MECHANISMS FOR DIGESTION IN TWO COMPETING FOREST CARNIVORES: FISHERS (PEKANIA PENNANTI) AND MARTENS (MARTES AMERICANA AND M. CAURINA) IN THE GREAT LAKES AND PACIFIC NORTHWEST

Abstract

Carnivores have evolved morphological and digestive traits to successfully exploit dietary items other than meat. However, observations of carnivores consuming fungi and insects in the wild have generally been dismissed as incidental consumption or from nutritional desperation. Animals that specialize on fungi (i.e. mycophagists) or insects (i.e. insectivores) have evolved unique digestive pathways to liberate nutrition from these sources. Notably, the digestive enzyme trehalase, which breaks down trehalose (the most common disaccharide of fungi and insects), is more abundant in mycophagists and insectivores, while amino-peptidase N (APN) is more abundant among strict meat eaters. Fishers (Pekania pennanti) and martens (Martes americana and M. caurina) belong to the same subfamily, are similar in size, occupy similar habitats, and generally consume the same foods, which is typically meat. However, fishers in the Pacific Northwest uniquely consume high volumes of fungi and insects. To determine whether fishers and martens have the endogenous capacity to digest fungi and insects, we sampled fishers and martens from Minnesota and California. We analyzed stable isotopes to determine the overall diet for each individual and quantified the trehalase and APN activity in gastrointestinal tracts. To explore the source of trehalase activity the small intestine, we isolated the brush border membrane from fishers and analyzed isolates via zymography and proteomics. We found that fishers consumed more fungi in their overall diet, which corresponded to higher trehalase activity compared to martens, which exhibited higher APN activity. Zymography

revealed that fishers’ trehalase activity appeared to be endogenous and trehalase sequences from fishers aligned with known trehalase sequences among other mustelid species. Overall, our results indicate that fishers have the capacity to digest dietary items such as fungi and insects and suggest that carnivore diets may have a greater plasticity which could explain competitive partitioning through divergence in diets among these closely related species.