Diet and Macronutrient Optimization in Wild Ursids: Grizzly Bears Versus Black Bears

When fed ad libitum, ursids can maximize mass gain by selecting mixed diets wherein protein provides 17 ± 4% of digestible energy.  In the wild, this ability is likely constrained.  By visiting locations of 37 individuals during 274 bear-days, we documented foods consumed by grizzly (Ursus arctos) and black bears (Ursus americanus) in Grand Teton National Park during 2004–2006.  Based on published data, we estimated foods and macronutrients as percentages of daily energy intake.  Using principal components and cluster analyses, we identified 14 daily diet types.  Only 4 diets, accounting for 21% of days, provided optimal protein levels.  Nine diets (75% of days) led to over-consumption of protein, and 1 diet (3% of days) led to under-consumption.  Highest protein levels were associated with animal matter (i.e., insects, vertebrates), which accounted for 46–47% of daily energy for both species.  As predicted: 1) daily diets dominated by vertebrates were positively associated with grizzly bears and protein intake was positively associated with body mass; 2) diets dominated by fruits were positively associated with black bears; and 3) mean protein was highest during spring, when high-energy foods were scarce, however it was also higher than optimal during summer and fall.  Although optimal gain of body mass was constrained, bears opted for the energetically superior trade-off of consuming high-energy, high-protein foods.  Given protein digestion efficiency similar to obligate carnivores, this choice likely supported mass gain, consistent with studies showing monthly increases in percent body fat among bears in this region

Diet and Macronutrient Optimization in Wild Ursids: A Comparison of Grizzly Bears with Sympatric and Allopatric Black Bears.

When fed ad libitum, ursids can maximize mass gain by selecting mixed diets wherein protein provides 17 ± 4% of digestible energy, relative to carbohydrates or lipids. In the wild, this ability is likely constrained by seasonal food availability, limits of intake rate as body size increases, and competition. By visiting locations of 37 individuals during 274 bear-days, we documented foods consumed by grizzly (Ursus arctos) and black bears (Ursus americanus) in Grand Teton National Park during 2004-2006. Based on published nutritional data, we estimated foods and macronutrients as percentages of daily energy intake. Using principal components and cluster analyses, we identified 14 daily diet types. Only 4 diets, accounting for 21% of days, provided protein levels within the optimal range. Nine diets (75% of days) led to over-consumption of protein, and 1 diet (3% of days) led to under-consumption. Highest protein levels were associated with animal matter (i.e., insects, vertebrates), which accounted for 46-47% of daily energy for both species. As predicted: 1) daily diets dominated by high-energy vertebrates were positively associated with grizzly bears and mean percent protein intake was positively associated with body mass; 2) diets dominated by low-protein fruits were positively associated with smaller-bodied black bears; and 3) mean protein was highest during spring, when high-energy plant foods were scarce, however it was also higher than optimal during summer and fall. Contrary to our prediction: 4) allopatric black bears did not exhibit food selection for high-energy foods similar to grizzly bears. Although optimal gain of body mass was typically constrained, bears usually opted for the energetically superior trade-off of consuming high-energy, high-protein foods. Given protein digestion efficiency similar to obligate carnivores, this choice likely supported mass gain, consistent with studies showing monthly increases in percent body fat among bears in this region

Scatter plots of the first two principal components (PC) constructed from daily diets of grizzly and black bears, Grand Teton National Park and vicinity, Wyoming, USA, 2004–2006.

<p>Top graph illustrates active variables, which were the daily percentages of total energy obtained from 7 food types (e.g., above-ground vegetation [AGVEG], below-ground vegetations [BGVEG]). Bottom graph illustrates supplementary variables of species (grizzly, sympatric black [black-S], allopatric black [black-A]), sex, and season (spring [1 May–30 Jun], summer [1 Jul–20 Aug], fall [21 Aug–31 Oct]).</p

Model-predicted mean percentage of protein (±95% confidence interval) within the daily digestible energy consumed by grizzly, sympatric black (Black-S), and allopatric black (Black-A) bears, by season and sex, Grand Teton National Park and vicinity, Wyoming, USA, 2004–2006.

<p>The dashed line depicts the optimal protein level (17 ± 4%) that has been shown to maximize body mass gain per unit of energy intake [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0153702#pone.0153702.ref008" target="_blank">8</a>].</p

Mean estimated macronutrient content, as a percentage of daily digestible energy, for the 14 daily diet types of grizzly and black bears identified by principal component and cluster analyses of foraging data collected during site visits of bear locations during a 24-hour period, Grand Teton National Park and vicinity, Wyoming, USA, 2004–2006.

<p>Foods were categorized into 7 primary food types (e.g., above-ground vegetation [AGBVEG], below-ground-vegetation [BGVEG]). The dashed line depicts the optimal protein level (17 ± 4%) that has been shown to maximize body mass gain per unit of energy intake [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0153702#pone.0153702.ref008" target="_blank">8</a>].</p

Mean estimated percentage of daily digestible energy provided by seven primary food types (e.g., above-ground vegetation [AGBVEG], below-ground-vegetation [BGVEG]) among grizzly, sympatric black (black-S), and allopatric black (black-A) bears, as documented from site visits to bear locations during a 24-hour period, Grand Teton National Park and vicinity, Wyoming, USA, 2004–2006.

<p>Mean estimated percentage of daily digestible energy provided by seven primary food types (e.g., above-ground vegetation [AGBVEG], below-ground-vegetation [BGVEG]) among grizzly, sympatric black (black-S), and allopatric black (black-A) bears, as documented from site visits to bear locations during a 24-hour period, Grand Teton National Park and vicinity, Wyoming, USA, 2004–2006.</p

The study area centered on Grand Teton National Park, Wyoming, USA, as defined by the spatial extent of all site visits during 2004–2006.

<p>Colored areas depict locations of site visits by species: grizzly bear (yellow); sympatric black bear (blue); and allopatric black bear (pink).</p

Positive (+) and negative (-) associations of the 14 identified daily diet types with supplementary categorical variables for season, sex, and species (χ² tests) and supplementary continuous variables for body mass and GPS fix interval (<i>t</i> tests), Grand Teton National Park, Wyoming, USA, 2004–2006.

<p>The number of symbols corresponds to <i>P</i>: single (0.01 ≤ <i>P</i> ≤ 0.05), double (0.001≤ <i>P</i> ≤ 0.01), triple (<i>P</i> ≤ 0.001).</p

Body mass range (kg) and sample size (bear-days) for sex and species categories of bears monitored to estimate daily diets, Grand Teton National Park, Wyoming, USA, 2004–2006.

<p>Body mass range (kg) and sample size (bear-days) for sex and species categories of bears monitored to estimate daily diets, Grand Teton National Park, Wyoming, USA, 2004–2006.</p

Mean estimated percentages of daily digestible energy provided by seven primary food types within 14 diets of grizzly and black bears identified by principal components and cluster analyses of field data, Grand Teton National Park, Wyoming, USA, 2004–2006.

<p>Homogenous diets were dominated by one particular food type, whereas mixed diets were composed of 2 or more principal food types. Within these categories, diets are ordered by number of bear-days observed.</p