Attracting a female can be expensive, and females may use this cost to weed out poor quality males. Male ornaments such as antlers are a reliable indicator of nurture, but what about nature? Recent research into in male ornamentation in stalk-eyed flies reveals a key role for genetics, and highlights the importance of mate choice in species survival.
Males throughout the animal kingdom go to some weird and wonderful lengths to try and attract females. This might involve an elaborate dance, a carefully wrapped gift, or an ornate head-dress, and these adverts are often costly to the male who performs them. A peacock’s tail makes him conspicuous to predators; a red deer’s antlers require food and energy to grow. And a female finds all of this very sexy, because it informs her about the quality of the male; only the highest calibre males can afford the handicap.
This phenomenon isn’t limited to birds and mammals, of course, even flies need to attract a mate, and stalk-eyed flies have a rather odd form of ornamentation: their eyes sit on the end of long stalks that protrude out horizontally from their head. Clearly this makes their lives difficult; long eye stalks take more energy to grow, and make the males more cumbersome during flight. So why do males grow these impractical eye stalks? Because, despite the costs, males with longer stalks have more luck with the ladies.
Females are thought to be attracted to males with longer stalks because they are reliable indicators of the quality of the male. Males with good genes, who grew up in plentiful environments, produce the largest ornaments and consequently get their pick of the ladies. In other words, these traits are strongly dependent on environmental conditions (e.g. food availability, temperature) and genetics. Or so the theory goes. There is good evidence that the appearance of male sexual traits is influenced by the environment in which the male grew up, but few empirical tests have been conducted to see whether ornaments also reflect genetic quality. And until recently, we had little idea of what impact this might have at the population level.
Nature and Nurture
Recent research in GEE has taken a major step towards answering these questions. Bellamy and colleagues conducted a long-term inbreeding experiment with the African stalk-eyed fly (Diasemopsis meigenii) to investigate how eye stalk length is influenced by genetic stress. Inbreeding is known to be harmful to populations, causing inbreeding depression by increasing the frequency of rare and damaging mutations, and thereby reduce the fitness of individuals. If long eye stalks are a sign of high genetic quality, then we would expect inbreeding to reduce the size of eye stalks over time. And this is just what 11 generations of mating with siblings: eye span decreased significantly during the experiment, more so than other traits such as wing size, which are not considered sexy. Thus, male traits important for attracting a female are more sensitive to genetic quality than non-sexy traits, supporting previous theory about why females are attracted to these traits in the first place.
Surviving the Genetic Storm
One rather unexpected result, however, was that male eye span at the start of the experiment was also a strong predictor of whether the line went extinct or not before the end of the experiment. Less than half the original lines survived the entire 11-generation experiment, and those who did survive were descended from males with longer eye-stalks. This again supports the idea that male eye span in stalk-eyed flies is linked closely to genetic quality; males with long eye stalks carried fewer deleterious mutations when the experiment began and so were more robust against inbreeding depression and less likely to go extinct. This result may have intriguing implications for conservation and for population dynamics in general. It shows that individual decisions such as mate choice may influence the fate of a whole population. Conservationists should take mate choice and sexual selection into account when designing programmes to conserve species, especially those with small or declining populations.
This project was made possible by funding from the Natural Environment Research Council (NERC), the Biotechnology and Biological Sciences Research Council (BBSRC) and the Engineering and Physical Sciences Research Council (EPSRC)