For people whose views on homosexual behaviour are that it is ‘not natural’, they would be pretty dismayed to look across the animal kingdom. Indeed, ‘same-sex sexual behaviour’ (SSSB) is pretty ubiquitous. Bearded vultures mount the same sex 11% of the time, seed beetles mount the same sex 50% of the time and Japanese macaques mount the same sex 46% of the time.
From an evolutionary point of view, such behaviour seems like it would be a waste of time, since it doesn’t result in offspring. However, its persistence implies that it does indeed have a function (rather than just being a mistake on the part of the animal), even if we haven’t worked out what it is in every case. In a recent study by Boutin et al. published in the journal Animal Behaviour, researchers sought to understand why field crickets show this seemingly maladaptive behaviour.
Male field crickets fight each other for territories. Once a male has a territory he then ‘sings’ to attract females by rubbing his forewings together. Females will check out the various males on offer and approach a male they’re interested in. The male then backs up towards a female, so that his abdomen is close to her head, and then if she chooses to, she mounts him to mate.
However, male crickets will sometimes also sing for other males and back up towards them when they approach (they don’t try to mount other males, because unlike other animals, in field crickets it’s the females that do the mounting).
So why do male field crickets sometimes elicit matings from other males? Boutin et al. had four ideas why this might be. First, perhaps mating with other males has a social function: mating with males may lead to less aggression and help establish or maintain social bonds or dominance hierarchies. Second, perhaps SSSB somehow actually improves the chances a male will mate with a female. This could be a way of showing off to females or practicing for when they actually encounter a female. It is also possible that when mating with a male, a field cricket transfers some of his sperm to him where it may then be passed on indirectly to a female (this is known to happen in some other insects and arachnids). Third, it could be that a male’s tendency to mate with other males is a restriction of natural selection. For example, selection may favour males that are more sexually active if they mate with more females and have more offspring. However, it is possible that part of the ‘personality’ of males that are more sexually active with females is that they are also more sexually active with males. Finally, it could be that male-male courtship is just a case of mistaken identity where males think that other males are in fact female.
The authors tested these four different possible explanations over a series of experiments, excluding the various possibilities until they could deduce what the most likely scenario was. First, they figured that if males back up and sing to other males to avoid aggressive interactions, then you would expect to see this behaviour most often directed towards more aggressive males. However, this was not the case. Males also did not seem to court other males in order to show off to females, as they were just as likely to court males when there were no females around.
In fact, the authors found that it was the most aggressive males that were most likely to court other males. However, they only did this when they were fed high-carb diets, implying that this behaviour requires a bit of extra energy. The most aggressive males are also generally the ones that tend to elicit more matings with females. Therefore, it seems that the same-sex sexual behaviour in this species may be caused by a limitation in natural selection: more aggressive behaviour is favoured by natural selection as it leads to more offspring being produced from males who mate a lot of females, however, these males also tend to mate more males. Thus the male-mating behaviour isn't selected against, as these males are still producing more offspring overall. However, the researchers also found that the least aggressive males were the ones most likely to be courted. Since females are also generally un-aggressive, this finding indicates that there is also a possibility that males were mistaking the non-aggressive males for females.
SSSB is sometimes assumed to be more common in lab-settings, where animals may have more restricted access to animals of the opposite sex. However, interestingly, the researchers also found that SSSB was just as common in wild field crickets.
Boutin, S. R., Harrison, S. J., Fitzsimmons, L. P., McAuley, E. M., & Bertram, S. M. (2016). Same-sex sexual behaviour in crickets: understanding the paradox. Animal Behaviour, 114, 101-110.
First field cricket: Arpingstone
Japanese macaques: bryansjs
Second field cricket: Jeffrey Reed