Molly Morris, associate professor of biological sciences, and colleagues found that older female swordtails spent more time with asymmetrically striped males than symmetrical males when offered a choice. These findings are the first to contradict previous studies showing that females tend to prefer males with symmetrical markings, which in this case are black bars on each side of the body. Scientists have suggested that symmetrical markings are a sign of genetic fitness.
The new study provides evidence that visual cues are not the only thing driving mate selection, however. The findings also suggest that "females may not have the same mating preferences throughout their lives," Morris said. Funded by the National Science Foundation and the Research Challenge Program at Ohio University, the paper has been published online in Biology Letters and will appear in next month's print edition.
Morris' findings raise the question of whether there may be a change in mating preference over time in other species as well. In a current study on human attraction, for example, the average age of the females and males tested for preference for symmetrical faces was only 20.3 years and 19.5 years, she said. The swordtail fish used in the latest research are two species of Mexican fish, Xiphophorus cortezi and Xiphophorus malinche. Because the genetic makeup of these fish is well known, researchers often use them to examine genetic causes of behavior.
To conduct the study, Morris and colleagues exposed females of both species to video animations of male fish, one with an equal number of bar markings on each side of the body, and one with an unequal number of bars on each side. The animations simultaneously were projec ted on opposite sides of a tank, and researchers measured the time the female spent with each animation. While younger and smaller females still showed a preference for the symmetrical male, older and larger females increasingly preferred the asymmetrical male.
In previous research conducted elsewhere, females were shown to prefer symmetrical over asymmetrical males when presented with a choice. Scientists suggested that symmetrical figures have more redundant information, which makes for a more powerful, easy-to-remember stimulus. Females also may be likely to choose symmetrical males because they are better able to deal with environmental stresses and could pass on that resilience to offspring.
Another explanation would be that the visual system "develops an internal prototype of an object that is based on an average of all the male fish [the females] have seen," Morris said. "Therefore, any fish they see that is closest to this prototype is preferred." The same phenomenon has been suggested to explain attraction in other animals, including humans.
As their lives progress, however, female swordtails may encounter more asymmetrical than symmetrical males, and so begin to prefer the pattern to which they have become accustomed in their environment, she said.
Further research based on this study might examine whether the experience of mating draws older females to asymmetrical males, or whether there is a genetic factor, activated at a certain age or size, that makes larger females select more asymmetrically striped males.
The asymmetrical barring pattern is likely a result of environmental stresses experienced before birth, Morris explained. Some males have the genetic makeup to deal with these stresses and still develop a symmetrical pattern, while others survive the stress but show marks of the struggle. A population with a large amount of asymmetrical individuals may have been subject to a lot of environmental stress, suc h as pollution.