The size of a male's penis is everything, at least according to some species of fish.
In a study to be published in a forthcoming issue of the journal Biology Letters, Andrew Kahn, Brian Mautz and Michael Jennions demonstrate that female Eastern mosquitofish (Gambusia holbrooki) spent longer time with males having longer gonopodia (intromittent organs).
The authors separated male mosquitofish into two size classes (small: less than 20 mm standard length and large: greater than 24 mm standard length). Within each size class, the gonopodia of all the males were experimentally shortened by cutting off the tips of the gonopodia to form a blunt-ended appendage.
Two forms of treatment were used: major and minor reductions, with the gonopodia undergoing major reduction being 15"17% shorter than those undergoing minor reduction.
Experiments were then conducted in which a male from each size class and treatment were placed in a compartmented test aquarium, and a single female fish was added to the aquarium.
The researchers measured the length of time the female spent associating with the male, with association defined as being less than 4 cm from the front of his compartment with her body oriented unambiguously towards him.
The authors found that, on average, females preferred to associate with large males with longer genitalia. They found female preference for longer genitalia to be absent when choosing between small males.
The authors conclude that more studies are needed on the effect of female choice on the evolution of male genitalia in poeciliids, but "cannot confirm whether female mate choice generates stabilising and/or directional selection with respect to the current average male genitalia length, because treatment males only had their genitalia shortened. At present, we lack a technique to experimentally lengthen the gonopodia of free-swimming males."
For more information, see the paper: Kahn, AT, B Mautz and MD Jennions (2009) Females prefer to associate with males with longer intromittent organs in mosquitofish. Biology Letters, doi: 10.1098/rsbl.2009.0637.