burying beetle
Burrying beetles have more gay sex when there are few females aboutgailhampshire/Flickr

Beetles have more gay sex when there are fewer females around as it serves an evolutionary strategy, experts have found.

Published in the journal Biology Letters, researchers looked at burying beetles in light of the acceptance threshold theory – a sort of cost analysis of mating.

The sexes of burying beetles is ambiguous – some females look and smell like males and vice versa. As a result, the male sometimes does not know if it is mating with a male or a female.

Why they do this instead of waiting for a beetle that is clearly a female depends upon how many of the opposite sex are around. If there are not many, males are more likely to engage in same-sex sexual behaviour (SSB).

"When recognition is not error-proof, the acceptance threshold used by males to recognise potential mating partners should be flexibly adjusted to maximise the fitness payoff between the costs of erroneously accepting males and the benefits of accepting females," the authors wrote.

Study author Sandra Steiger, from the Institute of Evolutionary Ecology and Conservation Genomics at the University of Ulm in Germany, told IBTimes UK recognition error – in this case where males have sex with other males thinking they are females – is based on a cost analysis and serves an adaptive purpose for male beetles that have little access to females.

She said: "If it's not costly – and instead is really costly to reject a female – then you should not reject any female because you just need one to reproduce. You should not be so restrictive because you might make a mistake and reject female just because it smells a little bit like a male.

"You should be more relaxed with your discrimination decision and accept more – it could end up being male but on the other hand you never reject a possible female."

Study findings supported the theory of the "prison effect", where homosexual sex increases because of few mating opportunities. They also lost the ability to discriminate between males and females because they could not "update their template of female cues".

Where females were abundant, SSB not only decreased, but males did not mate with every female they encountered.

"In nature, homosexuality is very common and it's very puzzling at first glance because it's very often you would think that if you are homosexual you can't reproduce, so there must be some explanation," Steiger said.

"In this case, we really think it's a strategy because you sometimes can't decide if it's a male or a female. If you have many females it doesn't really matter if you reject one because you've already mated and focus on females that look female. But if you're in a situation where you've never found a female, females are very valuable and you should never reject them. Then you should start to accept everything you encounter because it could be a female."

Findings of the study translate to instances of homosexuality in a number of species – but not humans as we can tell males apart from females. However, Steiger said it shows homosexuality serves an adaptive function.

She said: "It still shows it's natural and happens in nature all the time and even can be a strategy to circumvent problems."