Habitat quality and parasite assembly influence wildlife health, and they are key indicators of health and survivability of wildlife populations. To investigate the potential ecological relationships among habitat type, food nutrients, parasites and hormones in wild boar (Sus scrofa), we collected samples of wild boar feces and available plants in their habitat by line transects during winter. Along transects, we identified the composition of plants foraged by wild boar and measured the content of nutrients in available plants to estimate nutrient intake. We also quantified parasites and hormones in wild boar fecal samples. We compared food nutrients among different forest types and explored possible relationships among estimated nutrient intake, parasites and hormones. We found coniferous forest had positive effects on estimated fat intake and negative effects on estimated protein and fiber intake by wild boar. Furthermore, we revealed that estimated fat intake was negatively correlated with Metastrongylus elongatus parasites and positively correlated with triiodothyronine (T3). In contrast, estimated protein intake was positively correlated with M. elongatus and negatively correlated with T3. Finally, we found negative relationships between T3 concentrations and loads of Ascaris suum parasites and between cortisol (COR) and loads of Trichuris suis parasites. These insights on ecological relationships help identify potential dietary parameters in winter that could help predict and manage parasite and hormone responses for wild boar population recovery.Habitat quality and parasite assembly influence wildlife health, and they are key indicators of health and survivability of wildlife populations. To investigate the potential ecological relationships among habitat type, food nutrients, parasites and hormones in wild boar (Sus scrofa), we collected samples of wild boar feces and available plants in their habitat by line transects during winter. Along transects, we identified the composition of plants foraged by wild boar and measured the content of nutrients in available plants to estimate nutrient intake. We also quantified parasites and hormones in wild boar fecal samples. We compared food nutrients among different forest types and explored possible relationships among estimated nutrient intake, parasites and hormones. We found coniferous forest had positive effects on estimated fat intake and negative effects on estimated protein and fiber intake by wild boar. Furthermore, we revealed that estimated fat intake was negatively correlated with Metastrongylus elongatus parasites and positively correlated with triiodothyronine (T3). In contrast, estimated protein intake was positively correlated with M. elongatus and negatively correlated with T3. Finally, we found negative relationships between T3 concentrations and loads of Ascaris suum parasites and between cortisol (COR) and loads of Trichuris suis parasites. These insights on ecological relationships help identify potential dietary parameters in winter that could help predict and manage parasite and hormone responses for wild boar population recovery.

Diet composition and nutrient measurements

We processed 123 fecal samples of wild boar to identify food composition by fecal microhistological analysis (Shrestha et al. 2005, Bao et al. 2017). After identifying plant genus, we converted them to the dry weight (DW%) of the diets based on their relative frequency (Sparks and Malechek 1968). We determined the nutrients (fat, protein, fiber) of staple food consumed by wild boar following the methods of Ma et al. (2019). We ground samples in a Wiley Mill through a 1mm screen. We used Soxhlet method to determine the fat content by diethylether in a Soxhlet extractor at 60 ℃ for 4 h. We measured crude protein in plants using the Kjeldahl method (Kjeltec™ 8400, FOSS, Hillerød, Denmark). We measured crude fiber by sequential analysis using an A2000i fiber analyzer (ANKOM Technology Corp., Macedon, NY, USA). We estimated nutritional intake of each sample by multiplying the specific DW% of each plant by its nutritional content.

Parasites and hormone concentration detection

We counted the parasite eggs per gram (EPG) in fecal samples by McMaster method (Meyer-Lucht and Sommer 2005). We mixed 5 g of each sample with 40 ml of saturated zinc sulfate. Then, we filtered these mixtures through a wire mesh with a 250 μm aperture, and then injected filtered results into two McMaster Egg Slide Counting Chambers (Shanghai Veterinary Research Institute, Chinese Academy of Agricultural Sciences) and identified eggs to species based on morphological characteristics (Fig. 2). We calculated the EPG as follows: EPG = (n1+n2)/2 / 0.15 × V/m (Hu et al. 2018), where n1 and n2 are the numbers of eggs in two counting chambers, 0.15 is the volume of each counting chamber, and V and m are the volumes of the homogenized fecal sample and weight of feces, respectively (i.e., V = 45 ml and m = 5 g). Although freezing samples may alter the morphological integrity of parasite eggs (Gassó et al. 2015), all fecal samples were treated similarly and measurements provide a relative abundance. We sent fecal samples to Beijing Kemeixinyun Biotechnology Co. Ltd. to measure the fecal T3, COR, and P concentrations using methods described in Wasser et al. (2010).