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As mating season approaches, male animals are faced with a question that can make or break their chances at reproducing: does it pay to be a lover or a fighter? Or both?

Researchers at The University of Western Australia, working with academics at Syracuse University, New York, and The University of Manchester, UK found that males traded investment in weapons for testes when they were sure that females wouldn't fool around with other males when their back was turned.

The study, published in Nature Communications looked at more than 300 species from parasitic worms to gorillas and found that the males' ability to monopolise a female for continued mating drove the way they evolved.

John Fitzpatrick, an ARC Postdoctoral Fellow in the School of Animal Biology's Centre for Evolutionary Biology at UWA and now Lecturer in Animal Evolution at The University of Manchester, said the findings help to explain why some animals appear to invest most in expensive sexual traits but others are more frugal.

"We set out to see why some species show trade-offs in sexual traits and others do not - the answer lies in how successfully males are able to keep females from mating with rivals.

"We know animals try to get females in a couple of ways. When they fight for them they sometimes evolve weaponry - such as antlers or a really big body size or big teeth. The other way they do this is not to bother to compete before they mate but to have big testes and the highest sperm quality so that they can fertilise the most eggs," Dr Fitzpatrick said.

Co-author and Director of UWA's Centre for Evolutionary Biology Professor Leigh Simmons said the findings provide evidence for new evolutionary theory.

"Evolution has shaped animals to maximise reproductive fitness by the most economical route. The broad patterns we have observed offer strong evidence for recent theoretical models that aim to predict how animals should best balance their limited resource budgets."

Pheasants, minnows, and bush crickets invested in both weapons and testes, while pinnipeds, such as elephant seals, where males are almost five times the size of females, and acanthocephalan (a type of worm) invested more in weaponry and less in testes.

Other examples of males investing in weaponry are antlers in red deer, horns in dung beetles, spurs in pheasants and canine teeth in primates.

"Some species invest in both and that is a bit of a mystery," Dr Fitzpatrick said. "We will now look at whether maximising investment in sexual traits means you pay the price in some other aspect of life."

Media references

Winthrop Professor Leigh Simmons (Centre for Evolutionary Biology) (+61 8)  6488 2221

David Stacey (UWA Public Affairs) (+61 8)  6488 3229  /  (+61 4) 32 637 716