Sperm competition occurs when sperm of two or more males compete to fertilize a given set of eggs. Theories on sperm competition expect males under high risk of sperm competition to increase ejaculate size. Here we confirm this prediction experimentally in the three-spined stickleback (Gasterosteus aculeatus). In this species, sneaking (i.e., stealing of fertilizations by neighboring males) can lead to sperm competition. Sneaking males invade foreign nests, and the owners vigorously try to prevent this intrusion. In such fights, male body size is assumed to be an important predictor of success. Consequently, the risk of sperm competition may depend on the size of a potential competitor. We experimentally confronted males before spawning with either a large or a small computer-animated rival. We show that males ejaculated significantly more sperm after the presentation of the larger virtual rival than after the small stimulus. In addition, the time between the initiation of courting and the spawning was shorter in the large virtual male treatment. The results suggest that stickleback males tailor ejaculate size relative to the risk of sperm competition perceived by the size of a potential competito
