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Root grubbing

Root are the diagnostic food of grizzly bears, and indeed, where black and grizzly bears live together, many differences in diet can be ascribed to root grubbing by grizzlies.  This is not to say that black bears never dig roots, as some eastern black bears do when excavating jack-in-the-pulpit corms during the spring, but rather that this is an uncommon activity for them.  However, even for the better adapted grizzlies, the extraction of roots is a dynamic and sophisticated problem in balancing energy expenditure with energy return.  We consequently see some of the most compelling evidence for highly selective feeding when grizzly bears eat roots.


The quality and abundance of roots vary both within and among sites, as does the earthen matrix within which they are imbedded; and you can accordingly see temporal and spatial variation in bear grubbing that predictably maximizes benefits to the bear.  For example, take the case yampa (Perideridia gairdneri), a member of the carrot family that produces small but very tasty roots rich in starchy energy.  Grizzlies from the Yellowstone area seem to love this root.  However, they don't dig it at all times or in all places.  As might be expected from the preamble, peak grubbing occurs after the roots have reached peak starch content, shortly after the plants flower.  There is also a secondary peak in use during early spring, just prior to the draw-down of starch reserves that accompanies the first flush of spring growth.  Even during the season of peak grubbing, use roughly tracks episodes of rainfall and the easier digging conditions that follow.  Bears are also not indifferent to varied conditions at any given point in time, and consistently favor sites where yampa is denser and more easily dug.  Not only that, they further select for the most easily dug microsites where they can excavate more than one plant at a time; and even more remarkably, they select the largest plants (which also happen to have the largest roots) in a patch to dig.


Generalizing on the example of yampa, it is clear that grizzlies seek to maximize returns on their investment at all levels of selection when they feed on roots.  And, indeed, this is corroborated by observations of grizzlies digging for sweetvetch (Hedysarum sp.) and biscuitroot (Lomatium sp.) roots.  Although some details, such as peak season of use, vary from that of yampa grubbing, the observed differences can all be explained in terms of corresponding differences in the timing of peak starch content and optimal digging conditions.


Grizzlies are remarkably efficient at getting a root out of the ground, and find the desired root by a combination of sight and smell.  As indicated above, size is a factor in their selection as is the advent of flowering for some species.  However, in cases where several other species resemble the target food plant, sight may allow for only the first level of discrimination.  At these times and at times when the plant's above-ground portions are withered and gone, scent becomes the primary means by which bears target a prospect.  Again taking yampa as an example, this interplay of sight and smell proceeds as follows: the bear moves towards a cluster of small white flowers which may either be of yampa or yarrow (Achillea millefolium; another species that commonly grows admixed with yampa).  The bear smells the plant, and if yampa, then grasps the slender stem side-ways in its mouth, scoops the root out with a shallow dig while still grasping the stem, moves the root up into its mouth with its tongue and a sideways movement of the head, bites the root off, and finally spits out the remains.  As a result, it is common to find rootless stems with a kink in them lying atop or nearby a relatively small paw-sized excavation.


Art Pearson and Adolf Murie have also described the more labor-intensive excavation of larger sweetvetch roots by grizzly bears in the Yukon and Alaska.  The actual digging requires greater exertion by the main body, and involves a rocking motion after the claws of one or both front claws have been engaged.  The excavated root is then lifted out of the hole or from beneath the over-turned clod either with the claws or teeth.  The culminating move consists of the bear firmly grasping the top of the plant with its teeth and pulling the rest of the root through its firmly clasped claws, shredding the root prior to moving it into the mouth, " alternate tooth and tongue movements."  All of this may take as little as 20 seconds.  When all is said and done, they can proceed at a pace of 1 root/minute for sweetvetch and as many as 14 roots/minute for the much smaller yampah.

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