Tonight I did the Trinity City Hall run again. I listened to a Canadian science podcast that was talking about selection pressures that drive gonadotroph, the male reproductive organ, size in mosquito fish. Male mosquito fish have large gonadotrophs for the size of their bodies, with lengths ranging from 20% to 70% of their body length. They are internal fertalizers, hence the need for a gonadotroph. During courtship they "wave these around wildly" and females are very selective, favoring the males with the largest gonadotrophs. The question was then, why the large variation in gonadotroph size? Why don't they all just have large gonadotrophs? It turns out that a large gonadotroph is a big disadvantage if there are prediators around because the hydrodynamic drag it presents slows the burst swimming speed of the fish too much. Therefore, predators exert a selective pressure for smaller gonadotrophs, while females exert a selective pressure for large gonadotrophs. They demonstrated that it was the gonadotroph size that females were selective for, and not other attributes, by doing studies where they showed female fish side by side identical movie displays of a male fish doing his courtship dance, except that on one of the displays the gonadotroph had been digitally enhanced by 15%, (I wouldn't make this up) and they demonstrated that it was gonadotroph size that was the selected for attribute. | 