April 10 (UPI) -- Primates with showy traits, big teeth or bright red chest markings, tend to have smaller testicles, according to a new study.
Male primates are hyper-competitive, and no competition is of greater importance than the battle for most offspring. As revealed by new research, there are different strategies for sexual dominance.
One strategy is to invest evolutionary capital in large testicles and more potent semen, increasing the odds that copulation results in impregnation.
Another strategy is ornamentation.
According to the latest study, published Wednesday in the journal Proceedings of the Royal Society B, showy traits, like the hair capes of hamadryas baboons or the enlarged noses of proboscis monkeys, and weaponry, such as the enlarged canines of many apes, can help males intimidate their rivals and seduce more females.
When researchers at the University of Zurich closely observed male primates exhibiting showier sexual traits, they found the primates' preference for flash comes at a cost -- smaller testes.
"Ornament elaboration comes at the expense of testicle size and sperm production," Stefan Lüpold, an evolutionary biologist at the University of Zurich, said in a news release. "In a nutshell, the showiest males have the smallest testes."
Scientists analyzed the sexual ornamentation strategies of 100 primate species and found more competitive primate societies tend to yield flashier sexual traits.
Researchers also tested the semen of different male primates. They found bigger testicles produce larger quantities of semen. Males who produce more semen are more likely to win the race to a female's eggs.
Because the development of flashy sexual traits and big testicles both require energy, primates can't opt for both strategies. It's one or the other.
"It's hard to have it all," said Lüpold.