periods than those that gave birth early but seemed to compens ate for this by increasing the rate of mass transfer. Average transfer efficiency (pup mass gain/maternal mass loss) was 46 +/- 0.5%. Mother s lost, on average, 35% of their postpartum mass during lactation and 40% during the whole breeding period. Females whose postpartum mass in creased between seasons increased their expenditure on their pups; fem ales whose postpartum mass decreased, decreased their expenditure. The se data from mothers with single pups do not clarify whether differenc es in investment were controlled by mothers or their offspring. Howeve r on three occasions, study females raised two pups in a season. Despi te the increased demand, these females did not increase their expendit ure, suggesting that levels of investment are maternally controlled. T hese results show that levels of expenditure in southern elephant seal s appear to be determined largely by a single variable: female mass at parturition.