In chimps, grannies are sexy

Recently a 33-year-old reader asked me to investigate his curious attraction to older women — much older, in their 70s or 80s.

Varicose veins turn him on, he said, and he's a big fan of "granny porn."

What's interesting about this geriatric fetish, known as gerontophilia, is not that it happens but that it's considered so extraordinary. We take for granted that men prefer young women and that older women will buy all sorts of products to appear young. We all know what Joan Collins meant when she quipped that the problem with beauty is that it's like being born rich and getting poorer.

Yet the youth premium doesn't apply to other creatures. A paper published in Current Biology reports that chimpanzee females grow sexier with age. Chimp males are natural gerontophiles.

Boston University anthropologist Martin Muller and colleagues observed this pattern over many years following wild chimps at Kibali National Park in Uganda.

The finding makes sense, he says, if you know that chimps employ a completely different mating strategy than we humans. The females are not just promiscuous, they're almost frantically so. "It looks like their primary goal is to mate with all the males," Muller says.

This strategy probably evolved, he says, to protect their young. Males of many species tend to kill their rivals' babies — but not their own — before mating. In chimp evolution, the females who gave all the males a shot at being dad had the most surviving offspring to carry on their genes.

Anything less than total promiscuity could be deemed juvenile-endangerment.

Still, male chimps do have standards. Muller's team found that males in the troop tend to be indifferent toward the chimp equivalent of nubile teenage girls. Even more surprising, females get more and more male attention as they age. Grandmother chimps are the most chased and fought over.

We find that odd, perhaps, because of a biological quirk known as menopause.

The females of most other species, including chimps, remain fertile for more than 90 percent of their lives, says Bobbi Low, an evolutionary biologist at the University of Michigan. For female diamondback terrapins, that can mean more than a century. When a 20-year-old male meets a 100-year-old female, he mates with the same enthusiasm he'd show a turtle his own age.

In our menopausal species, however, men with a taste for older women are less likely to procreate, thus breeding into the human race a preference for nubile women.

Another component of our young-female fetish can be traced to long-term relationships, says David Buss, an evolutionary psychologist at the University of Texas in Austin. It's not that all humans are monogamous, but many do mate for years, and some for life. A male is likely to produce more children if he picks a wife of 19 than one of 45.

As a long-term mate, says Buss, a woman in her late teens offers the highest "reproductive potential."

You can argue all you want about inner beauty and the value of experience, but studies across cultures find female allure is just about always tied to youth, if not synonymous with it.

Harder to explain is why male chimps find the oldest females even hotter than the merely middle-age. Both are equally fertile, and both have the experience necessary to protect their offspring. Muller says one possible explanation is that chimp females climb the dominance ranks with age, giving them and their babies better access to territory and food.

Luckily for our older women, humans are a varied species with many individuals who break the rules.

Faye Flam's Carnal Knowledge column appears Sundays in The Seattle Times