Adult seals (over five years old) grow to an average length of and an average weight of around . Females are on average longer and around heavier than males, though their weights fluctuate substantially according to season; females can lose up to 50% of their body weight during lactation, and males lose a significant proportion of weight as they attend to their mating partners and fight off rivals. During summer, males typically weigh , and females . A molecular genetic based technique has been established to confirm the sex of individuals in the laboratory. Large crabeater seals can weigh up to . Pups are about in length and at birth. While nursing, pups grow at a rate of about a day, and grow to be around when they are weaned at two or three weeks.