Data from: The influence of plant defensive chemicals, diet composition, and winter severity on the nutritional condition of a free-ranging, generalist herbivore

When consuming plants, herbivores must deal with both low nutritional quality from cell wall constituents and potentially toxic plant secondary metabolites, which are often inversely related. Herbivores that consume a highly nutritious, but chemically defended plant, may consume high levels of toxins that require energy for detoxification. Alternatively, herbivores may avoid consuming high levels of toxins by consuming a diverse diet that may be lower in overall nutritional quality. In this study, we assessed the relationship among nutritional restriction, detoxification and diet diversity in a free-ranging wild herbivore. We collected urine deposited in the snow (hereafter, snow-urine) and feces by free-ranging moose Alces americanus, a generalist browser, during winter. We used the ratio of urinary urea nitrogen to creatinine (UN:C), measured in snow-urine samples, as an indicator of nutritional restriction, and the ratio of glucuronic acid to creatinine (GA:C), as an indicator of investment in detoxification. We used microhistology to determine diet composition from fecal pellets. GA:C and UN:C were positively associated, suggesting that nutritional condition tends to be worse for individuals investing more in detoxification. We found, after accounting for the influence of winter severity, diet diversity and UN:C to be negatively related, suggesting that increasingly diverse diets were associated with improved nutritional condition. Overall, the most important predictor of UN:C was winter severity and proportion of diet comprised of balsam fir Abies balsamea. Physiological indicators of nutritional restriction tended to be worse during severe winters and among individuals that had consumed more balsam fir. These results highlight complex relationships among environmental conditions, foraging decisions, and costs of detoxification that can influence nutritional condition of herbivores.

植食动物在取食植物时，需同时应对由细胞壁组分导致的低营养品质，以及潜在有毒的植物次生代谢物——二者往往呈负相关关系。取食高营养但具有化学防御的植物的植食动物，可能会摄入高剂量毒素，而对这些毒素进行解毒代谢需要消耗能量。反之，植食动物可通过取食整体营养品质较低的多样化日粮，来避免摄入过量毒素。本研究针对一种自由活动的野生植食动物，探究了营养限制、解毒作用与日粮多样性之间的关联。研究期间，我们于冬季收集了自由活动的广食性啃食者——美洲驼鹿（Alces americanus）在积雪中遗留的尿液（下称雪尿）与粪便。我们以雪尿样本中测得的尿尿素氮与肌酐比值（urinary urea nitrogen to creatinine, UN:C）作为营养限制的指示指标，以葡萄糖醛酸与肌酐比值（glucuronic acid to creatinine, GA:C）作为解毒投入的指示指标。我们通过显微组织学方法分析粪便颗粒，以确定日粮组成。GA:C与UN:C呈正相关，表明解毒投入更高的个体，其营养状况往往更差。在控制冬季严酷程度的影响后，我们发现日粮多样性与UN:C呈负相关，表明日粮多样性越高，个体的营养状况越好。总体而言，影响UN:C的最重要预测因子为冬季严酷程度以及日粮中香脂冷杉（Abies balsamea）的占比。营养限制的生理指标在严酷冬季以及取食更多香脂冷杉的个体中，往往表现更差。本研究结果揭示了环境条件、取食决策与解毒成本之间的复杂关联，而这些关联可影响植食动物的营养状况。