A roaring African lion with his mane flowing full is for many people the very icon of wild nature. But precisely why males have manes has never been nailed down. Now researchers provide evidence that the mane is a signal advertising the animal's condition, which females use to choose mates and males use to assess rivals. In an apparent evolutionary tradeoff, however, manes also impose a cost on males by increasing their heat load.
Animal behaviorist Craig Packer of the University of Minnesota, Twin Cities, has studied lions at Serengeti National Park in Tanzania for more than 2 decades. Over the years, his group has amassed a huge database on the animals. Matching photographs of individual lions to records of their age and condition, Packer and graduate student Peyton West found that lions with longer and darker manes were more mature, had higher testosterone levels, lacked injuries, and were better nourished.
Lions know this as well, apparently. West and Packer discovered this by setting out life-sized models of male lions with manes of different colors and lengths. Males avoided models with dark, long manes--apparently to sidestep conflict with macho alpha males, the researchers reason. Females reacted differently, sidling up to the dark-maned males. The females' preference makes sense, the researchers point out, if dark manes reflect maturity and good physical condition. A male in top condition can better defend his females and cubs from attack by other males.
But great manes come at a cost: Using an infrared camera, the researchers could plainly see that manes, especially long and dark ones, were a major source of heat, a liability on the hot savanna. Indeed, the long-term data revealed that manes grow shorter and lighter colored when seasons are warmer. This suggests that manes impose a significant physiological cost on the animal.
"This is one of the few cases of a sexually selected trait where a physiological cost has been demonstrated," says evolutionary biologist John Endler of the University of California, Santa Barbara. But behavioral ecologist Göran Spong of Uppsala University in Sweden is less enthusiastic, asserting that the study still doesn't resolve whether the mane is for signaling or for protection in fights.