As a male amongst a crowd of receptive and unreceptive females, finding a mate to woo and carry your offspring is hard work! The common fruit fly (Figure 1), Drosophila melanogaster, must deal with this complex social scenario in addition to competing with other males for a receptive mate. You might even infer that successful courtship in fruit flies requires complex cognitive processing, including the ability to identify a receptive female though sensory cues. It has been previously shown that female Drosophila use olfactory cues to identify potential mates, though whether males also use these cues to identify female mates is less known (Dickson, 2008). In a recent publication, Hollis and Kawecki (2014) hypothesized that sexual selection contributes to the evolution and maintenance of cognitive abilities, such as olfactory discrimination, in male fruit flies. In other words, “smarter” flies may fare better in the mating game and consequently create more smart offspring.
One way to approach the above hypothesis is to compare the behavior of male flies with and without sexual selection. To do this, Hollis and Kawecki maintained monogamous and polygamous (also known as the control) populations of flies for over 100 generations. Males in the polygamous group would naturally undergo sexual selection for increased ability to court receptive females, while this pressure was removed in the monogamous group, where each male was randomly paired with a receptive female. Here mating-adaptive abilities in males should be beneficial under polygamy and irrelevant under monogamy.General male attractiveness and motor activity did not differ between groups when the authors observed male courtship with only one receptive female present. However, males in the monogamous group sired less offspring than males in the polygamous group when both groups were combined in a population with females, indicating lower competitive reproductive fitness in the monogamous group. After subjecting flies to a courtship task and an olfactory learning task, Hollis and Kaweki also found that performance was lower for the monogamous group in both contexts. In the courtship task, males in the monogamous population took about 19 min longer to court the receptive female in the presence of one receptive and five unreceptive females (Figure 2). Further, these males were likely to spend more time in vain, courting unreceptive females, than their polygamous counterparts. The above findings suggest that absence of sexual selection decreases mating ability in male flies; the next step is to identify the potential role of sensory cognition in this impairment. Olfactory learning is a sensory-cognitive process commonly thought to help males gauge female receptiveness for mating. Receptive females may give off pheromone cues that signal to males of their readiness to mate. Inability to detect such cues might prevent male flies from identifying receptive females, leading to decreased mating success. It would make sense, then, that sexual selection favors males with heightened olfaction. Results from the olfactory learning task support this. Males in the monogamous group showed decreased olfactory learning, demonstrated by lower performance in identifying and avoiding odors associated with an aversive shock (Figure 3). These findings reaffirm that sensory-cognitive ability plays a role in how sexual selection affects mating success in male fruit flies. Interestingly, females in the monogamous population did not exhibit decreased olfactory learning, suggesting that sensory-cognitive decline from the absence of sexual selection is sex-specific and that there are sexual differences in how flies respond to environmental stimuli.
So why is it important to study the effects of sexual selection on cognitive ability in fruit flies? These insects are easy-to-maintain model organisms with relatively short reproductive cycles, making them ideal for evolutionary experiments. Moreover, the competitive sexual selection scenario that flies encounter is not much unlike what other organisms, including humans, experience. The next time you meet a potential mate, you might even consider the role sexual selection over generations has led to the cognitive abilities of the specimen in front of you.
Dickson, B. J. (2008). Wired for sex: the neurobiology of Drosophila mating decisions. Science (New York, N.Y.), 322(5903), 904–9. doi:10.1126/science.1159276
Hollis, B., & Kawecki, T. J. (2014). Male cognitive performance declines in the absence of sexual selection. Proceedings of The Royal Society: Biological Sciences, 281(1781), 20132873. doi:10.1098/rspb.2013.2873