Sperm precedence in a hermaphroditic nematode (Caenorhabditis elegans) is due to competitive superiority of male sperm.

When male and hermaphrodite Caenorhabditis elegans mate, the male's sperm outcompete the hermaphrodite's own sperm and fertilize a majority of the offspring. Here, we investigate the mechanism of male sperm precedence. We rule out the possibility that male sperm are stronger and more competitive because they are activated later than hermaphrodite sperm. We also find that a previously known gender difference in sperm activation does not influence sperm competition. Male sperm, rinsed free of seminal fluid, retained the capacity to take precedence after artificial insemination. Therefore, we conclude that male sperm themselves are competitively superior to hermaphrodite sperm. This trait maximizes outcrossing after mating and may increase both genetic diversity and heterozygosity of offspring whose parents, due to self-fertilization, may be highly homozygous.