Female fish dance for love

Researchers have long thought of males as the lone dancers in courtship display, leaving females to judge--but in one species of cichlid fish, the opposite occurs, according to new research.

A cichlid female displays her purple-shaded pelvic fin.
Image: Baldauf et al., BMC Evolutionary Biology

In this species, females seek to impress potential mates as well by fanning out their large, violet pelvic fin. The linkurl:results,;http://www.biomedcentral.com/1471-2148/10/301/abstract published this month in BMC Evolutionary Biology, also suggest male choice can drive females to evolve exaggerated traits, a finding that disputes the traditional belief that sexual selection is a one-way road allowing only females to affect male appearance. This paper "is certainly a significant step ahead in our understanding of the evolution of female ornamentation," linkurl:Andrea Pilastro,;http://www.bio.unipd.it/%7Epilastro/ evolutionary biologist at the University of Padua in Italy, said in an email to The Scientist. Scientists have widely studied the effect of female choice on male traits, such as the peacock's extravagant tail or the stag beetle's massive horns, but until relatively recently neglected the effect of male choice on female appearance. Typically females of a species will invest a great deal of energy into eggs or young and males will invest nearly none, causing females to be highly selective with their mates and males to take whatever they can get. However, in biparental species, where both males and females invest equally in their young, "males will become very choosy as well," said linkurl:Sebastian Baldauf,;http://www.evolution.uni-bonn.de/SBaldauf.htm postdoctoral student at the University of Bonn in Germany and first author on the paper. Using computer animations of a female cichlid, Baldauf investigated the evolution of female ornamentation in the African cichlid fish (Pelvicachromis taeniatus), a biparental species where females fan out their enlarged, violet-colored pelvic fin alongside male displays during courtship. Overall, Baldauf found that males displayed more often to female animations with larger, brightly colored pelvic fins than to females with less pronounced traits. Baldauf also found that the pelvic fins grew disproportionately larger than other fins in females, suggesting that male preference for enlarged pelvic fins may have caused females to evolve the trait in the first place. The caudal fin or tail was also disproportionately large in females, but can be explained by natural selection, Baldauf said, because it is the main fin that controls movement. "So there is strong natural selection on this fin to stay larger." The next step is to understand what kind of specific information the exaggerated pelvic fin could convey to males, said Baldauf--such as, perhaps, a correlation between a larger fin and egg number. It's also worth exploring the role of the caudal fin in male choice and the pelvic fin in female-female competition, Pilastro added. "Male mate choice and female ornaments functioning in mate attraction are ideas that have been slowly blossoming for over two decades," linkurl:Geoffrey Hill,;http://www.auburn.edu/academic/science_math/cosam/departments/biology/faculty/webpages/hill/index.html evolutionary biologist at Auburn University in Alabama, said in an email to The Scientist. "Studies like this one will help entrench these concepts in mainstream thinking."
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[29th January 2009]