This blog post is provided by Allison M. Roth and tells the #StoryBehindThePaper for the article “Sexual selection and personality: Individual and group‐level effects on mating behavior in red junglefowl“, which was recently published in Journal of Animal Ecology.

Over the past two decades, behavioral ecologists have become increasingly interested in consistent between-individual differences in behavior, otherwise known as “animal personality”. It is, however, unclear why such between-individual differences exist, given that the personality of an individual has been shown to influence fitness in a variety of species, with individuals of some personality types exhibiting increased survival and/or reproductive performance, compared to individuals with different personality types. In other words, if selection favors a particular personality type, why do other personality types continue to persist? One possibility is that individuals with certain personality types have higher fitness under some social environmental conditions, while individuals with different personality types have higher fitness under different social environmental conditions.

In a recent study in the Journal of Animal Ecology, we investigated how personality influences sexual selection, examining how two common measures of personality, exploration in a novel arena and boldness towards a novel object, influence various measures of reproductive performance in male and female red junglefowl (Gallus gallus). We also explored how the reproductive performance of an individual is affected by the personality of its sexual competitors and potential mates. Furthermore, we examined whether there was assortative or disassortative mating with respect to personality type. In other words, whether birds mated more frequently with individuals with similar or dissimilar personality types, respectively. Lastly, we explored the potential mechanisms that might underlie differential reproductive success. Specifically, we examined whether an individual’s personality influenced whether males spent more time courting or sexually harassing females, how much time males spent associating with females, and female solicitation of copulations.

We classified individuals as faster- or slower-exploring, based on how quickly they explored a novel arena, and bolder or shyer, based on how quickly they approached a novel object. We assembled the junglefowl into 24 mixed-sex groups of 12 individuals. Eight of these groups had a female-biased sex ratio, eight had a male-biased sex ratio, and eight had an even sex ratio. In addition to manipulating sex ratio, we manipulated the personality composition of males and females in the groups, so that some groups had a higher proportion of individuals with fast-exploring personality types, and some groups had a higher proportion of individuals with slow-exploring personality types.

We found that, in female-biased groups, faster- and slower-exploring males had increased reproductive performance compared to intermediate-exploring males; they mated with more females and experienced lower sperm competition intensity. Males with lower sperm competition intensity are expected to enjoy higher paternity share. Interestingly, we also found that, in female-biased groups, faster-exploring males mated with more females when they were in groups where the other males (i.e., their competitors) were slower-exploring, on average, and vice versa. Intrigued by these results, we wanted to test more explicitly for negative frequency-dependent sexual selection. We examined whether the proportion of matings obtained by fast-exploring males in female-biased groups was influenced by the proportion of fast-exploring males in the group and found that, as the proportion of fast-exploring males in a group increased, the proportion of matings by fast-exploring males decreased, indicating negative frequency-dependent sexual selection. Our results demonstrate the importance of considering the social environment when studying links between individual personality and fitness and suggest that spatial or temporal variation in an individual’s social environment may assist in allowing variation in exploration to persist in red junglefowl.

Interestingly, we also found evidence to suggest that males with different exploration types had different routes to achieving reproductive success. We found that, across sex ratios, 1) faster- and slower-exploring males spent a greater proportion of their efforts sexually harassing females, rather than courting them, compared to intermediate-exploring males, 2) faster-exploring males associated with females more than slower exploring males and associated most with females that they mated more frequently with, and 3) females solicited copulations from faster-exploring males more than they solicited copulations from slower-exploring males, suggesting a female preference for faster-exploring males. Lastly, although boldness did not appear to influence reproductive performance, we found that bolder males were more likely to mate with shyer females, and vice versa (i.e., disassortative mating). In the long run, this could help maintain genetic variation in boldness.

The findings of this study help explain how variation in personality may be maintained. We provide empirical evidence of negative-frequency dependent sexual selection acting on personality. We hope that this study will inspire other researchers to consider how an individual’s social environment might influence relationships between an individual’s personality and its fitness.