For 13 months, biologist Leah Domb lived in the wilds of Tanzania's Gombe National Park watching baboons--paying extra special attention to the females with big, red rear ends.
Domb went there in the mid-1990s to help solve a bit of a mystery. Why do the rumps of female baboons swell up each month around the time that they are fertile? And what do males see in such a vivid display?
Today, thanks to lots of detailed observation and a novel method Domb developed for measuring the size of baboon rumps, she and her colleague Mark Pagel have some answers.
As it turns out, the larger the swelling, the more interested the male becomes. And with good reason. Females with the largest swellings are a better bet reproductively. They produce more surviving offspring over the course of their lives.
"In the past, the assumption had been that this signal simply said, 'OK guys, it's time to mate,' " said Robin Dunbar, an evolutionary biologist at the University of Liverpool in Manchester. "This shows it signals something more than that--something about the quality of the female. It's saying: 'Stick with me guys, I'm the best of the bunch.' "
Domb and Pagel's finding, published in Nature earlier this year, feeds into a thriving field of research in evolutionary biology--one that scrutinizes things as diverse as the symmetry of a swallow's tail or the shimmer of a peacock tail, the bellow of the red deer or the size of its antlers.
All are features, scientists believe, that evolved to help animals attract members of the opposite sex or to help fight off competition from members of the same sex. Evolutionary biologists have only begun to appreciate how important this phenomenon of sexual selection has been in shaping animal life, says Dunbar: "[Charles] Darwin floated the idea, but people never understood its implications."
Sexual swellings may seem less lovely than the magnificent tail of a peacock, but Domb, as a student of anthropologist Richard Wrangham at Harvard University, soon became intrigued by the baboons' backsides. Pagel, a visiting professor, believed the females were using their red rumps to compete with each other for mates. Domb decided to test this idea.
She traveled to Gombe National Park, famous for primatologist Jane Goodall's chimps but also home to some well-studied troops of olive baboons.
Each day, Domb would set out with an assistant to find baboons. Then she would sit and watch them for hours at a time, carefully noting how much attention the males paid to the females who were in the fertile phase of their cycle and had swollen rumps.
Domb also measured the size of each rump--and while she certainly wasn't the first scientist to do so, her colleagues admire the precision she brought to bear. ("Very clever," comments Sarah Blaffer Hrdy, emeritus professor of anthropology at UC Davis.) Instead of simply eyeballing rumps and rating them from 1 to 10 for size, Domb videotaped the rumps, similarly videotaped a meter stick, then digitally compared the two images.
Analyzing the data, Domb found that males spent more time fighting over the females with the biggest rumps. And they spent a lot of time hanging around grooming those females so other males couldn't get near them.
And when Domb studied the detailed historical records for the 29 females studied, she found that the females with the biggest monthly swellings had matured earlier and had more offspring each year. The offspring they had were more likely to survive.
"It was really quite wonderfully surprising for us that we found such a strong correlation with so many measures of female fitness," Domb says.
Baboon sexual swellings, in other words, are signaling a real, useful message--one that's very similar to the showy peacock's tail. Both are cumbersome-looking. And both are flashy signs of reproductive quality.
Only a healthy, well-fed peacock with sturdy, desirable genes has enough energy to fritter away on a magnificent tail. Less-healthy peacocks will have ratty, straggling plumages. Females, prudently, pick the large-tailed peacocks so their offspring can inherit "good" genes. And it's known that large-tailed peacocks do father healthier offspring.
In a similar way, only the healthiest of females will be able to tolerate a large sexual swelling, say Pagel and Domb. The swellings get easily cut and are so large that females can't sit down properly. They increase a female's body weight by as much as 14% (as much as 25% in some primate species). That will make a female less nimble and more vulnerable to predators.
The female baboon is taking on a role more commonly reserved for the male of a given species--showing off to attract a member of the opposite sex.
"If you think about it, in other species all the female has to do is present her bum to the male or flicker her eye or swish her tail--that's all she has to do for the male to become interested," said Domb. "You don't need a large, elaborate ornament."
Females, after all, are the ones who spend months growing a fetus and then raising young--big investments of time and energy often made without help from the father. Thus, evolutionary biologists reason, females should be picky about whom they mate with and males should be strutting about trying to get attention.
Scientists don't know for sure why things are different for baboons. There are competing theories. Perhaps, says Domb, the male baboons are cautious about mating because the fights with other males can get so vicious.
"Competition is so fierce," Domb says. "The females might need to bring males in and convince them they're worth that cost of fighting."
It's also not entirely clear why some primate species have sexual swellings while others don't. Only about 10% of primates have them. Baboons swell. Chimps swell. Gorillas don't. People don't. Social structure appears to be key. Swellings are a feature of sexually promiscuous species that live in troops with more than one male.
And yet, sexual selection has clearly helped shape human biology, say evolutionary biologists.
For instance, studies suggest that men find women with hourglass figures, small chins and plump lips more attractive--those kinds of features are a signal of higher estrogen levels and thus higher fertility.
And men and women alike prefer mates who have symmetrical faces and limbs, perhaps because such symmetry means the body has developed properly, signifying robust health in an uncertain, disease-infested world.
We may not have bright red rumps, "but we are animals--we are totally animals," said Randy Thornhill, evolutionary biologist at the University of New Mexico in Albuquerque. "We are the products of evolutionary processes just as much as baboons and earthworms."
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Choosing a Mate
Animals and humans have evolved many traits that enable them to mate more successfully. Some traits make them more desirable to the opposite sex. Some make them better able to compete with others of the same sex. Here are some examples of what scientists believe are sexually selected traits.
Peahens prefer peacocks with the largest, showiest tails.
Antlers evolved in many species to help males compete for mates.
Males of many species, including this sage grouse, compete for the attention of females with elaborate courtship dances.
Large lips, small chins and symmetrical faces are believed by evolutionary biologists to be ingrained notions of beauty.
Scientists think the larger body size and greater musculature of a man evolved because of competition between males.