Southern elephant seals (Mirounga leonina) typically give birth to a single pup and raise it over a short 24-day lactation period. Lactation is characterised by: maternal fasting, rapid pup growth and abrupt weaning after which the weaned pups rely on stored fat for growth and survival. Females are not able to transfer all of their stored resources to their pups because they themselves need to use some to return to their remote foraging grounds after the breeding effort. Therefore the amount of energy expended by a female during lactation may affect not only the survival of her pup, but her own survival and future reproductive success. Female southern elephant seals are therefore under strong selective pressure to allocate finite amounts of resources to their pups. In the rare event of producing twins, females that wean both pups are likely to experience reduced reproductive success. Twin births accounted for 0.38% of all the observed elephant seal births at Macquarie Island in 1999. The mean birth masses, weaning lengths and lactation duration for twin and singleton pups did not differ significantly but weaning mass, weaning girths and lactation growth rates did differ significantly. In all cases, singleton pups were larger and grew faster than twin pups. Twin pups suffered greater pre-weaning mortality than singletons (16.7% and 4.6% respectively) and fewer were seen alive after 18 months (20% compared with 47% respectively).