Females prefer sword-less males

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Females of a swordtail species that has lost its sword prefer to mate with sword-less males when given a choice between those with or without a sword on the tail, says new research.

The majority of the members of the livebearer genus Xiphophorus have males bearing a sword-like extension of the caudal fin, and numerous studies have shown that females select for this trait in male fish.

However, Xiphophorus birchmanni is a little unusual in that it has secondarily lost its sword. So it's been assumed that the females have also lost the preference for males bearing a sword on their tails.

To test the theory, Wong and Rosenthal of the Department of Biology at Boston University played a titilating computer-animated film of "virtual males" of Xiphophorus birchmanni to females to test their preference for the presence of a sword on the tail.

The paper, which is due to be published in The American Naturalist later this year, says that when offered a choice between natural-looking animated males and unnatural males with a digitally-attached sword, the female birchmanni selected the natural looking partners.

Wong and Rosenthal say their results suggest that female preferences need not always select in favour of elaborate male secondary sexual characteristics, like a pointy tail. They believe that the findings also provide a possible explanation for the lack of introgression of a sexual trait in a naturally occurring hybrid zone.

For more details see the paper: Wong, BB and GG Rosenthal (In Press) - Female disdain for swords in a swordtail fish. American Naturalist. 2006 Jan; 167(1): 136-40.