Abstract
At northern latitudes, large spatial and temporal variation in the nutritional composition of available foods poses challenges to wild herbivores trying to satisfy their nutrient requirements. Studies conducted in mostly captive settings have shown that animals from a variety of taxonomic groups deal with this challenge by adjusting the amounts and proportions of available food combinations to achieve a target nutrient balance. In this study, we used proportions‐based nutritional geometry to analyze the nutritional composition of rumen samples collected in winter from 481 moose (Alces alces) in southern Sweden and examine whether free‐ranging moose show comparable patterns of nutrient balancing. Our main hypothesis was that wild moose actively regulate their rumen nutrient composition to offset ecologically imposed variation in the nutritional composition of available foods. To test this, we assessed the macronutritional composition (protein, carbohydrates, and lipids) of rumen contents and commonly eaten foods, including supplementary feed, across populations with contrasting winter diets, spanning an area of approximately 10,000 km2. Our results suggest that moose balanced the macronutrient composition of their rumen, with the rumen contents having consistently similar proportional relationship between protein and nonstructural carbohydrates, despite differences in available (and eaten) foods. Furthermore, we found that rumen macronutrient balance was tightly related to ingested levels of dietary fiber (cellulose and hemicellulose), such that the greater the fiber content, the less protein was present in the rumen compared with nonstructural carbohydrates. Our results also suggest that moose benefit from access to a greater variety of trees, shrubs, herbs, and grasses, which provides them with a larger nutritional space to maneuver within. Our findings provide novel theoretical insights into a model species for ungulate nutritional ecology, while also generating data of direct relevance to wildlife and forest management, such as silvicultural or supplementary feeding practices.
Highlights
Wild herbivores living at northern latitudes must deal with large spatial and temporal variation in the nutritional composition of available foods
Taken together with evidence for macronutrient balancing by captive moose (Felton et al, 2016), and indications of nutrient balancing from fecal analysis (Ma et al, 2020), our study confirms nutritional balancing by a ruminant herbivore species in the wild
Our findings here from free-ranging moose in southern Sweden support earlier conclusions from moose in captive settings that their food selection during wintertime is governed by macronutrient balancing as opposed to alternative strategies such as energy or protein maximization (Felton et al, 2016)
Summary
Wild herbivores living at northern latitudes must deal with large spatial and temporal variation in the nutritional composition of available foods. To obtain a large enough sample to compare diets across a range of foraging environments, we analyzed the contents of winter collected rumen samples of 481 free-ranging moose belonging to multiple populations in southern Sweden These populations live in a region with extensive human modification of the landscape (Lindbladh et al, 2014), and the populations face quite different foodscapes primarily defined by intensity of forestry, agriculture, and urbanization, leading to natural browse being highly concentrated in space and time. To assess the nutritional state of moose living in these varying foodscapes, we used proportions-based nutritional geometry, an analytical framework for examining nutrient balancing that is well established in the literature of primate nutrition (Raubenheimer, 2011) In this framework, nutrient balancing is indicated if different members of a group (e.g., a species, population, sex, age-class) compose diets with similar nutritional composition despite eating different combinations of foods (Raubenheimer et al, 2015). Our second hypothesis was that the pattern of regulation in the polygastric wild moose would involve not just protein, nonstructural carbohydrate, and lipid, and the fermentable structural carbohydrates, cellulose, and hemicellulose (Felton et al, 2018; Van Soest, 1994)
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