Moose selecting for specific nutritional composition of birch places limits on food acceptability

Abstract

Despite decades of intense research, it remains largely unsolved which nutritional factors underpin food selection by large herbivores in the wild. We measured nutritional composition of birch foliage (Betula pubescens) available to, and used by, moose (Alces alces) in natural settings in two neighboring regions with contrasting animal body mass. This readily available food source is a staple food item in the diet of moose in the high‐fitness region, but apparently underutilized by moose in the low‐fitness region. Available birch foliage in the two regions had similar concentrations of macronutrients (crude protein [CP], fiber fractions, and water‐soluble carbohydrates [WSC]), although a notably lower variation of WSC in the low‐fitness region. For minerals, there were several area differences: available birch foliage in the low‐fitness region had less Mg (depending on year) and P, but more Ca, Zn, Cu, and Mn. It also had higher concentrations of some plant secondary metabolites: chlorogenic acids, quercetins, and especially MeOH‐soluble condensed tannins. Despite the area differences in available foliage, we found the same nutritional composition of birch foliage used in the two regions. Compared to available birch foliage, moose consistently used birch foliage with more CP, more structural fiber (mainly hemicellulose), less WSC, higher concentrations of several minerals (Ca, Zn, K, Mn, Cu), and lower concentrations of some secondary metabolites (most importantly, MeOH‐soluble condensed tannins). Our study conceptually supports the nutrient‐balancing hypothesis for a large herbivore: within a given temporal frame, moose select for plant material that matches a specific nutritional composition. As our data illustrate, different moose populations may select for the same composition even when the nutritional composition available in a given food source varies between their living areas. Such fastidiousness limits the proportion of available food that is acceptable to the animal and has bearings on our understanding and application of the concept of carrying capacity.

The complexity of interacting nutritional drivers behind food selection, a review of northern cervids

“…Even condensed tannins are a mixed group, and a recent study shows that subgroups of tannins may influence moose food selection differently (Wam et al. ). As a typical browser, moose particularly face seasonal variation in food types.…”

“…Looking only at tannins in a study may therefore render quite different conclusions than looking at total phenolics. Even condensed tannins are a mixed group, and a recent study shows that subgroups of tannins may influence moose food selection differently (Wam et al 2018). As a typical browser, moose particularly face seasonal variation in food types.…”

Subtle foodscape displacement of a native ungulate by free‐ranging livestock in a forest agroecosystem

“…, Wam et al. , b). In our study, we did observe a slightly higher diet content of browse for moose in control areas compared to sympatric moose, potentially explaining their higher fecal N. It could be that because browse includes more perennial plant parts, they also have more condensed tannins than other plant groups.…”

Using by‐catch data from wildlife surveys to quantify climatic parameters and timing of phenology for plants and animals using camera traps

“…; Post and Stenseth ; Wam et al. ). These species were relatively easy to study as they have clear seasonality and visually identifiable phenophases at the resolution of our camera‐trap images.…”

“…; Post and Stenseth ; Wam et al. ) and (3) link these to animal phenology related to fitness, namely onset of moult, calving date and growth of antlers (Linnell and Andersen ; Price et al. ; Mills et al.…”