parental investment
the amount of energy a parent expends in parental care. Because female mammals go through gestation and subsequent lactation, it is often assumed that they have greater parental investment than do males. However, there is considerable male parental investment in cooperative-breeding species, such as marmoset and tamarin monkeys, in which males can lose up to 10% of body weight during the period when offspring must be carried constantly. Male parental investment may also be high in species in which males defend the young from predators or attacks from other members of the same species.