Scientific Method / Science & Exploration

For female fish, a choice between beauty, brawn, and brains

A new study shows how female choice, predator choice, and mating strategies …

Imagine a scenario in which there were five different kinds of men, each of which looked and behaved drastically differently. How would the women of the world choose between all these eligible bachelors? According to a new study in BMC Evolutionary Biology, this complicated situation is a way of life for the South American freshwater fish Poecilia parae.

Male P. parae come in five genetically determined varieties: yellow, blue, red, parae (which have vertical stripes), and immaculata (which are very drab). Parae males are large and antagonistic, while immaculata males are small and meek. Red, yellow, and blue males are all medium-sized. In this species, females ultimately have the choice of which male to mate with.

To determine how these different morphs fare with the ladies, the authors ran several experiments with wild-caught P. parae from the Republic of Guyana. First, they determined which kind of male females preferred by placing different males in a tank and calculating the amount of time females spent with each morph. Most females strongly preferred the red and yellow males over the other types.

Then, the various morphs’ behavior was tested in combat. Two males of different morphs were placed in a single tank and allowed to fight while a female watched through a clear partition. In almost all cases, parae and yellow males were dominant over all the other morphs, especially the subordinate immaculata. After witnessing the outcome, the eavesdropping female was then placed in the tank with the winner and the loser and could choose her lucky mate.

Interestingly, although parae and yellow males crushed the other morphs in male-male combat, they weren’t as dominant with the ladies as predicted by their wins. Instead, several females still chose to mate with the red males that they liked before they witnessed the fight.  While yellow males were somewhat successful in aggressively restricting access to females, some females still chose red losers over yellow winners.

Multiple morphs and alternative mating strategies are usually maintained in a population by frequency-dependent selection, in which the success of any one morph depends on how common it is. Rare morphs do better than common ones, but then suffer once they become abundant. Among P. parae, however, the frequencies of various morphs don’t change between years, so the existence of multiple mating strategies can’t be due to frequency-dependent selection. There must be another explanation for how and why the multiple morphs all coexist.

The authors believe that balancing selection may be at work in this system. Balancing selection occurs when different selective pressures work in opposite directions. For example, P. parae females prefer red and yellow males at first sight, but so do cichlids, their main predator. So, while sexual selection favors red and yellow males, natural selection keeps these morphs in check and allows other morphs a chance. 

The fact that yellow guys don’t always get the girl (despite winning most of the fights) suggests that balancing selection is also taking place between male antagonistic behavior and female choice: male aggression promotes the reproductive success of yellow males, but female preference favors red morphs. With all these opposing selective pressures, no one morph dominates, but none are driven to extinction.

So what about the wimpy immaculata males? Females don’t like them and they never win fights, so how do they persist in the population? It turns out that these sly little guys adopt a “sneaker” tactic. The appearance and behavior of immaculata males mimic those of juvenile females, so they can sneak in and grab a quick copulation with the female before she figures out what they really are.

BMC Evolutionary Biology, 2010. DOI: 10.1186/1471-2148-10-391  (About DOIs).

Listing image by Syracuse University

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