Horny beetles have tiny testes
Beetles with the biggest horns have the smallest testes, say scientists who show that in evolutionary terms you can't have it all.
They say their finding is clear evidence of an evolutionary trade-off between the ability to fight off sexual competitors and reproductive potency.
Or put simply, the ability to find a mate and the ability to fertilise her.
Professor Leigh Simmons of the University of Western Australia and US researcher Professor Douglas Emlen of the University of Montana publish their findings online today in the Proceedings of the National Academy of Sciences.
The researchers looked at beetles of the genus Onthophagus, dung beetles known for the size and variety of their horns.
"What we did was test a fundamental assumption underlying evolution ... that males face a trade-off between competing for access to lots of females and investment in gaining fertilisation with those females," Simmons says.
"They need to have big horns to win fights and get females and they need to have big testes in order to win in sperm competition.
"But they can't do both, so species which invest very heavily in their horns tend to invest less in their testes."
The study also demonstrates the theory of sperm competition, which occurs when females mate with more than one partner, Simmons says.
According to this theory, male rivalry continues after a mate has been found as sperm from different males compete to fertilise an egg.
"Theory predicts that males should increase their investment in sperm production as sperm competition in increased," he says.
Simmons tested the theory by cauterising the area of the developing beetle pupa, where the horns would grow, destroying cells and preventing horn growth.
He found that if horn development was inhibited, the pupa compensated by developing larger testes.
The principle is known as a resource allocation trade-off, he says.
"They got a fixed amount of resources to allocate to various structures, to their legs, their wings, their horns and to their testes and other important organs," he says.
"If [the developing beetle] doesn't produce horns those males then produce bigger testes because they have resources that weren't used for horn growth."
Simmons says the principle of resource allocation has been demonstrated in other animal studies.
For example, there is some evidence that bats trade the size of their testes for brain power.
And a recent study showed that stalk-eyed flies, in which eye span width is a measure of sexual desirability, trade testes size for the width of their eyes.
"There have been suggestions of a trade-off between gaining mates and gaining fertilisation but our study is probably one of the most conclusive studies to date to have demonstrated this," Simmons says.