Constraints on Frugivory by Bears

Abstract

Abstract. Bears consuming wild fruits for fall energy accumulation are constrained by several factors, including intake rate, the physiological capacity of the gastrointestinal tract, and the metabolic efficiency of gain in body mass. We measured these relationships through foraging and feeding trials using captive and wild black bears (Ursus americanus) and grizzly bears (Ursus arctos). Four fruit types covering a range of sizes and clustering were offered to captive bears to determine the effect of density, size, and presentation on intake rate. Intake rate (in grams per minute) and bite rates (in bites per minute) increased curvilinearly with increasing fruit density in singly spaced fruits. Maximum intakes ranged from 30 g/min for 0.5-g berries to >200 g/min for 4.2-g fruits. The highest bite rates were obtained during the initial encounter with each patch as bears consumed all visually apparent fruits on the surface. Bite rates quickly dropped by 15-20% as foraging continued within the patch. Maximum bite rates were not depressed until initial fruit density fell to <50 berries/M3. Maximum daily fresh fruit intake for the captive bears averaged 34 ? 6% (mean ? 1 SD) of body mass. The dry-matter digestibility of wild fruits, particularly preferred species, was as high as 72%. While large captive bears could gain body mass very rapidly when given fruit ad libitum, foraging efficiencies increasingly constrained growth rates of wild bears >100 kg. We concluded that large bears, such as grizzlies, must depend on plants that permit large bite sizes or high bite rates through fruit clustering and bush configuration that reduce leaf-to-fruit ratios.

Seasonal and annual variation in the diet of brown bears Ursus arctos in the boreal forest of southcentral Sweden

“…This supports previous findings that the majority of autumn fat reserves come from carbohydrates from berries (Welch et al 1997). We found that berries constituted 68% of the estimated dietary energy content in autumn, which was intermediate between the 49% reported from northeastern Norway, ca 1100 km north-northeast of our study area (Persson et al 2001) and the 80-81% documented from Jämtland, Sweden, ca 300 km northwest of our study area (Dahle et al 1998).…”

“…During the two years of berry inventory, crowberry showed the greatest concentration of berries (annual means of 458 and 1304 berries m -3 foliage), lingonberry was intermediate (149 and 260), and bilberries showed the lowest concentration (57 and 106). Welch et al (1997) showed experimentally that maximum bite rates of brown bears were depressed when initial fruit density fell to  50 berries m -3 . They conluded that tall, leafy shrubs, such as bilberry in our study, produce fewer, singly spaced berries that are obscured by relatively large leaves, causing bears to search more for and to select individual berries, resulting in lower bite rates and smaller bite sizes.…”

“…We picked all the berries in the quadrate, sorted them to species and degree of ripeness, and counted and weighed them. This enabled us to calculate the number of berries m -2 , berries m -3 foliage, and yield of ripe berries per m 2 for each species, as these are important parameters for foraging bears (Welch et al 1997).…”

“…In our study, we found that bilberry was most strongly preferred and that lingonberry was least preferred, based on occurrence of berries, which is a better parameter to measure to evaluate selection. Bears should select berry species that maximize energy and provide a balanced nutrient intake (Coogan et al 2014) in addition to growing in a clustered manner, allowing the bears to reduce search time, take larger bites, and thus forage more efficiently (Welch et al 1997). Eriksson and Ehrlén (1991) reported that both the carbohydrate content and mean fresh berry weight were much higher in bilberries (31.1% of dry weight and 0.48 g, respectively) than in lingonberries (15.4%, 0.25 g) or crowberries (14.4%, 0.14 g).…”

Missing and forbidden links in mutualistic networks

“…in plant-hummingbird networks [9,17], plant-insect networks [12,18] and plant-frugivore interactions [2]). Foraging constraints are determined by rates of search and handling of, for example, fruits [19], and biochemical constraints are operating if a frugivore is unable to process secondary compounds in fruit pulp. Even a multiple-trait or multiple-constraint model may be needed to make solid predictions about link patterns in real-world networks [20].…”