When it comes to the size of your genitals, if you're a mosquitofish, it helps if you're well-endowed.
New research from scientists at Washington University's Department of Biology has shown that female mosquito fish prefer males with larger genitalia, a trait which has led to the males evolving much larger private parts than many other livebearing fish.
Unlike egglaying fish, male livebearers such as the mosquito fish, have a special organ called a gonopodium which is used to internally fertilise the female, in much the same way as a penis.
However, having enormous genitals can make you vulnerable to predators if you're a mosquito fish. This means that there's an evolutionary trade-off for the male mosquito fish in avoiding predators and having smaller genitals, or attracting the ladies with more impressive front bottom parts.
The team from Washington University studied the mechanisms that dictate the size of a male mosquito fish's mating equipment in an experiment on two related species of mosquito fish, Gambusia affinis and Gambusia hubbsi, and have just reported their findings in the Proceedings of the National Academy of Science (somewhat aptly this journal goes under the acronym PNAS).
Their results suggest that both predators and females have an effect on the size of bottom parts that males of the species evolve.
In the lab, female G. affinis preferred males with larger genitalia - a form of premating sexual selection, but natural selection in the presence of predatory fish favoured those fish with smaller genitals.
The bigger your genitals the harder it is for you to escape from predators, due to the effect large bottom parts have upon burst swimming performance.
The study challenges previous research, which has suggested that it's postmating sexual selection that's driven the evolution of large genitals:
"Although postmating sexual selection is widely presumed to be the most important mechanism for driving genital diversification, these findings suggest alternative mechanims, particularly for organisms that cannot retract their genitalia, may also prove important."
For more details see the paper: Langerhans, RB., Layman, CA. and TJ Dewitt (2005) - Male genital size reflects a tradeoff between attracting mates and avoiding predators in two live-bearing fish species. Proceedings of the National Academy of Science. 2005: May 13.