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Observations

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The grandmother factor: Why do only humans and whales live long past menopause?

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killer whales live past menopause like humans doMost mammals don't live long past their reproductive years, failing to serve much evolutionary purpose after they can stop passing on their genes to offspring.

Only three long-lived social mammalian species are known break that mold. Killer whales (Orcinus orca), pilot whales (Globicephala macrorhynchus) and humans (as well as possibly some other great apes) all have females that generally live for decades after they cease being able to bear young. So what might we have in common with these cetaceans?

A new study, published online June 30 in Proceedings of The Royal Society B: Biological Sciences, describes a strong link with specific social patterns that might predispose females to live beyond their fertile years.

"Whether it is younger or older individuals that are most likely to refrain from breeding and adopt the role of kin-selected helpers, depends on the pattern of dispersal and mating," the researchers concluded.

In species in which the males leave home to breed, over a female's lifetime, she is surrounded by an increasing number of males to which she is not related (as male offspring leave home and others die). Thus, this pattern increases her options for new mates and decreases the incentive for helping to provide for young that are not her own (or carry any of her genes), the researchers, Rufus Johnstone, of the Department of Zoology at the University of Cambridge, and Michael Cant, of the Center for Ecology and Conservation at the University of Exeter, noted in their paper.

For mammals in which the female leaves the group to mate—or in which breeding happens away from the group—a female will find herself surrounded by an increasing number of males to which she is related (as sons, grandsons and other generations of males stick around). In this scenario, it actually behooves her—and the group—to stop mating (and competing for breeding resources that could increase the fertility of younger females) and help younger females raise her progeny. Other research has shown that having a grandmother around to help out confers extra benefits on younger generations.

Although contemporary human societies have given rise to all kinds of moving and mating arrangements, the researchers point out that in traditional human forager societies, "female transfer to the husband's family at marriage is more common," and genetic analysis shows an extended human history of "female-biased transfer." For the two whale species, the researchers noted that both are thought to mate outside of their local groups, leading to "an increase in local relatedness with female age."

Johnstone and Cant explained that mating patterns are not likely the only factor to contribute to the evolution of menopause and a long post-reproductive life in these three species. But learning more about females' changing social groups helps to reveal "the underlying similarity between the ape and whale cases… which would otherwise be obscured by the differences in their social structure."

The jury is still out on whether these marine mammals also endure hot flashes.

Image courtesy of iStockphoto/Lazareva

The views expressed are those of the author and are not necessarily those of Scientific American.

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