Sexual size dimorphism is a common phenomenon in mammals, and researchers have been trying to demonstrate the evolutionary causes leading to sexual dimorphism. Two main hypotheses emerged: (i) the sexual selection hypothesis and (ii) the sexual competition hypothesis (also called resource partitioning hypothesis). Here, we attempted to link sexual dimorphism in fishers (Pekania pennanti (Erxleben, 1777)) with their fall diet using stable isotope profiling and body and skull measurements. We used the carcasses of 39 fishers which were caught in eastern Québec during fall 2014 by volunteer trappers as well as several potential prey items ranging from small rodents to cervids. We expected minimal niche overlap between sexes, as males should be able to exploit different prey species than females. We also expected to observe an effect of age class (adults vs. juveniles) on trophic niche. As expected, we found great evidence of sexual dimorphism in both body mass and skull measurements: males were heavier and longer than females and had a larger zygomatic and intracanine width and a longer skull. While proportions of prey in diet according to sex and age did not vary greatly, we found some evidence of niche partitioning using Layman's metrics. Indeed, females tended to have a less diversified and more similar diet compared to one another, whereas males showed more diversified and contrasted diets. Despite our limited sample size, our findings provide partial support to the sexual competition hypothesis, as the difference in body and skull size based on sex could have evolved to lessen intraspecific competition in fishers.

We benefited from the collaboration of local volunteer trappers during the 2014–2015 trapping season (25 October–31 January). Trappers collected fishers using Conibear body traps with a diagonal frame size of 160–220 mm to sell furs on international markets. Captures were conducted in accordance with the provincial regulations of sport trapping in Québec and according to the Agreement on International Humane Trapping Standards. We collected tissue samples of potential prey species through sport trapping, sport hunting or fruit picking. We selected prey items based on the known diet of fishers, such as large, medium, and small rodents, cervids, small game and fruits.

SIMMr and SIDER packages on R.