Male guppies sporting a tad more orange on one side of their bodies than on the other tend to flash that better side at females.
That’s the conclusion of a new study of asymmetric male guppies flirting with females in adjoining tanks, says Mart R. Gross of the University of Toronto. In reaction to a dead female suspended in a tank, males in a neighboring tank still courted but didn’t favor their good sides.
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There’s something about a live female’s response that encourages a male to show the most orange, Gross and his colleagues report in the Sept. 7 Proceedings of the Royal Society B.
Gross says that the test’s emphasis on behavior is unusual for an experiment concerning what biologists call fluctuating asymmetry. Such studies generally explore minor differences between the two sides of an organism that show up in all but Hollywood-perfect individuals. A 1992 test involving barn swallows suggested that females prefer males with the most-symmetric tails. Although evidence since then has been mixed, some theorists have proposed that high levels of symmetry indicate a prime physical specimen in many animal species.
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“Research has focused on preference for symmetry in the opposite sex but has ignored what individuals that are asymmetric actually do about it,” says Gross.
To check for effects of asymmetry, he and his colleagues worked with lab guppies descended from fish collected in Trinidad. The researchers observed courting fish for periods of 10 minutes, noting whether a female showed interest by crowding close to the side near a male’s tank. Every 15 seconds, the researchers noted which side of his body the male was displaying.
The researchers photographed their 53 males and analyzed the digital images. Males received a score for their total amount of orange on each side and how symmetrical their coloring was.
“The females were not interested in symmetry,” concluded Gross. “They were looking at orange.” The more, the better.
This insight helped researchers make sense of male displays. The 26 males with more-than-average symmetry displayed their very slightly better sides only 49 percent of the time.
The 27 males with pronounced lopsidedness, however, showed their good sides 60 percent of the time.
Gross says that the study challenges the idea that an organism can’t respond to its own asymmetry. Evolutionary biologist Stefan Van Dongen of the University of Antwerp in Belgium says the possibility that animals can hide symmetry flaws “is something to take into account when designing experiments.” He adds, however, that he’d like to see a test of whether male guppies showing their better sides father more offspring.