As mating season approaches male animals are faced with a question that can determine their chances of reproducing: shouldwalrus (1) they be a lover or a fighter? A recent study, led by Faculty researcher Dr John Fitzpatrick, has found that where animals fall on the lover/fighter scale depends on the extent to which they are able to ensure continued mating rights with females.

In species where fighting for the right to mate means greater control of the female, males invest more in weapons and less in testes size. But males produce large weapons and testes in species where fighting for females occurs both before mating – with weapons – and after mating – with sperm. Some males found fighting the most successful method. Others found fighting was only the first step in sexual relations and also had to rely on large testes to ensure their fertility.

The study looked at over 300 species and found that male ability to monopolise females for continued mating drove the way they evolved. Looking at mammals, birds, fish, insects, and flatworms, they discovered that males only traded-off investment in weapons and testes when they were sure that females wouldn’t fool around with another male when their back was turned. Dr Fitzpatrick said:

“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, 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 Stefan Lüpold, from Syracuse University, said:

“You don’t get something for nothing in evolution. We wanted to see which species invested in weapons over testes. Some of these species invest in both, and that is a bit of a mystery. We will now look at whether maximising investment in sexual traits means you pay the price in some other aspect of life. Understanding the way animals reproduce is important as it helps us understand how species evolve and can prove important for conservation.”

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