Why bears consume mixed diets during fruit abundance

Abstract

In many ecosystems, grizzly bears (Ursus arctos) and black bears (Ursus americanus) feed heavily on berries and fruits in the fall. While a few birds and mammals are exclusively frugivorous, bears and most other animals consume fruit as part of a mixed diet. The mixed-diet strategy avoids potential calcium, protein, amino acid, or other nutrient deficiencies that can occur on fruit diets. However, we hypothesized that the high carbohydrate - low protein content of fruit would increase energy metabolism and force bears to use dietary mixing to meet protein requirements and, thereby, reduce energy metabolism. We examined the effects of six plant-based diets containing from 2.3 to 35% crude protein on intake, maintenance costs, and efficiency of gain of captive grizzly and black bears. In addition, the food habits of six populations of wild grizzly and black bears were analyzed, to determine the crude protein and digestible dry matter content of their diets. Efficiency of gain (0.53 ± 0.02 (±SD) g gain/g digestible dry matter intake) did not differ across diets. However, maintenance costs differed, ranging from 24 g·(kg0.75·day)-1 (120 kcal (1 cal = 4.1868 J) digestible energy (DE)·(kg0.75·day)-1) on the 35% protein pelleted diet to 80 g·(kg0.75·day)-1 (340 kcal DE·(kg0.75·day)-1) on fruit diets containing 2.3-5.6% protein (P = 0.0001). Supplementation of the fruit diet with additional protein increased mass gain but did not completely reverse the growth-depressing effect of the fruit-only diet. Protein limitations or other characteristics of fruit diets that increase energy metabolism and intake may be strategies that also directly benefit plants, by increasing either seed dispersal or propagation.