Researchers have found that where animals' position on the lover/fighter scale depends on how much they are able to ensure continued mating rights with females.
The study looked at over 300 species and found that males' ability to monopolise a female for continued mating drove the way they evolved.
The study looked at sexual behaviours in male mammals, birds, fish, insects and flatworms and has found 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.
Senior author John Fitzpatrick, a Lecturer in Animal Evolution at The University of Manchester, said that these finding help to explain why some animals appear to invest maximally in expensive sexual traits but others are more frugal.
He said that they 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.
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 worms) invested more in weaponry but not 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.
The study has been published in the journal Nature Communications.
