Male Fiddler Crabs will quite happily protect a female neighbour, but do so partly in exchange for sex, according to a new study from The Australian National University.

The study by Richard Milner, Professor Michael Jennions and team leader Dr Patricia Backwell of the Research School of Biology at ANU looked at how female fiddler crabs – without the large claw that the males are armed with – go about protecting their territories. The results of the study are published in Biology Letters.

The researchers found that even though the female crabs are physically no match for attacking males they do have something that can work to their favour – offering sexual favours to neighbouring males in exchange for protection. These sexual offerings lead to neighbourhood coalitions, where a male crab will protect female neighbours from other homeless males seeking a new territory.

“In fiddler crabs, both males and females defend territories that are essential to their survival,” said Professor Jennions.

“Males have a weapon in the form of a giant claw that is the largest weapon relative to body size in the animal kingdom. In contrast, females are weaponless and possess two small feeding claws.”

“This study shows, for the first time, that in exchange for sex and other benefits, males protect their female neighbours from territory-seeking male intruders. The paper provides the first evidence of ‘defence coalitions’ between territorial males and females,” he said.

Trading sex for material benefit is not unheard of in the animal world. Other examples where this happens include female red-winged blackbirds, where mating with an extra-pair male opens the chance to forage on his territory, and the Adelie penguin, where females exchange sex for highly sought-after stones used for nest building. This is the first study to show a similar process in fiddler crabs.

“Males protected their female neighbours in 95 per cent of instances where the intruder was male, and only 15 per cent of instances where the intruder was female,” said Professor Jennions. “This suggests that males don’t care who their neighbour is, as long as they are female. Additionally, by ensuring that their neighbour is female, males will benefit through increased opportunity for reproductive success and reduced boundary maintenance costs – because it is less costly to maintain a territory boundary with a female than a male.”

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