London, Oct 17: Male beetles with the biggest horns have the smallest testicles, scientists at the University of Montana in Missoula, US have found.
Douglas Emlen, who led the research, said that the findings are a clear evidence of an evolutionary trade-off between the ability to fight off sexual competitors and reproductive potency.
The researchers looked at beetles of the genus Onthophagus - dung beetles known for the size and variety of their horns. In some species, these make up 40 per cent of males’ body length.
These iridescent beetles use their flashy ornaments to battle against one another and block access to tunnels where they mate with females. But the competition does not end there, as females often mate with more than one male. So while they are inside the female, one male's sperm must compete with other males' sperm to fertilize eggs.
According to researchers, it is generally thought that the males that produce the most sperm are more likely to achieve a fertilization so, besides the horns, testicle capacity is important in competition between males too, reports Newscientist.
The study also found out an inverse relationship between horn and testes size.
"This study is the first solid experimental demonstration that adaptations to compete for mates trade-off with what it takes to compete for fertilizations," Scott Pitnick at Syracuse University in New York, US, said.
"Because of energetic constraints, you really can't be good at all things," said Pitnick, who last year revealed a trade-off between relative brain and testes sizes in 300 different species of bat. (ANI)