conclude that some bears were insectivores. Ants are the most commonly eaten insect, and sight
probably plays an important role in helping bears track down likely spots to eat them. The logs, debris
mounds, and rocks typically excavated or over-turned by bears in their pursuit of ants are usually
prominent and are no doubt readily discernable to the eye of a bear. In any case, a typical bear's
approach to using ants, once a likely spot has been found, is anything but haphazard given the uncertain
rewards often hidden beneath a mound or log and the uncertain costs associated with getting there.
Bears sample a lot of logs and ant hills, and as a consequence, you find a lot more small ant digs
than you do big ones. Several factors seem to influence where bears end up expending a substantial
amount of effort to acquire ants from logs, including its size and hardness, its aggregation with other
suitable logs, its exposure to the sun, and the numbers and size of ants contained within. Thus, the
canonical excavated log is >5 dm diameter, moderately decomposed under a moderately brittle rind,
located in a forest opening surrounded by numerous other favorably endowed logs, and contains large
numbers of large ants. There are plausible reasons why all of these features favor bear use, but
ultimately they devolve down to one key issue: the greatest ant biomass in return for the least energetic
investment. A similar diagnosis of excavated anthills shows disproportional bear use of large hills
containing large numbers of large ants, and ultimately leads to the same conclusion.
Bears rely upon their claws to gain access to ants, whether they are under a rock or in a log or
hill. In fact, 3 to 5 parallel gouges on the upslope side of a log or anthill dig is diagnostic of bear
involvement. Paws are also used to facilitate the capture of ants once access to a nest has been gained.
Typically, a bear will let the ants swarm over its paw and then lick them off (a handy way to minimize the
ingestion of soil), or if the ants are less aggressive, it will lick them directly off the substrate. The high
fraction of dirt and wood debris associated with ant remains in bear feces, however, is testimony to the
fact that the consumption of undigestible material is an unavoidable by-product of ant use.
The benefits that bears derive from eating ants is another topic that has defied the ready
comprehension of most researchers. Individual ants are small and, in total, typically comprise a small
part of feces that result from bears foraging on them. The rest of these feces is typically nest debris. The
energy required, especially to break into ant nests sheltered in logs, is also not trivial. This is graphically
brought home to anyone who has tried to emulate a bear's efforts with a modified five-pronged potato
rake, dubbed the "clawometer" by researchers who developed it. It is thus hard to understand how bears
can come out ahead energetically on much of the ant grubbing that they do. Researchers have
consequently invoked other non-energetic explanations, such as nutrient requirements, a taste for formic
acid, or possible aids to digestion. Many ants have a pleasant, almost citric, flavor eaten raw and might
appeal to afficiandos of lemons, and they can provide protein to bears during times of year when protein
is remarkably deficient in the rest of their diet, but this speculation is far from convincing, and any more
confident conclusions will have to await further study.
The sight of a hornet's thorax or head protruding from a bear feces is a somewhat disconcerting
sight. Few people do not pause and contemplate the potential unpleasantries of eating a hornet,
including graphic images of the angered meal administering stings to the esophagus. Nonetheless, bears
do eat substantial numbers of hornets, especially during dry years when hornet numbers multiply. Nests
of the ground-dwelling hornets typically used by bears are not as obvious as those of ants, and the most
likely means of detection is by scent, perhaps in combination with tracking hornets that are leaving or
entering a nest. Bears excavate the paper nests typically from a recess at the base of a tree or from
under semi-decomposed deadfall, and may consume nest and hornets together. Based strictly upon
circumstantial evidence, it is probable that bears minimize personal discomfort and maximize meal sizes
by more often attacking nests during the chill early morning hours, when hornets are lethargic and more
likely to be home. This tactic has similarly been observed to work when bears eat similar-sized
grasshoppers and mormon crickets, with greatest success during cool days or the cooler crepuscular
hours. Peter Krott also observed some brown bear cubs that he was raising in the European Alps to
sneeze or blow at hornets when they were eating them, perhaps as a means of keeping them at bay until
they could be eaten.
Foraging Behavior of Bears . Mattson 7
Acquiring Fruits and Seeds
The main problem that confronts bears in their pursuit of fruits and seeds is getting them off the bushes or
out of the tree tops either before other competitors get there or before the fruits decompose. After finding
a suitable patch, a bear may face more immediate problems such as gaining access to fruits and seeds
elevated above the ground, removing seeds from some undigestible protective covering, gleaning fruits
from out of a less digestible matrix of leaves and twigs, or simply the rate at which it can consume and
digest readily accessible berries. But ursids are as well equipped as any organism to deal with these
difficulties and draw upon an acute sense of smell, relatively well-developed color vision, facile lips, paws,
and tongue, and varied climbing capabilities to ingest sometimes phenomenal amounts, 10-45 kg, of
energy-rich fruits and seeds in a given day.
The simplest foraging scenario for a bear eating fruits is probably the removal of berries from
ground-hugging or nose-level bushes. In this case the bear often tries to maximize the number of berries
ingested relative to leaves and twigs. This endeavor is obviously limited by energetic considerations and,
at some point, it is not worth the extra time and effort to be picky. However, this break-even point clearly
varies among individual bears (depending upon their skills and tolerances) and with the abundance and
type of berry being consumed. Although it has not been clearly demonstrated, there are observations
supporting the logical expectation that bears are more selective when sated or when foraging upon large
abundant berries. Observers have thus reported behavior ranging from the sloppy to the fastidious.
Some bears wrap their lips around a stem and indiscriminately strip off leaves, twigs, and berries. Other
bears daintily pluck berries out from among less desired portions by a combination of lip and tongue work,
sometimes aided by manipulation of stems with their paws. Accordingly, we find berry feces that
sometimes contain nothing but the remains of fruits and others that contain mostly leaves.
Bears also consume a substantial number of tree fruits and seeds that have fallen to the ground,
either by their own efforts (as described below), wind and natural dehiscence, or the efforts of
competitors. They detect such things as acorns and whitebark pine cones by sight and scent, and move
along alternately nose to the ground and scanning the nearby forest floor. They typically use their lips
and tongue to pick up nuts from the ground litter, crush the shells in their mouth, and spit out the hull.
Acquisition of seeds from whitebark pine cones can be a little more complicated, but can be as simple as
chewing and swallowing the cone, seeds and all. At the other extreme, bears often break the cone bracts
off with their claws or teeth, after bracing the cone with another paw, and facilely lick the seeds out of the
debris with their tongue. The opportunity to indulge in this more fastidious consumption of seeds seems
to be greater when bears are raiding cone caches made by red squirrels compared to when they are
scavenging sometimes rancid seeds from cones dispersed over the forest floor. We accordingly see
feces that consist wholly of crushed seeds when bears are using squirrel middens and messier feces
containing lots of other cone remnants when bears are engaged in the energetically more costly pursuit of
wind-thrown cones.
A number of berries are produced towards the top of tall bushes. Mountain ash, elderberry,
chokecherry, and hawthorn are good examples of this type of fruit that is potentially quite abundant but
often beyond immediate reach of tongue and lips. Bears resolve this problem quite simply by squatting or
standing on their hind legs and pulling more flexible fruit-laden stems to within range of their mouth. More
robust stems may be subdued by either grabbing them with the paws and pulling on them until they break
or walking along them from proximal to distal end until either the branch breaks or the fruit is reached.
Bears employ variations on this technique to get fruits and seeds out of trees. There are many
incentives to make a trip into the tree tops, including potential meals of sugar-rich cherries and fat-rich
acorns and beechnuts that would otherwise have to wait until later in the year, or be sacrificed altogether
to competing rodents and birds. The option of arboreal foraging is largely denied to grizzly and brown
bears, and is perhaps the price they pay for being able to live in austere northern habitats where digging
and large body size are important to survival. This not to say that grizzlies never climb trees, and, in fact,
some closely-related European brown bears have been observed to forage for fruits or leaves in tree
canopies in Norway, the Alps, and Afghanistan. Nevertheless, tree foraging is the definitive domain of the
smaller-bodied black bear, whether in North America or Asia.
Foraging Behavior of Bears . Mattson 8
Once in a tree, black bears most often try to get the items of gastronomic interest down from the
canopy by either breaking or chewing off branches or by simply shaking the fruit off. They then descend
to eat on the ground. Under other ill-defined circumstances, black bears will stay in the canopy to feed.
This type of foraging is characterized by the bear securing itself in a fork or on a broad branch near fruitladen
stems, and then consuming the fruits from branches that have been pulled inward with the paws.
Branches thus handled often break, and are then either dropped or accumulated in a pile beneath or near
the bear. This accumulation has often been described as a "bear nest", but has nothing to do with either
rearing young or resting, and is simply an artifact of feeding. In any case, black bear foraging is usually
clearly betokened not only by claw marks on tree trunks, but also by broken limbs on the ground and
dangling in the canopy.
Grazing and Browsing
Bears are not able to digest much of the fiber that they eat. This fact is key to understanding how, when,
and where bears graze grasses and forbs or browse the leaves and flowers of shrubs, given that all these
items have a relatively fleeting period in their seasonal development when fiber content is low enough
that bears can benefit by much of what they ingest. This low digestibility, as well as minimal mastication,
is reflected in the bulk and structure retained by foliage in bear feces. The basic strategy employed by
most bears when grazing seems to be: eat large volumes when the net energetics of digestion are in your
favor, and incur as few additional costs associated with acquisition and processing as possible. As with
roots, we thus see a lot of selective feeding when bears are serious about grazing. Bears are unstudied
about some grazing, but this could be fairly characterized as "incidental" or "auxiliary". In the latter case
incentives may be to aid digestion or clear the digestive tract, and is often the interpretation applied to
grazing that accompanies use of meat.
Early in the growing season bears seem to be limited more by the biomass and height of grazable
foliage than they are by its quality. Grasses and forbs usually emerge through the dried-up remnants of
last-year's growth, or "hay", which limits access to this more nutritious new growth. On rare occasions
bears have been seen raking or muzzling through detritus to expose spring growth, but more often bears
either graze where the new foliage is more robust or where there is less obscuring hay. Bears most often
graze with their incisors, aided by manipulations of their flexuous lips and tongue, and are capable of
cropping material as low as 4-8 cm. In the colder climates typical of most current bear range, this minimal
growth is typically first achieved on exposed south-facing slopes where the snow melts first and where
much of the previous year's foliage has been removed by winter-active herbivores. It is thus common for
bears to be seen grazing on this kind of site during early spring, literally throughout the northern
hemisphere.
It isn't long, however, before nutritional quality and growth characteristics of the forage begin to
limit where bears can beneficially graze. Among the grasses and sedges bears seem to favor species
that have broader succulent leaves concentrated farther up the main flowering stems, and avoid species
that concentrate short slender leaves at the base of taller sparse culms. Bears also begin to restrict their
grazing to moist and shaded sites or sites at higher elevations where plant maturity, and associated
increase in fiber content, is delayed. They also tend to shift their grazing to broad-leaved species, or
"forbs", which tend to be more nutritious later in the growing season. Clover is a favorite mid- and latesummer
forage, and in some places bears graze mixed patches of bluegrass and clover so intensively
that they look as if they've been mowed by a ground's keeper. However, the typically numerous bear
feces are a dead give away to the true cause.
Bears are especially picky when it comes to eating forbs. Compared to grasses and sedges,
bears are much more selective about the species that they eat, both because of variable nutritional
quality and highly variable levels of potentially noxious secondary compounds. These compounds may
not only complicate digestion, they may also be mildly toxic. By contrast, use of grasses and sedges is
limited primarily by the amount of indigestible fiber and silica. Thus, we see little or no use of most forb
species and relatively heavy consumption of a select few, including cow-parsnip, angelica, and sweetcicely
at mid-latitudes, and boykinia and sourdock farther north.
Foraging Behavior of Bears . Mattson 9
However, selection is not limited to choosing a species, but is also extended to the parts of a
plant that are eaten. Bears seem to relish dandelion flowers, and in most places restrict themselves to
the stems, blossoms, and petioles of cow-parsnip except during early spring. This selective consumption
of plant parts is especially evident when bears eat thistle. Thistle stems are quite succulent shortly after
elongation and can taste like the sweetest celery. However, from a bear's point of view this otherwise
tasty morsel is disadvantaged by a spiny exterior, and the bear accordingly tries to maximize ingestion of
the succulent interior while minimizing ingestion of spines. A bear will do this one of several ways. One
approach is to knock the spiny flower head off with a swat of the paw and strip off the equally spiny
leaves with the claws prior to eating the stem, or eat the stem, leaves and all, but spit the leaves back out.
Alternately, some bears will break the stem over, strip the leaves off of the facing surface with their claws,
and then eat the exposed stem by precise nips with their incisors. This behavior is fascinating not only
because of the involved technique, but also, like much bear behavior, because of the varied approaches
taken by different bears.
As a final item, it is worth noting that bears browse the leaves and flowers of trees and shrubs,
especially during early spring. Bears are most commonly observed eating the leaves and catkins of
aspen and willow throughout higher latitudes, but can also be seen eating the leaves of such plants as
devil's club in coastal Alaska and beech trees in the northeastern United States. Some of this browsing is
complicated by comestibles that are beyond immediate reach. In these cases bears employ the same
techniques that they use to get berries from tall shrubs or small trees: they either bend the branches to
within mouth's reach, walk out along more robust branches to the succulent growth, or break the
branches down. Again, flexible paws are key to these strategies.
Consuming Cambium
Both black and grizzly bears have the annoying habit of eating cambium from commercially-valued trees.
Cambium is the succulent and spongy growth that conveys most of the tree's nutrients between roots and
leaves, just beneath the tree's bark. Simple carbohydrates are part of this arboreal freight, and spring
sugar content of cambium in some tree species can be as high as that of berries. So there is an
understandable impetus for bears to use cambium, but with the unfortunate side-effect of retarding tree
growth or even killing a tree if the cambium is exposed and eaten around the entire trunk (i.e., "girdled").
Regardless of the fact that this usually antagonizes foresters and other people who hope to profit from the
production of wood fiber, the means by which bears gain access to cambium is fascinating.
Bears seem to prefer different trees for a variety of reasons related to bark thickness, tree size,
and sugar and ash content of the cambium. Nutrient content varies by season, species, and vigor of the
individual tree. We accordingly see bears preferentially stripping bark in the spring or early summer,
when sugar content is highest, from more vigorously growing trees, as in stands that have been thinned
or fertilized, of species that tend to have thinner bark and higher average sugar content. Trees 15-50
years old and 15-50 cm in diameter typically fill this bill, as does Douglas-fir, lodgepole pine, redwood,
and various species of true firs.
Bears most commonly gain access to cambium by working their claws under the bark near the
base of the tree and pulling back and upward. A series of tugs with claws and teeth eventually pulls off
an elongate vertical strip of bark 10-15 cm wide and 6-30 dm long. These strips tend to be broader at the
base and taper to a point where the bark either detaches and falls to the ground or remains dangling by
an attenuated connection. Less often bears will start stripping from the top and initiate the tear with their
teeth. Whatever the specific technique, bears will almost always attack from uphill if the tree is on a steep
slope. After removing the bark, most bears then consume the cambium by vertical scrapes with their
lower incisors. Some bears apparently do not eat the cambium, and merely lick the exposed sap,
whether because of the particular tree species or idiosyncratic behavior is not clear. In any case, a bear
may content itself with one tear or progressively work its way around the entire tree, in which case the
tree inevitably dies.
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sbsc.wr.usgs.gov/cprs/research/projects/grizzly/pdf/FORAGINGBEHAVIOROFNORTHAMERICANBEARS.pdf