Introduction

Among sexually reproducing species, the penis evolved as an internal fertilization device. But across different species, penises exist in a bewildering array of shapes and sizes (see Eberhard, 1985). Among primates, the human penis is distinctive by virtue of both its size and its enlarged glans and protruding coronal ridge (see Gallup & Burch, 2004). There has been some speculation that the human penis evolved not only as an internal fertilization device, but also as a mechanism for displacing semen left by rival males in the female reproductive tract (e.g. Baker & Bellis, 1995).

In a series of studies designed to simulate sexual intercourse under laboratory conditions using artificial genitals, we found that when latex vaginas contained simulated semen, phalluses that approximated the configuration of the human penis displaced 80% or more of the semen by drawing it away from the cervical end of the vagina (Gallup et al., 2003). Through a series of experimental manipulations, we determined that the coronal ridge may be an important feature of the penis in mediating semen displacement. Thus, as a mechanical means of affecting sperm competition, the human penis may enable successive males to displace foreign semen from the female reproductive tract and substitute their semen for those of their rivals.