Study Documents Menopause in Gorillas

The results suggest that "there's no evolutionary benefit to menopause -- it's simply that there's no cost," said Packer. That is, as an individual ages, the reproductive system is the first to go, but that's okay at the point when the individual won't live long enough raise an additional baby, he said. "Since humans have a more prolonged period of infant dependency than other species, we'd expect menopause to occur earlier in life."

The theory predicts that reproductive decline will begin once the mother's life expectancy drops below the time required to raise additional offspring. For example, if women in pre-technological societies could expect to live 50 years, and if a child, in order to survive, needed its mother until the age of 10, then reproductive decline could begin at age 40. Packer's data illustrates this concept in baboons and lions. Female baboons don't live past age 26 or 27, and their infants require at least two years of maternal care. Baboon reproductive rates decline around age 21, which allows ample time for the youngest infant to reach independence.

Packer said that he found no evidence that menopause frees older females to help younger females raise offspring. Such behavior should result in higher survival of individuals whose grandmothers are still alive but no longer reproductive. Although grandmother lions and baboons both engage in what's called kin-directed behavior, they had no measurable impact on the survival or reproduction of their grandchildren or adult daughters.

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