Gastrointestinal helminth abundance in the Eurasian beaver (Castor fiber) is affected by sex, pregnancy and habitat type

Abstract Internal parasites exploit host resources and may reduce host fitness and population viability. Parasite prevalence and abundance may be associated with intrinsic factors including sex, age and body condition, and extrinsic factors including habitat type and population density. Our study quantified gastrointestinal helminths abundance in 49 Eurasian beavers ( Castor fiber ) shot during spring hunting season in Norway and examined associations with host sex, age, body condition, habitat type, and population density. Two host-specific gastrointestinal helminths were identified: the intestinal trematode or fluke ( Neostichorchis subtriquetrus ) (prevalence 81.8%; N = 42) and the stomach nematode ( Travassosius rufus ) (prevalence 100%). Males exhibited higher parasite abundance compared to females, for both parasite species. Pregnant beavers harboured higher fluke abundance than non-pregnant females. Beavers inhabiting stream type habitats had significantly lower fluke abundance than those in rivers and small lakes, whereas Travassosius rufus abundance was significantly lower in river habitats compared to streams and lakes. No significant associations were detected with age, body condition or population density. Host sex, reproductive status and habitat type were associated with gastrointestinal helminths abundance. Further studies across broader geographic regions and demographic groups are needed to clarify the ecological consequences of parasitism in beavers.