Parasitism is sex-biased in many animal species. In mammals, males often appear to be more severely affected, possibly as a result of costs associated with sexual selection. River otters (Lontra canadensis) and other members of the Mustelidae are definitive hosts for nematodes of the genus Skrjabingylus, which have been found to cause lesions and deformation of the frontal bones of the skull. Infection has also been shown to reduce braincase volume in 2 mustelid species; thus, we hypothesized that a similar relationship exists in otters. Furthermore, we predicted that this effect of nematode parasitism would be biased toward males, which are the larger sex in otters. We used 130 male and female otter skulls collected throughout Ontario to test whether skulls with lesions attributable to nematode infection would show a male-biased reduction in braincase volume and other changes to skull morphology. We found that braincase volume was reduced in male otters with lesioned skulls and to a lesser extent in female otters with lesioned skulls. There was no detectable effect of age on braincase volume. We concluded that parasite-induced damage to otter skulls includes reduced braincase volume, and that this effect appears to be male-biased. This might affect behaviour of otters, reducing survival, and contributing to a pattern of sex-biased mortality.

Nomenclature: Carreno, Reif & Nadler, 2005; Wilson & Reeder, 2005.