Sexual selection under duress


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By: Elsa Youngsteadt

The theory of sexual selection is one explanation for the evolution of showy traits in male animals-- things like antlers and peacock tails. Females searching for good mates use these displays as indicators of vigorous males with good genes, and female preference for such traits drives their evolution. Unless, that is, it all becomes too dangerous. If showy guys start attracting the wrong kind of attention, or if there are a lot of predators around that make mating risky, females would rather just get it over with in a hurry, than risk their lives evaluating males and choosing the best one. But it turns out that in fiddler crabs, females at great risk of being eaten by birds are still very picky-- and they prefer males who build structures that help them hide from birds.

Fiddler crab males build burrows in which they mate, and they invite females to visit their burrows by standing beside the entrance, waving their oversized claws. Sometimes the males build a little hood over the entrance to the tunnel, and biologists working in Panama found that males with hooded burrows are more likely to score a mate than those with plain burrows. In the May, 2007 issue of the journal PloS ONE, the biologists report that, unlike other known cases of sexual selection, increased risk of predation actually makes the female crabs pickier. That's because the sheltered burrows help them avoid being eaten by grackles hunting for tasty crabs on the beach: the more grackles there are, the more threatened the females feel, and the stronger their preference for males with hooded tunnels.

This study illustrates principles of sexual selection-- but does so with a twist, in that the male display provides direct survival benefit to the females rather than serving simply as a symbol of the male's own fitness and likely contribution of good genes to the next generation.


PLoS ONE for original article

News from New Scientist

Live Science story

Webpage of John Christy, one of the authors of the study

Questions for review and discussion

1. Summarize the theory of sexual selection. How does it explain the evolution of showy ornaments and displays in male organisms?

2. Why would females pay less attention to sexually selected traits when they are at high risk of predation?

3. How does the sexual display of male fiddler crabs keep females' attention even under risky circumstances (i.e. lots of predators)?

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