By Anna Rees
When we think of sexual selection in animals, we traditionally imagine that all potential mates will try to get with the most desirable mate available to them. Well, just like in humans, punching above your weight isn’t always the best option. In a study done on fish by Stephen Baudalf et al., they wanted to know if there was evidence that fish will prefer mates with similar traits to their own in the presence of other, more desirable fish.
In the Cichlid fish species they were studying, size directly correlated to fitness, and so the largest males and females are sexually preferred. Researchers put fish of varying sizes in front of a computer simulation of potential mates and evaluated which ones they preferred. Unsurprisingly, without any competition, all the fish showed the most interest in the large fish, regardless of their own size. However, when the fish were put in an environment with competition, the smaller fish either faced rejection from the larger fish or made advances on fish of a similar size to them.
These fish, like people, are characterized by a high level of biparental cooperation. In other words, both the males and females spend much of their energy caring for their offspring. Since this is such a commitment and energy-expensive process, the fish have incentive to be choosy with their mates. Both males and females are able to court or reject the other sex, and so their behavior is a great model for sexual selection.
The researchers who conducted this study believe size preference is related to reducing conflict with other fish around them. While all the fish would prefer a larger mate since this trait generally demonstrates better fitness, conflict between other fish for mates is a driving force in mate decision. The larger fish were able to mate with each other and reject the smaller fish, and the smaller fish were left to either not mate at all, or mate with each other. Although this isn’t a particularly romantic outcome, this does lead to fish generally ranking themselves among potential mates and often preferring to mate with other fish who are of a similar size to them. These fish would rarely mate with fish who were smaller than them when given other options.
Baldauf, S.A., Kullmann, H., Schroth, S.H. et al. You can't always get what you want: size assortative mating by mutual mate choice as a resolution of sexual conflict. BMC Evol Biol 9, 129 (2009). https://doi.org/10.1186/1471-2148-9-129