Foraging Efficiency

The rate at which a bear of a given size can ingest, masticate, and swallow food is a critical aspect of nutrition. It sets the stage for how much can be passed through the gut, which is particularly important when it comes to vegetal foods. Bears depend upon passing large volumes of foliage through their digestive tract if they are to stay ahead energetically when eating such a fibrous food (see Digestion). And, when it comes to berries and roots, they likewise need to ingest large volumes to compensate for the very low protein content of these foods--but with the derivative benefit of ingesting relatively high concentrations of digestible energy (see Protein & energy effects).

Given this imperative to ingest large volumes when bears eat vegetal foods, it is with such foods that the consequences of fine-scale variation in foraging efficiency are starkest--at the scale of bite size and bite rate. Which is why researchers such as Charlie Robbins and Lisa Shipley have focused on berries and foliage in their investigations of foraging efficiency by bears. And it is their research that I feature here, most of which was executed under Dr. Robbin's tutelage by Christy Welch and Karen Rode.

Effects of forage density & stature

The two graphs at right show the effects of berry density on the size and rate of bites by bears. Bite rate and size are perhaps self-evidently the two immediate determinants of the total rate at which food is ingested, at least as far as the mouth cavity. After that, the rate and efficiency of mastication take over; e.g., chewing.


The two graphs feature two berry-producing species that are of widespread importance to bears (see Vegetal foods): serviceberry (Amelanchier alnifolia; salmon-colored dots) and huckleberry (Vaccinium membranaceum; blue dots). There are substantial differences between these two species in how berries are presented to a potential forager. Serviceberries grow in attentuated terminal bunches which allow large-mouthed foragers such as bears to harvest multiple berries in a single bite. By contrast, huckleberries tend to grow singly and dispersed, which means that bears need to employ a strategy of taking numerous small bites that are often contaminated with the accompanying detritous of leaves.


These morphologic differences are clearly evident in the relations of bite rate (A) and bite size (B) to variations in berry density for each species, where each dot represents one feeding trial involving a single bear. Put succinctly, bite size, but not bit rate, increases substantially as densities of serviceberries increase. The opposite is true for huckleberry. Bite rate, but not bite size, increases with density. More to the point, different strategies are imposed on bears by physical configurations of the berries they eat. As a result, morphologic features are as important as digestible energy in determining the overall energetic benefits of exploiting different patches of berries.

 

The same is true for grazed foliage. The figure to the right shows how bite sizes of clover vary with both the size of the involved bear and, more to the point here, with stature of the grazed plant--this under circumstances where the clover is uniformly dense. Basically, the relatively small sizes of bites taken by small bears are essentially unaffected by the stature of the grazed plant. By contrast, even though large bears are constrained to bites no larger than those of small bears when grazing plants <9-13 cm (3.5-5") tall, they reap huge comparative benefits (at least in terms of bite size) if they can find patches of herbaceous plants that average >15 cm (6") tall.

 

In short, the small dentition and jaws of small bears mean that they don't benefit from increases in the stature of grazed foliage, whereas the larger dentition and jaw of large bears allow them to exploit and benefit from greater vertical expression of biomass. Which amounts to a constraint imposed by foliage stature on large but not small bears, which exacerbates the intrinsic energetic problems for large bears when they try to subsist on a diet of stems and leaves. Which is to say, they typically can't, whereas small bears more commonly can (see Body mass effects).

 

 

The implications? If a bear, it probably makes more sense to seek out patches of foliage to graze that allow for small rapid bites. And, if you are large, you are probably not going to benefit much in the end from being able to take larger bites of taller-statured foliage (as above). Which partly explains why, in places such as Yellowstone, smaller bears seek out and heavily graze very dense short-statured patches of clover and bluegrass--commonly referred to as "grazing lawns." Interestingly, intense grazing by bears and other herbivores maintains such "lawns" by stimulating on-going regrowth of protein-rich digestible foliage (see Grazing in Yellowstone). 

The relationship between bite size and bite rate further elucidates the basic mechanics of bear grazing and, along with this, some fundamental contraints imposed on bears trying to subsist on foliage. The graph at left shows this relationship; again, each dot represents a single trial involving a single bear.

 

The inverse relationship is not surprising. A larger bite requires more processing time, which intrinsically constrains bite speed (i.e., bite rate). But the important feature here is the rapid diminishment in bite rates with increases in bite size.

Bite rate vs Bite size

Total rate of intake

Regardless of proximal biomechanics, the bottom line for bears is the total rate at which they can ingest foliage, berries, or any other vegetal food. Again, total rate of intake is especially critical for bears when it comes to vegetal foods (see above)--in contrast to when they eat foods such as meat.

 

The graphs immediately above show total rate of intake (grams of dry matter per minute) as a function of key constraining factors. In the case of both serviceberry and huckleberry (B) intake (not surprisingly) increases with the density of each type of berry in a given patch. But the response for serviceberry is dramatically greater than the response for huckleberry. Why? Because serviceberries (as I describe above) grow in terminal clumps that make it much easier for bears to harvest when compared to the more dispersed single berries of a huckleberry bush. That being said, huckleberry is considerably more digestible than serviceberry (see Digestion), which probably negates the benefits of the latter species rooted solely in ingestible volumes.

 

Graph A shows intake rate as a function of bite size for clover, one of the choicest of grazed bear foods. Why bite size rather than forage density? Simply because the research that produced these data essentially held density constant by insuring that bears had access to thick patches of foliage within which researchers varied composition and stature. Perhaps the point of this being that anything less would presumably be a non-starter for a food as fibrous and relatively indigestible as foliage (see Digestion). But the key point of graph A is that ingested volumes increase with bite size at first, but then rapidly reach a plateau (i.e., asymptote). This pattern is consistent with the depressive effect of bite size on bite rate (see above), ultimately rooted in the limits imposed by a need to chew foliage and then, after that, rate of passage through the gut.