A swollen backside is the mark of a good mother (in baboons).
Elaborate and wondrous sexual ornaments abound in the animal kingdom. The peacock's tail and the nightingale's song both advertise their bearer's quality as a mate. Now we must usher a female trait into this gallery of marvels -- the baboon's rear end.
Leah Domb, of Harvard University, and Mark Pagel, of Britain's University of Reading, have found that the degree to which a female baboon's bottom swells up when she is sexually receptive is a good guide to her reproductive potential 1.
Females with larger swellings begin breeding at a younger age, and breed more frequently. They also have more offspring, a higher proportion of which survive.
Males compete with each other more fiercely to mate with large-bottomed females, incurring more injuries in the process -- suggesting that the best females are trying to attract the best males.
This is the first time that sexual advertisements have been discovered in a female mammal. It is a surprise that female baboons need to advertise, says Kristen Hawkes, an anthropologist at the University of Utah in Salt Lake City, as "the assumption is that getting male attention is not a problem".
Domb and Hawkes both believe that adverts might have evolved because males weigh the costs of mating -- more fighting, for example -- and measure out their effort accordingly. This would drive females into competing indirectly with each other.
Producing a swelling, comments Hawkes, is "clearly a very expensive thing to do". Swollen females' body weight rises by about 14%, swellings hamper movement, are vulnerable to infection or parasites, and are probably uncomfortable to sit on. "It really interferes with how you would normally spend your day," says Hawkes.
It's important that these signals should be expensive to produce, as otherwise low-quality females could cheat the system, and flashy advertisements wouldn't be a reliable guide to reproductive potential.
So how do you measure a baboon's backside? Domb and Pagel analysed videotapes of female baboons in Gombe National Park, Tanzania, where the baboons have been studied since 1967, enabling them to match the size of a baboon's behind to her breeding history.
Robin Dunbar of the University of Liverpool, who studies decision-making in humans and other primates, thinks that more studies are needed to confirm that all primate swellings are advertisements. "One would like to see more data for other species before jumping onto that particular bandwagon," he says.
About 10% of primate species have sexual swellings. They seem to have evolved at least three times, and tend to be found in promiscuous species.
The rears of human females don't swell up when they are at their most fertile, but the bustles beloved of Victorian and Edwardian fashion might be seen as an artificial means to a baboon-like end. And some researchers believe that characteristics of human females, such as large breasts or waist-hip ratio, might signal reproductive quality.
So do human males read these signs? Perhaps -- the Kipsigi of East Africa pay more for a bride who began menstruating young. They have no direct knowledge of this, but use qualities such as plumpness and skin tone as a guide to their prospective bride's potential.
Domb,L. G.& Pagel, M. Sexual swellings advertise female quality in wild baboons. Nature 410,204- 206 2001.
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Whitfield, J. Bum deal for baboons. Nature (2001). https://doi.org/10.1038/news010308-11