Pigeons are normally a heterosexual and monogamous species (the exception proves the rule). But what happens if you skew the sex ratio to more female-biased or male-biased? Łukasz Jankowiak, et al. (2018) did such experiments and found out that same-sex pairs can occur in such situations. It is known that female-same-sex pairs have a higher likelihood in monogamous species compared to polygamous species. In contrast, male-same-sex pairs are more likely in polygamous species. Indeed, this trend is supported by the experiment, which showed that there are more female-female pairings in a female-biased sex ration as male-male pairings in male-biased sex ratios. Moreover, the female-female partnerships were longer lasting and resulted in successful breeding whereas the male-male partnerships where short and showed no egg adoption behavior. Unfortunately, it is hard to find driving force behind same-sex pairs: there are many different theories (from mistaken sex identity to intersexual conflicts). The authors guess, that in this pigeon experiments, the same-sex pairing in males can be explained by the "heterosexual deprivation" (meaning: there are just no females available, therefore they go for males), while the female-female pairs are more driven by "alloparenting" (you need two adults for successful breeding so you better "bind" a female to your nest as breeding alone). But of course, it is just a guess. So message of the day: female-female pairs more in monogamous species, male-male pairs more in polygamous species. According to the paper, it has something to do with parental care: when the male provides a lot of parental care in monogamous species, females tend to compensate missing males with females, while males show a smaller chance of male-male pairing. In contrast, in polygamous species, if the males care less, the females do not have to compensate the missing men (less female-female pairs) while the frequency of male-male pairs increases.
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