In many animal species, males must compete fiercely with other males for access to mates. As a result, males often develop elaborate traits that will help them conquer other males, or make them more attractive to females. However, sexual competition can also select for traits that are more than just skin-deep. Courtship and mating are complex tasks requiring memory and problem-solving skills, which selection could also act on.
Brian Hollis and Tadeusz Kawecki from the University of Lausanne in Switzerland decided to investigate whether sexual competition influences cognitive performance in fruit flies (Drosophila melanogaster). Fruit flies have relatively simple courtship and mating rituals, but the researchers hypothesized that even in this system, competition for females might select for increased cognitive performance in males. In order to eliminate sexual selection, the authors raised three replicate populations of fruit flies for over 100 generations by randomly pairing single males with single females. In doing this, the authors eliminated all male–male competition, as well as all mate choice. The authors then tested the flies raised in enforced monogamy against males from the original population that had been kept under the naturally polygamous conditions, where males compete with each other to mate with multiple females.
First, the scientists challenged the flies to compete for mating opportunities. They found that when multiple males had to compete over females, the males from the polygamous lines were far more likely to mate successfully with females than the males that came from 100 generations of enforced monogamy. However, there were no differences in locomotion between the flies and no differences between the flies in their mating success when a single male was paired with a single receptive female. The researchers concluded that the monogamous males had no gross abnormalities, but had lost their ability to compete with other males for mates.
In order to identify what these monogamous males might be doing wrong, the scientists put varying numbers of receptive and non-receptive females with either a single monogamous or a single polygamous male. They found that males from the lines of enforced monogamy would waste substantial effort trying to court unreceptive females, while the polygamous males targeted their courtship efforts appropriately. The scientists concluded that over the generations, the monogamous males had lost the cognitive ability to correctly identify the females that would be the most receptive to their amorous advances.
Finally, the researchers asked whether this decline in cognition was specific to courtship, or whether the monogamous males had reduced performance during other cognitive tasks. They trained the flies to associate a specific odour with an unpleasant shock and then challenged the fruit flies to solve a maze where the flies had to chose between the adverse odour and a neutral smell. The researchers found that the monogamous males were worse than the polygamous males at solving the maze. Interestingly, when they tested females from both the monogamous and polygamous lines, the scientists found that females of the two lines were equally capable of solving the maze. This result suggests that the males' decline in cognition is specific to the lack of sexual competition, rather than a result of genetic drift or a decline in overall performance in the absence of mate choice. Thus, it appears that sexual competition selects for cognitive traits, and competition is necessary to keep males sharp.