(A) When a normal male (outlined in blue) mates with a hermaphrodite (red), his sperm (blue) are larger than the hermaphrodite sperm (pink) and are better able to compete for positions in the two spermathecae (purple). Sperm located in the spermathecae are in a better position to fertilize the hermaphrodite's oocytes. Some sperm and a young embryo are shown in the uterus (brown), and the passage of an oocyte through the spermatheca on the right has dislodged the sperm. The region inside the dashed box is expanded in B–D. (B) Male sperm can normally outcompete hermaphrodite sperm and enter the spermathecae (middle), but Hansen et al. show that comp-1 mutant male sperm (green) are outcompeted by hermaphrodite sperm (bottom). (C) Furthermore, although comp-1 sperm can fertilize female oocytes in the absence of competition (bottom), they cannot compete with normal male sperm (middle). (D) When both the male sperm (green) and hermaphrodite sperm (orange) are made by comp-1 mutants, the larger male sperm have a competitive advantage. This shows that the COMP-1 protein and sperm size independently affect competition.