The Nutritional Balancing Act of a Large Herbivore: An Experiment with Captive Moose (Alces alces L)

The nutrient balancing hypothesis proposes that, when sufficient food is available, the primary goal of animal diet selection is to obtain a nutritionally balanced diet. This hypothesis can be tested using the Geometric Framework for nutrition (GF). The GF enables researchers to study patterns of nutrient intake (e.g. macronutrients; protein, carbohydrates, fat), interactions between the different nutrients, and how an animal resolves the potential conflict between over-eating one or more nutrients and under-eating others during periods of dietary imbalance. Using the moose (Alces alces L.), a model species in the development of herbivore foraging theory, we conducted a feeding experiment guided by the GF, combining continuous observations of six captive moose with analysis of the macronutritional composition of foods. We identified the moose’s self-selected macronutrient target by allowing them to compose a diet by mixing two nutritionally complementary pellet types plus limited access to Salix browse. Such periods of free choice were intermixed with periods when they were restricted to one of the two pellet types plus Salix browse. Our observations of food intake by moose given free choice lend support to the nutrient balancing hypothesis, as the moose combined the foods in specific proportions that provided a particular ratio and amount of macronutrients. When restricted to either of two diets comprising a single pellet type, the moose i) maintained a relatively stable intake of non-protein energy while allowing protein intakes to vary with food composition, and ii) increased their intake of the food item that most closely resembled the self-selected macronutrient intake from the free choice periods, namely Salix browse. We place our results in the context of the nutritional strategy of the moose, ruminant physiology and the categorization of food quality.营养平衡假说（Nutrient Balancing Hypothesis）提出，在食物充足的条件下，动物选择日粮的核心目标是获取营养均衡的膳食。该假说可通过营养几何框架（Geometric Framework for nutrition, GF）进行检验。营养几何框架可帮助研究者探究营养素摄入模式（如宏量营养素：蛋白质、碳水化合物、脂肪）、不同营养素间的相互作用，以及在膳食营养失衡时期，动物如何解决过量摄入一种或多种营养素与摄入不足其他营养素之间的潜在冲突。本研究以草食动物觅食理论发展中的模式物种——驼鹿（Alces alces L.）为研究对象，依托营养几何框架开展摄食实验，结合对6只圈养驼鹿的连续观测与食物宏量营养素组成分析。研究人员允许驼鹿混合食用两种营养互补的颗粒饲料，并辅以有限量的柳属饲用嫩枝（Salix browse），以此确定驼鹿自主选择的宏量营养素摄入目标。此类自由选择摄食的时段，与仅允许食用两种颗粒饲料之一辅以柳属饲用嫩枝的限制摄食时段交替进行。对自由选择摄食时段驼鹿食物摄入量的观测结果支持了营养平衡假说，因为驼鹿会以特定比例混合各类食物，从而获得特定比例和含量的宏量营养素。当被限制仅食用两种单一颗粒饲料搭配的日粮之一时，驼鹿表现出两项特征：其一，维持相对稳定的非蛋白质能量摄入，同时使蛋白质摄入量随食物组成发生变化；其二，增加摄入最接近自由选择时段自主选定宏量营养素摄入目标的食物——即柳属饲用嫩枝。本研究将结果置于驼鹿营养策略、反刍动物生理学以及食物质量分类的研究背景下进行讨论。