||Testis size variation within sneaker males of the dusky frillgoby Bathygobius fuscus (Gobiidae): effects of within-tactic competition
Kawase, Shoma ,
Hayashi, Takahiro ,
Matsumoto, YukioTakegaki, Takeshi
Biological Journal of the Linnean Society
399 , 2017-10 , Oxford University Press
A ‘sneaking tactic’ is an alternative reproductive strategy that usually results in sperm competition among males with different tactics. Relatively large testes are a sneaker-specific trait that has generally been thought to have evolved due to sperm competition between sneaker males and bourgeois (guarding) males. However, here we show that competition among sneaker males can also affect testis enlargement in the dusky frillgoby (Bathygobius fuscus) sneaker males. The competitive advantage of focal sneaker males was experimentally manipulated by placing them in tanks with either relatively smaller or larger males. Testis enlargement was conspicuous in focal males that were cohoused with larger males. Smaller sneaker males may invest more in testicular growth because they are at a competitive disadvantage in physical contests for sneaking opportunities among sneaker males and consequently may be confined to making relatively late intrusions during spawning into nests that have a higher risk of sperm competition. Another possible reason for the relative size-dependent energy investment in testes may be increased investment by large sneaker males in aggressive interactions for sneaking opportunities. This is the first evidence that testis size variation among sneaker males is affected by the competition among sneaker males.