Sperm size matters

Elaborate courtship displays often occur between rival males before mating, for example the bloody battles between male elephant seals, but relatively little has been known about sperm competition after mating. In November 8 Science, Gary Miller and Scott Pitnick at the Department of Biology, Syracuse University, New York, USA, show that the dimensions of the female reproductive tract exerts selection pressure on sperm size and fertilization success in Drosophila (Science 298:1230-1233, November 8, 2002).

Miller & Pitnick examined populations of Drosophila melanogaster selected for divergent sperm length or female sperm-storage organ length. They observed that sperm length evolution occurred as a response to selection on the female reproductive tract. Giant sperm tails have evolved because female reproductive tracts selectively bias paternity in favor of males with longer sperm.

"Although we now understand what drives sperm length evolution, we do not know what is driving the evolution of seminal receptacle length. ...