Sexual Dimorphism in the Three-toed Amphiuma, Amphiuma tridactylum: Sexual Selection or Ecological Causes

Abstract

Sexual dimorphism is widespread among vertebrates, and may be attributable to sexual selection, differences in ecology between the sexes, or both. The large aquatic salamander, Amphiuma tridactylum, has been suggested to have male biased sexual dimorphism that is attributable to male–male combat, although detailed evidence is lacking. To test this, data were collected on A. tridactylum head and body size, as well as on bite-marks inflicted by conspecifics. Amphiuma tridactylum is sexually dimorphic in several characters. There was no sex difference in body length, but males had heavier bodies than females of the same body length. Larger males had wider and longer heads than larger females, but whether any of these sexually dimorphic characters are attributable to differences in diet is unknown because diet data (by sex) are lacking. There was no difference in the number of bite-marks between males and females, and juveniles also possessed bite-marks, suggesting that the biting is not necessarily related to courtship or other reproductive activities. In addition, fresh bite-marks were present on individuals during months well outside of the breeding season. Biting was observed in the field and lab to occur by both sexes on both sexes, during feeding-frenzy type foraging. Thus, biting is likely related to foraging rather than to courtship. The sexually dimorphic characters remain unexplained, pending sex-specific diet data, but there is no evidence that they are related to male–male combat or to courtship.