Older Female Fish Prefer Imperfect Male Mates, Study Finds
There’s hope for the less-than-perfect male – if you’re a swordtail
fish, that is. As the size and age of female swordtail fish
increase, so does the preference for males with asymmetrical
markings, according to a new Ohio University study.
Biologist Molly Morris studies the mating preferences of swordtail
fish. (Photo by Rich Fatica, Ohio University)
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
projected 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, such as pollution.
Co-authors of the study were post-doctoral fellow Oscar
Rios-Cardenas and undergraduate student Mary Scarlett Tudor, both
of Ohio University.
