Whale of a hot flash

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

Comments

[T. Proce]

"why do females go on living beyond their reproductive years? "

That sounds misogynist. Shouldn't the question be reversed? That is, since the entire organism typically lives for X years, why does the reproductive system lose its efficacy after Y years? Given the research subject, this is by far the more productive perspective.

[Brian W.]

Agreed

[Greg M]

I disagree, and I don't think that is misogynist.

There are several good hypotheses why a female looses its efficacy after Y years. Males have evolved sexually to produce an astronomical number of gametes, where the fastest (an indicator of the sperm having good genes) is the one that makes it to the egg. Females have evolved sexually to produce very few gametes (actually all the gametocytes are already present at birth (a few thousand) in a female, each one finishes meiosis monthly). There is no physical way of telling if an egg is good (they do not "race" as in sperm) and it is very taxing on an organism to become fertilized and have to abort due to bad eggs.

Thus because of the high price of an egg (in biology terms, as well as in our economy, I've seen several ads in NYC for females to donate their eggs and receive $10,000, whereas guys usually get $50 for their sperm) it is not efficient to keep producing gametes in females as they get older since they increase the chance of auto abortion as well as damage to the female parent due to the decreased efficacy of the reproductive system (as in every system in every organism as they get older). Yes, males do loose efficacy of their sperm as well, but in males there is sperm competition which increases the chance that the best sperm fertilizes the egg. Also, there is no physical damage done to males because of auto abortion so there is no evolution pressure for males to stop producing sperm.

[Chelsea]

I agree, the phrasing of "why do females go on living beyond their reproductive years? " chafes a bit.

Thinking about mammals or animals in general, it seems interest in whales comes from the fact that few others go through menopause. Interesting that humans and whales have few predators.

The simple answer may be there is no selective pressure against it - women (in these cases) don't become slower or less quick to respond to stress as a result of being infertile (though they may because of the advanced age that prompts it).

There are lots of ways to extend the lifetime of an entire organism, since most cells can be replaced if they are damaged, nutrition can be enhanced due to agriculture, antibiotics curb infection, blah blah blah. There are very few ways to extend the life of individual cells that slowly and inevitably are harmed by oxidative stress. And so the twain can't track together. Makes perfect sense to me.