| Summary: | The northern fur seal (Callorhinus ursinus) inhabits the North Pacific Ocean and Bering Sea, where it feeds on a variety of prey. In this study, we studied a captive male to determine how different prey affect the composition of and testosterone metabolites in the faeces. Faecal steroid metabolite levels are often expressed per unit dry weight (DW) of faeces, but dry faeces contain both organic and inorganic matter, so we also measured the ash-free dry weight (AFDW), which can more accurately express the levels in samples with organic matter. The mean organic component of faeces was lower when the seal fed on fish than when it fed on squid. Testosterone metabolite levels in the AFDW and serum testosterone levels showed a significant relationship in faeces collected one day after the blood withdrawal. When the seal fed on squid, differences in testosterone metabolite levels between DW and AFDW were small. These results suggest that faecal testosterone metabolite levels expressed in quantities per unit of AFDW are more useful than DW to understand the reproductive physiology of seals. departmental bulletin paper
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