HABITAT SUITABILITY IN MUSKRATS - A TEST OF THE FOOD LIMITATION HYPOTHESIS

Data on habitat selection, overwinter survival, and litter survival ra te of muskrats indicate that fitness differs among habitats in marsh e nvironments. We measured nine nutrient-dependent phenotypic variables of muskrats and one independent assessment of forage quality during th e ice-free period (May-October) from 1992 to 1994. Data were used to t est the hypothesis that differences in food resources limit the demogr aphic performance of muskrats in marginal versus prime habitats (food limitation hypothesis, FLH). None of the null predictions relating hab itat quality with reproductive parameters could be rejected statistica lly. Adult body mass and length were greater in prime habitats, but fa t content did not differ, which suggested that the difference was asso ciated with larger structural size of dominant animals in prime habita ts. Growth rate of weaned juveniles was not different among habitats, which represented the strongest evidence for rejecting the FLH. Faecal crude protein content, which was used as an index of quality of food ingested, was marginally different between the two most suitable habit ats, but the direction of the difference did not support our predictio n. Overall, the temporal variation in faecal crude protein content was similar among habitats. We conclude that food is not a key factor lim iting the demographic performance of muskrats among habitats during th e ice-free season.