Exploitation of rodents

Red squirrels, pocket gophers, and voles are often involuntary provisioners for grizzly bears because of a helpful tendency to accumulate large (1-5 liter) caches of roots and seeds, which the bears later raid at their convenience.  In addition, the ground squirrels, gophers, and voles may themselves be a significant source of meat.  But regardless of whether the bear's reward is a food cache or the animal itself, acquisition almost always requires some amount of excavation.  As a consequence, black bears are less often the beneficiaries of rodents compared to grizzly bears, which are better built for digging (see Front limbs).

 

Grizzlies enhance their chances of catching rodents by seeking out areas with high densities of vulnerable rodents.  Subsequent detection of this often subterranean prey is then typically dependent upon scent.  This becomes especially clear when a bear digs down directly to a pocket gopher root cache buried under 1 m of snow and dirt in an otherwise featureless snow field, or when a bear is observed to stick its nose down a ground squirrel hole before committing itself to an excavation.  However, scent is not the only means by which bears find rodents or their food stores.  It is very likely that bears first orient toward the whitebark pine cone caches of red squirrels by listening for the chatter of these territorial creatures, which typically emanates from somewhere near their central hoard.  Upon finding this spot, called a "midden", the bear then uses a combination of scent and sight to seek out the cones buried in the spongy mass of debris that has accumulated from generations of squirrel meals.

 

Bear excavations for rodents are quite variable.  Although grubbings for voles are typically shallow and often of limited extent, they can also be so extensive that a field may look like it has been plowed.  Pocket gopher excavations similarly vary, in a seasonally predictable manner.  Digs are shallowest but most extensive during spring, when the gophers are restricted to shallow depths by frozen ground and saturated soils, and deepest but least extensive in mid-summer, when the gophers are established at greater depths (see the publication linked here).  Different types of digs are also diagnostic of whether a bear has been seeking a root cache or pursuing an animal.  A grizzly in pursuit of a pocket gopher or vole that is escaping down a tunnel will literally hop side-ways while furiously excavating a runway with its front paws.  On the other hand, excavation of a root cache is a more leisurely affair and involves the studied digging of more circular holes that often reveal both a nest and a cache.  Bears will typically rake through a cache before consuming the roots directly off their claws, presumably as a means of reducing the amount of ingested dirt.  They may be so relaxed that they lay on their bellies while engaged in this activity.

 

Grizlies most often capture rodents with their paws, typically by pinning them to the ground but occasionally by stunning them with a swat.  Not unlike the canids, bears will also pounce on above-ground rodents prior to pinning them, and sometimes achieve this despite an obscuring blanket of snow.  Adolf Murie described the use of a similar pounce by grizzly bears that was directed at the remaining over-burden of soil between them and an elusive ground squirrel, presumably with the intent of startling the squirrel into making its escape.  Whatever the means of capture, a bear usually makes short work of eating a rodent after capturing it.

 

This small reward has mystified a number of researchers trying to understand the benefits that bears derive from consuming rodents.  The "carloads" of earth moved by some grizzlies in pursuit of a relatively diminutive ground squirrel has especially caused its share of wrinkled brows.  This seeming paradox is perhaps best resolved by invoking normal bear behavior and the potential power of habit.  For example, most excavations for ground squirrels occur in the spring and fall, when they are most vulnerable, and may take only a few minutes each.  However, many observations of bears digging for squirrels are made during mid-summer, when grizzlies are more likely to undertake 30-60 minute digging marathons.  Excavations for the even smaller pocket gophers and voles are rewarded not only by their capture but also by the discovery of root caches.  It is thus likely that the high costs ascribed to rodent use by the casual observer may be the result of both unrepresentative observations and, indeed, the tendency for bears to engage in habitual activities even under energetically unfavorable conditions; much like the human who walks an additional two blocks to a favorite restaurant for a cup of coffee.