In fish, sexy mates can be easy meals, study finds
http://www.sun-sentinel.com/news/custom ... ews-fringe
By Tina Hesman
St. Louis Dispatch
Posted May 10 2005, 1:47 PM EDT
Females of two species of fish prefer males with
larger genitalia, a Washington University researcher
has found. But while males with large sex organs are
more likely to get mates, they're also more likely to
be eaten by predators because their genitalia are a
drag on their swimming speed.
The discovery could shed light on how predators
influence the formation of new species.
``It's a groundbreaking study,'' said Duncan Irschick,
an evolutionary biologist at Tulane University.
Sometimes male sex organs are the only thing that
distinguishes one species from another, Irschick said.
But scientists have been unable to fully explain the
tremendous variation seen in male genitalia.
The new study, published this week in the online
version of the Proceedings of the National Academy of
Sciences, is the first to focus on predators as an
influence on genital evolution, Irschick said.
R. Brian Langerhans, a graduate student at Washington
University, captured small fish related to guppies. He
studied two species of fish that give birth to live
young _ Gambusia affinis, which he caught in ponds in
Texas, and Gambusia hubbsi from the Bahamas.
Some of the Texas ponds also contained predators, such
as bass, bluegill and green sunfish. Other ponds were
free of bigger fish. In the Bahamas, gobi and
barracuda prey on the inch-long guppy relatives in
some of the habitats Langerhans sampled.
Both species of fish have tubelike sexual organs,
called gonopodia, extending from their bellies. The
organs can't be retracted, and male fish slowly wave
their gonopodia in front of females to attract mates.
Langerhans found that males of both species taken from
predator-containing habitats had smaller sexual organs
- by about 15 percent - than males from predator-free
locations.
``It's not so dramatically obvious to our eyes,''
Langerhans said, but for a female fish, a 15 percent
difference is huge. But it's not always easy to tell
whether noticing the difference means greater
attraction for mates.
``Female choice is tough, because how do you figure
out what a female wants?'' Irschick said.
To solve the age-old mystery, Langerhans made a video
of a male and played the movie for the female side by
side with film of the same male with a digitally
enhanced gonopodium. Females spent about 80 percent
more time with the well-endowed version.
But the attraction came at a cost for the males. Males
with large genitals were about 20 percent slower than
males with smaller gonopodia. And slower is bad for an
animal trying to evade a predator.
``If there are predators around, good luck surviving
long enough to even get a chance to mate,'' Langerhans
said of the more-endowed males.
The finding may help explain the development of tail
swords in some species of fish, Langerhans said. Males
sometimes grow extensions on their tails, which humans
have likened to swords, but may appear as gonopodia to
female fish. Females gravitate to males with larger
swords, but predators are also attracted to the
tail-fin accessory.
Males produce offspring with similar-size gonopodia
even when raised in the laboratory without predators.
That means that the trait is mostly determined by
genetics and not by environment. But the presence or
absence of predators can push a population toward one
size extreme or another by selecting for either fleet
fish or males who can attract the most mates.
The tendency to gravitate to males with larger organs
also seems to be hard-wired. Even females raised in
environments with predators found the larger gonopodia
more attractive, Langerhans found. The preference
seems to defy logic - a female who gives birth to
offspring with large genitals risks losing her brood
to predators. But the females just can't seem to
resist the males with bigger gonopodia.
``They're sexy, and their sons are going to be sexy,
and that's what females want,'' Langerhans said.
"Duct tape is like the Force. It has a light side, a dark side, and it holds the universe together...." - Carl Zwanzig
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