The Japanese filefish Paramonacanthus japonicus has extreme sexual dimorphism in its overall shape, even though its mating system is monogamy with biparental care. This sexual dimorphism is mainly due to the development of secondary sexual traits in males. Males become more slender in body with elevated soft dorsal and anal fins as they mature. We examined the function of such male secondary sexual traits by field research and fluid-dynamic analysis. Underwater observations showed that movement rate and steady swimming speed of males were higher than those of females. Male and female P. japonicus showed similar feeding habits and egg-tending behavior, although males attacked potential egg predators more frequently. A wind-tunnel experiment using the air bearing and spring system showed that the drag coefficient of males was significantly lower than that of females, indicating a lower male hydrodynamic drag performance. Also, male elevated soft dorsal and anal fins are considered to give rise to higher thrust performance in monacanthids. Thus, these results suggest that male secondary sexual traits are hydrodynamic devices for enhancing swimming performance that seem to be actually functional under natural conditions. We discuss the evolution of such conspicuous male sexual traits in P. japonicus.
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