Endurance rivalry in capelin: insights from semen quality and regeneration

Male-biased adult sexual size dimorphism is often the result of intra-sexual selection driven by male-male competition. Capelin ( Mallotus villosus ) exhibit male-biased sexual size dimorphism but lack contest competition or clear scramble competition, and female mate choice, if present, is unrelated to male size. Consequently, it is hypothesized that adult sexual size dimorphism in capelin is due to a mating system that favours larger males with greater energy reserves, allowing them to compete for prolonged access to mating opportunities through endurance rivalry. To enable this, males need a continual supply of semen throughout the spawning season. However, they have unusually small testes and are predicted to deplete stored semen rapidly. Furthermore, their unique sperm physiology (i.e. the only known externally fertilizing vertebrate to release pre-activated semen) may constrain their ability to regenerate it. We found that the majority of capelin sampled on beaches had adequate semen, but towards the end of the spawning season, some fish had none. Capelin held in laboratory tanks could regenerate semen within two days, maintaining their ability to seize continual mating opportunities. These results support the endurance rivalry hypothesis.