The effect of size sexual dimorphism on diet in American martens (Martes americana) was investigated. Three hundred and eighteen gastrointestinal tracts were collected from trapped martens 2000-2004 in the Upper Peninsula of Michigan. Prey species were similar to those found throughout North America. Ten prey species were identified in 151 martens and classified into six prey groups based on body size. The proportion of prey groups to kilocalories consumed was similar between male and female marten diets, as well as between juvenile and adult diets. The average dietary breadth (0.49) was similar to other North American studies and dietary overlap (0.99) was high between sexes and between age groups. This suggests that the diet of males and females, and of adults and juveniles was identical and there was no prey partitioning.

The effect of unequal sample sizes was examined by reducing sample size (by 50%) of one sampled group and repeating the statistical analyses. Comparison of ten constructed datasets (half the number of randomly selected stomach samples and all intestinal samples) showed that stomach and intestinal contents were similar, indicating that using either stomachs only, intestines only, or a combination (in equal proportions) of stomach and intestinal contents would not change test results. Ten identically designed comparisons of stomach samples (half the number of randomly selected females and all males) showed no differences between male and female stomach contents. Ten comparisons of stomach contents between half of the juveniles and all of the adults were also similar; indicating that unequal sample sizes of sex and age classes did not bias diet estimates of stomach contents.