A team of researchers used mathematical models and existing animal data to determine why the males of some species develop flashy but impractical ornaments. For example, peacocks have large, colorful feathers but related species lack those showy features. The findings are in a paper that was just published in the journal Proceedings of the Royal Society B.
In some animal species, the males have flashy and extravagant features such as large antlers, colorful feathers, or giant horns. While some of these features could potentially help males fight, others seemingly serve no purpose other than to appeal to females. The peacock has long, bright colored feathers that are actually quite impractical. The feathers make the bird stand out to predators while slowing him down when he’s trying to escape. Even if females prefer these feathers, the trait seems evolutionarily costly. A theory called Zahavi’s handicap principle states that these traits continue to evolve because they signify that the male is strong enough to deal with the negatives that come with the showy ornaments. For example, a male with extremely long feathers might have a difficult time escaping predators–but his existence means he was strong enough to survive even with the extra burden. This tells an interested female that the individual is strong and healthy.
Researchers from the Northwestern University decided to investigate the idea of Zahavi’s handicap principle. They had first observed that some species had extravagant, completely impractical ornaments while others lacked ornaments or had more subdued traits. Most species fit one of the extremes with none in the middle. The team used mathematical models to confirm this idea and found that animal species tended to split into two groups. In one group, the cost of a showy ornament was made up for with better fitness because the healthy males could handle the handicap, gaining access to more females. In other species, the males didn’t evolve any traits that would put them in such a dangerous position. The team concluded that sexual selection and natural selection were both at play but one dominated in the end for each species. In animals with showy features, sexual selection exaggerated those features and natural selection then allowed only the healthiest, strongest males to survive.
The team’s findings provide more evidence of the existence of Zahavi’s handicap principle. This explains why in some animal species, males have flashy but impractical ornaments to attract mates. In other species, this is too costly and the males develop less showy features. The team compares this idea to wealthy humans buying impractical items they don’t need in order to demonstrate their wealth and value. The researchers hope that their findings will help scientists better understand how sexual selection and natural selection work together in evolution.
Clifton SM, Braun RI, Abrams DM. Handicap principle implies emergence of dimorphic ornaments. Proceedings of the Royal Society B (2016).