One evolutionary response to sperm competition is the variety in penis morphology of many species. For example, the shape of the human penis may have been selectively shaped by sperm competition. The human penis may have been selected to displace seminal fluids implanted in the female reproductive tract by a rival male. Specifically, the shape of the coronal ridge may promote displacement of seminal fluid from a previous mating via thrusting action during sexual intercourse. A 2003 study by Gordon G. Gallup and colleagues concluded that one evolutionary purpose of the thrusting motion characteristic of intense intercourse is for the penis to “upsuck” another man's semen before depositing its own.