by Piter Kehoma Boll
Homosexual behavior, as you may know, is a widespread phenomenon across the animal kingdom, especially male-male sexual behavior. Like many other aspects of life, this behavior likely evolved independently many, many times and plays different roles in different species.
A study recently published in the journal Animal Behaviour investigated the male homosexual behavior of the red flour beetle, Tribolium castaneum. The team wanted to verify whether a large supply of females or a large number of rival males would influence the occurrence of males having sex with other males.
In order to test that, the researchers directed the evolution of some populations of the red flour beetle in the lab. Some populations were maintained in a male:female ratio of 1:9 and others of 9:1, i.e., in the first group the population consisted of 10% males and 90% females, so males had a greater chance of finding a female than a male and competition between males was very week. In the second group, 90% of the population was made up of males, so females were harder to find and males had to fight for them.
After about 100 generations in which the sex ratios were maintained, the researchers compared the occurrence of male-male sex in both treatments. The results show that males that evolved in an environment where females were abundant and competition between males was low were more likely to engage in homosexual behavior than males that evolved in an environment were competition between males was high and the chances of finding a female were much lower.
The most likely explanation for this difference is that males in highly competitive enviroments need to be better in identifying female individuals to mate with, while in environment where there are plenty of females available, most of the encounters are with females, so the strategy to “have sex with whomever you find” is good enough. The few instances in which such males find other males and mate with them is not enough to reduce their reproductive fitness. In other words, when you live among few females, it is crucial for you to recognize someone as a female and mate with her, otherwise you may end up not passing your genes to the next generation. Now if there are plenty of females you may have sex whenever you want and you certainly will have some children, even if you sometims have fun with your male pals.
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Sales K, Trent T, Gardner J, Lumley AJ, Vasudeva R, Michalczyk Ł, Martin OY, & Gage MJG 2018. Experimental evolution with an insect model reveals that male homosexual behaviour occurs due to inaccurate mate choice. Animal Behaviour 139: 51–59. https://doi.org/10.1016/j.anbehav.2018.03.004
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