It's a question that stumps evolutionary biologists and women of a certain age:
What's the deal with menopause?
OK, so maybe the evolutionary biologist puts the question a little differently: If the goal of any organism is to pass on his or her genes, why do females go on living beyond their reproductive years?
Researchers in Washington and British Columbia sought an answer in killer whales, the black-and-white beauties that cruise their inland and near-shore waterways. Among little-known killer-whale factoids: They are extremely long-lived. And not only do female killer whales go through menopause, they have the longest post-menopause lifespan of any mammal, including humans.
Male killer whales rarely live to be 50, although they can father calves up to their last spout. Females enter their reproductive years around age 10 and lose fertility rapidly after age 40, but live to be 55 and older. One of the female killer whales in the study was known to be more than 90 years old.
One possible explanation for menopause is the grandmother hypothesis: Females who are past their own reproductive stage can help their daughters or other kin raise their offspring, allowing the daughters to produce even more babies because grandma is around to babysit.
But in the study published in the journal Frontiers in Zoology, the scientists saw no effect of grandma whales on either the fecundity of their daughters or the survival of their grand-calves.
They did find some support for another explanation: the attentive-mother hypothesis. Like humans, killer-whale calves remain dependent on their mothers well beyond weaning. Living, on average, 10 years beyond menopause allows a mother to see her final offspring through to maturity (assuming that the "mature" young adult whale doesn't try to move back into the pod basement).
The researchers also found that the calves of the oldest mothers had a 10% greater chance of survival than the offspring of younger mothers -- a comforting thought for any older mammal undertaking motherhood and menopause back-to-back.
-- Mary Engel
Photo credit: AP