Diet and Foraging

The brown bear is both an omnivore and a predator. In Scandinavia, bears get most of their energy intake from berries, ants, and ungulates (particularly moose calves), but they also eat many other types of grasses, forbs, and fungi. Their diet varies between seasons and years coinciding with the different pulses of food resources on the landscape and their nutritional needs (1). For example, during spring, bears need food that can help them grow and build muscle mass, while in the summer and fall they must renew their fat reserves for the coming winter. Therefore, protein-rich foods tend to dominate the diet during spring and early summer while bears seek foods rich in carbohydrates, which are more easily converted to fat, during later summer and fall (2). Bears also generally have a high-fiber diet throughout the year (2). Interestingly, bears seem to have similar nutrient intakes regardless of their foraging strategy and their sex/reproductive status, which is attuned for preparing for the hibernation period (2). However, bears in Scandinavia appear to eat less meat than might be expected, or observed in other systems (3).

Here, we cover the wide variety of foods they generally use and describe how diet can vary between seasons and years when different food sources are more or less available. For this section, we define the seasons as: spring = April-May, summer = June-July, fall = August-October.

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Insects

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Ants: During spring, ants are an important food source for bears in Scandinavia. Ants are a relatively abundant and available food source on the landscape, which makes them particularly important during the spring post-denning period (4). Red forest ants (specifically Formica aquilonia and Formica polyctena) and carpenter ants (Camponotus herculeanus) are consumed the most by bears in Scandinavia (4). Although red forest ants represent the majority of ant biomass consumed during spring, carpenter ants are more preferred, meaning relative to their availability, they are selected the most often (4). Furthermore, bears continue to eat carpenter ants during summer and fall, while their intake of forest ants drops off. Red forest ants (high in protein) are mound-builders and live in large above-ground colonies while carpenter ants (easily digestible and high in fat) live in dead wood, form smaller colonies, are less abundant, and therefore more difficult to get. In general, the carpenter ants preferred by bears tend to be high in energy content, low in formic acid, and behave relatively passively when their nests are disturbed (4).

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Altogether, ants make up about 10-20% of the total annual energy intake of the Scandinavian brown bear and about ~7% of their major diet when compared with berries and moose meat (3, 5). However, it is possible that shifting habitat conditions based on landscape practices and climate change have changed ant predation patterns over the last 25 years and may continue to do so. For example, carpenter ant availability is likely linked to forestry practices, with intensive clear-cutting resulting in more dead wood on the landscape in which the carpenter ants thrive (4, 6). Scandinavian brown bears feed more on ants than bears in North America, perhaps because red forest ants are more widely available; the amount of available ant biomass was 3 times greater than moose biomass (including both calves and adults) in the Southern Study Area (4). Thus, this protein-rich food source is very important to bears in this region during spring and summer which is reflected in the diet. For example ants appear to contribute twice as much to their diet compared to moose (3).

Other: Other insects that have been found in small amounts in their diet include other ants (Formica exsecta and Formica lugubris), moth lacewings (Oliarces clara), maggots (Diptera spp.), bees, bumblebees and wasps (Hymenoptera spp.), and beetles (Coleoptera spp.) (1, 4, 7, 8).

Ungulates and other sources of meat

Ungulates, or hooved animals, play an important role in the diet of the Scandinavian brown bear. The nature of this relationship has far-reaching consequences for both the ungulate prey of bears as well as the general management of the bear population. For more information on the nature and consequences of bear predation on ungulates in Scandinavia, see Bears as predators. Below, we summarize the extent to which these different ungulates contribute to bear diet.

Moose: Moose calves are the most heavily utilized ungulate by bears in Scandinavia and are a key part of their energy intake during the moose birthing period (mid-May to early June). Bears are generally best at hunting newborn moose within the first 4 weeks of the newborn’s life (9), which means there is a short window during which bears incorporate moose into their diet (from mid-May through the end of June). Research suggests that, on average, bears take between 6.5 and 7.6 moose calves per year, which equals about 14-30% of their annual energy intake (9, 10). Nevertheless, there is large variation in bear predatory behavior; some bears do not kill a single moose calf, while others take 10 or more per spring (11). Thus, moose meat may be a very important part of the diet of some bears, but relatively unimportant for others.

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Bears are also capable of killing adult moose, which happens more often during April and May when moose are in generally weaker condition from winter and during the birthing period when females are more vulnerable (12). Although predation on adult moose seems to happen more rarely in Scandinavia than for example in North America (12), the protein provided from an adult moose kill during spring is a rich resource for the successful hunter. Bears are also adept scavengers and protein can be acquired from both bear-killed and wolf-killed remains, where bears and wolves overlap, as well as from moose slaughter remains left by hunters (13, 14). However, scavenging behavior may vary between individuals. For example, females with cubs of the year seem to scavenge less often on slaughter remains than other bears, likely to avoid interactions with other bears (8).

Reindeer: In semi-domestic reindeer herding areas, reindeer calves are also an important contributor to bear diet, and a continual source of conflict for reindeer herders. In our Northern Study Area, we estimated each adult bear takes about 10.2 reindeer calves per year (range 8.6 – 11.5) (15). Similar to moose, bears also hunt newborn reindeer during their first 4 weeks of life. Reindeer calves are therefore also available in a pulse from early to late May, after which bears likely switch to moose calves (15). Bears also kill adult reindeer, the majority of which also occurs during the birthing period (15). Thus, where the two overlap, reindeer provide a pulse of protein-rich food during the post denning period.

Sheep: Compared to Sweden, there are about 1 million domestic sheep that free-range in Norway which means that sheep play an important role in the diet of bears in Norway, but not Sweden. For example, an early study conducted in the late 1980’s and early 1990’s suggests that bears in Nord-Trøndelag received as much as 65-87% of their annual energy intake from ungulates, which was mostly made up of sheep (7). How much sheep currently contribute to bear diet in Norway remains unknown. However, a 2022 pilot study in Trøndelag explored the potential for using molecular methods to monitor sheep consumption via scat collection (16). They found sheep DNA in only 5% of the sampled scats, most of which was detected in August (16).

Other: Bears are opportunistic feeders and have a wide diet breadth. Other sources of meat have been found in their diet including a variety of bird and rodent species (7) and domestic livestock such as pig and cattle (8).

Plants

Berries: Berries are one of the most important food sources for bears in Scandinavia and constitute the majority (89%) of their diet (3). Berries have matured by the late summer to early fall, during which time the bear switches diet yet again. As previously mentioned, berries contain high-calorie carbohydrates that can be easily converted to the fat bears need to endure the hibernation period. Bears preferentially eat blueberries (Vaccinium myrtillus), crowberries (Empetrum spp.), and lingonberries (Vaccinium vitis-idaea) with their preference changing between seasons (1, 17). For example, crowberries and lingonberries are selected more during spring, likely because there are berries remaining from the previous year, while blueberries are preferred during fall when they have come into season. An adult bear can eat up to a third of its own body weight in berries in one day, which equals about 20,000 calories. Altogether, berries make up about 45% of a bear's annual energy intake in Sweden. Although berry production can fluctuate from year to year in Scandinavia, the general occurrence of berries in bear diet appears to remain relatively stable (1). This is because bears compensate for decreased availability of one berry species by switching to another. For example, when blueberry production is low, bears eat more crowberries (1). However, berries are not an equally important food source for all bears across Scandinavia. In Norway, for example, only 6%-17% of a bears annual energy intake comes from berries; the majority comes from sheep (7). They also appear to be less important in the far north of Scandinavia where bears eat more meat (Persson et al. 2001). This variation illustrates that food availability is a very important driver of the seasonal and geographical changes in bear diet. For example, in southern Europe bears rely on so-called hard mast (oak and beech nuts) in the fall, and less so on soft mast (berries), reflecting the availability of different food items at different latitudes, e.g., in Slovakia (18).

Grasses, forbs, and other plants and fungi: Scandinavian bears also use a wide range of grasses (graminoids) and herbaceous flowering plants (forbs) to supplement their diet throughout the year (7, 8). They appear to eat grasses rather steadily through spring and summer but slightly less in fall while their use of forbs peaks during the summer months when flowers are in bloom (1, 7). Bears also eat spore producing plants (cryptograms) such as ferns, horsetail reeds, and thistles (1, 7). Mushrooms (fungi) appear more in their diet during fall (1, 7).

Brown bear diet in context

The majority of the studies on bear diet in Scandinavia have been conducted in our Southern Study Area, with the exception of the reindeer predation studies that were conducted in the Northern Study Area. This is important as bears in Scandinavia have access to different food resources as well as varied amounts of those resources depending on where they live. That means different foods may be more or less important for local bear populations across Scandinavia. For example, an early diet study based on scat collect between 1978 and 1982 in the Pasvik Valley in northeastern Norway suggests that bears are more carnivorous in the far north (19). Unlike southern Scandinavia, meat from ungulates, particularly adult moose, were the most important foods during spring and summer, with meat from ungulates contributing 85% of their energy content (19, 20). Furthermore, berries were also important during fall, but ungulates remained an important source of energy for northern bears as well, and the contribution of insects and vegetation was minimal (19). Thus, the diet of Scandinavian brown bears as described above should be understood with the caveat that in general, it describes the diet of bears in the southern population, unless otherwise noted.