Grazing

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. All of these items have a relatively fleeting period in their seasonal development when fiber content is low enough that bears can benefit from much of what they ingest.  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, you 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, which is often the interpretation applied to grazing that accompanies consumption 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 short 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 these kinds of site during early spring, throughout the northern hemisphere.

 

It isn't long, however, before nutritional quality along with the architecture 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 increases in fiber content are 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 late-summer forage, and in some places bears graze mixed patches of bluegrass and clover so intensively that they look as if they've been mowed.  However, the typically numerous bear feces at such a site 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 because of highly variable levels of potentially noxious secondary compounds.  These compounds may not only complicate digestion, but also be mildly toxic.  By contrast, use of grasses and sedges is limited primarily by the amount of indigestible fiber and silica.  As a consequence, grizzlies make little or no use of most forb species, but heavily consume a select few, including cow-parsnip, angelica, and sweet-cicely at mid-latitudes, and boykinia and sourdock farther north.

 

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.

 

Finally, 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.  Some of this browsing is complicated by the location of comestibles 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.