Oh, Research.

From Nature September 2009 (the most recent issue):

"Evolutionary biology: Well endowed

BIOLOGY LETTERS/THE ROYAL SOCIETY 2009

Biol. Lett. doi:10.1098/rsbl.2009.0637 (2009)

The wide variation in male genitalia size in animals is thought to have evolved mostly in response to selection pressures that come into play during or after copulation and increase the male's share of paternity.

But it seems that females of the mosquito-fish Gambusia holbrooki choose mates before copulation on the basis of the size of their genitalia — and for them, bigger is better. Andrew Kahn and his team at the Australian National University in Canberra tested female preference for males that had had their genitalia considerably reduced in size by surgery (pictured, bottom) compared with those with only a minor reduction (pictured, top). They found that females spent, on average, around one-and-a-half times longer associating with the better-endowed males."

Really, research can be quite fun(ny) sometimes. :V Obviously we want to pay money to read this kind of exciting study!