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.