With the historic supreme court hearings this week, there has been much discussion about homosexuality*. One of the ‘arguments’ that you often hear from the anti-gay rights side is that being gay isn’t natural. Evidence from the animal kingdom would refute this however, as same-sex behaviour is common and found in many different animals.
There’s the famous example of ‘Roy’ and ‘Silo’, two male chinstrap penguins that formed a pair bond and raised a chick (‘Tango’) together, later turned into a distinctive children’s book (you can also read about their tragic breakup here – this part has yet to be made into a children’s book).
Homosexuality is also common in many insects, and some flour beetle males actually mate 50% of the time with other males. But why does same-sex behaviour occur? How is it maintained by evolution? This is a complex question, and the answer is likely to differ from species to species. For example, flour beetle males that mate with other males can actually transfer to females this way. Other male insects like weevils or fruit flies may just not realise that the individual they’re mating with is also a male (it being better to mate with more animals, and get it ‘wrong’ sometimes, than be too discriminating and miss out on potentially fruitful mating attempts). However, in addition to specific cases, there may also be overarching patterns across species in how homosexuality is selected for and maintained by evolution.
While many studies have concentrated on male-male sexual behaviour, females also engage in same-sex behaviour. Laysan albatrosses form female-female partnerships, performing the same mating rituals as in male-female pairs of this species, and these couples can last a lifetime.
Many female animals show behaviours towards other females that they wouldn't normally direct towards males. This produces a quandary for the scientists studying them, as such behaviours can’t be explained by the females making a ‘mistake’ in thinking it was a male.
For example, in the seed beetle Callosobruchus maculatus, females mount other females in the same way that the males of this species mount the females. As the females of this species do not normally do this to males, why might they do this to females?
C. maculatus is a pest to humans found almost world-wide as it’s hitched lifts across the globe on the beans that we eat. When a female wants to reproduce, she lays her eggs on the surface of beans. When the larvae hatch, they burrow into the bean and hang out inside there for a while together, eating away at the bean, and altogether having a jolly good time. The male adults then emerge from the beans before the females, and patrol the area waiting for them. Once the females emerge, the males mate with all of them. On a side-note, males of this species have spines on their penis that cause serious damage to the females, and so the females kick the males during mating to try to keep it as short as possible (although no one has suggested this as a reason for the prevalence of female-female sexual behaviour in this species).
To work out why females might mount other females, the researchers compared the tendency of brother and sister beetles to mount other females. They found that, although males tended to mount females more often than the females did, families where the males mounted more females also had females that mounted more females. In other words, females engaged in more same-sex behaviour when their brothers were also more sexually active with regard to this particular behaviour.
This relationship in mounting behaviour between brothers and sisters indicates that there could be the same genes underlying this behaviour in both sexes. Genes for behaviours can often correlate between males and females of a species, when it's not too costly for the the other sex to have them. In this case, females that mount other females more do not suffer any negative consequences compared to females that don't (they have just as high reproductive success). Therefore, there would be no selection against females having this behaviour, and indeed those that mount more may produce sons that also mount more, and produce more offspring.
Whether this mechanism exists in other species is yet to be seen, but this study will help stimulate further studies and research into this intriguing question.
Main paper discussed:
Burgevin, L., Friberg, U., & Maklakov, A. A. (2013). Intersexual correlation for same-sex sexual behaviour in an insect. Animal Behaviour 85: 759-762.
Aiken, R. B. 1981. The relationship between body-weight and homosexual mounting in Palmacorixa-nana Walley (Heteroptera, Corixidae). Florida Entomologist, 64, 267-271.
Edvardsson, M. & Tregenza, T. 2005. Why do male Callosobruchus maculatus harm their mates? Behavioral Ecology 16(4): 788-793.
Harari, A. R., Brockmann, H. J. & Landolt, P. J. 2000. Intrasexual mounting in the beetle Diaprepes abbreviatus (L.). Proceedings of the Royal Society B, 267, 2071-2079.
Serrano, J. M., Castro, L., Toro, M. A. & Lopez-Fanjul, C. 2000. Inter- and intraspecific sexual discrimination in the flour beetles Tribolium castaneum and Tribolium confusum. Heredity, 85, 142-146.
Cover of And Tango Makes Three, published by Simon & Schuster.
Laysan albatross pair: Eric VanderWerf
beetle eggs on cowbeans: Richard001
Seed beetle mating photo: F. Champion de Crespigny
Beetles for marriage equality: Cole Eskridge
* On a slightly unrelated note, see this interesting article on the psychological roots of attitudes to same-sex marriage and other beliefs.