The function of sound production by Atlantic cod as inferred from patterns of variation in drumming muscle mass

We quantified variation in the mass of drumming muscles (the sound-producing musculature) among individual Atlantic cod (Gadus morhua L., 1758) for comparison with other evidence on the role of sexual selection in the evolution of sound production by this species. Based on 913 cod sampled from the Western Scotian Shelf in 2001-2002, we found that males had drumming muscles that were larger than those of females and that increased in mass prior to spawning and declined thereafter. Drumming muscle mass was highly variable, particularly among males, and generally more variable than other morphological traits (somatic mass and body length). This pattern of drumming muscle variation is consistent with the hypothesis that drumming muscles are influenced by sexual selection and suggests that sound production by males during the spawning season has fitness benefits, perhaps through a role in mate competition. Drumming muscle mass of spawning males was also positively associated with body size, condition, and fertilization potential, suggesting that sound production may be an indicator of the size of the signaler and may reveal information about individual quality. In conjunction with previous studies of sound production by Atlantic cod, our study underscores the potential importance of sound production to cod spawning behaviour.