The Gut Envionment
The "gut" euphamistically refers to the digestive tube that, in mammals, consists of the mouth, esophagus, stomach, small intestine, large intestine, and anus. The shape and size of the gut have a major influence on digestive processes, as do the chemical and microbial compositions of the intra-gut fluids and mucosa. Together, these features comprise the gut environment. I focus here on the environment of the intestines of bears, given that the intestines support the greatest digestive action in most animals. And, as always, the greatest insights come from contrasting bears with other taxa.
Gut morphology
Morphology focuses on the shape and form of anatomical features. Some of the most obvious dimensions of morphology include length, volume, and shape. Bears are of carnivorous descent (see Evolution) and, because of that (as well as evolutionary conservatism), they have a digestive tract not that much different from other carnivores--including dogs and cats. Which means that they have no specialized chambers within which digestion of plant fiber (e.g., cellulose and hemicellulose) occur, and that the overall length of the intestine is less than that of herbivores of comparable size. In the arcane terms employed by disciplinary specialists, bears and other carnviores have a digestive tract modeled after a plug-flow reactor; i.e., a tubular reaction vessel through which ingested materials flow in an orderly fashion, with little mixing. Such digestive systems are not friendly to bacteria that can ferment otherwise undigestible fiber, which require sequestered oxygen-free (anaerobic) environments within which fermentation can progress in its necessarily leisurely fashion.
From a morphological perspective, some key features of the ursid digestive tract include lack of rumen, cecum, or enlarged colon. Given that the cecum and colon are part of the large intestine, this translates into a dominance of internal lower digestive organs by the small intestine. Bears (along with other carnivores) have a comparatively short and underdeveloped large intestine that is roughly 10-20% of the length of the small intestine, the site of most uptake and digestion of easily-digested nutrients such as fats and proteins. By contrast, herbivores have large intestines that are 70 to 120% the length of their small intestines--or 270 to 710% the volume. This in addition to the potential contribution of the foregut (e.g., rumen) to digestion of fiber in many larger herbivores.
As a side-note, other omnivores such as pigs exhibit a fair amount of plasticity in gut morphology in response to diet composition. Pigs fed a diet rich in fiber can have guts as much as 10% larger than pigs fed diets rich in fat and protein, with most of the increased volume attributable to enlargement of the colon and cecum, and in spite of shrinkage of the small intestine. Although this kind of morphologic variability has not been shown for bears, it is likely to occur, especially in light of the fact that intestinal microbiota vary in response to diet composition among ursids (see below).
Black, grizzly, and polar bears (of the subfamily Ursinae) tend to have longer intestines relative to their body length compared to other carnivores, including members of other ursid subfamilies such as the Ailuropodinae(i.e., giant pandas). The figure at left is illustrative.
The top graph shows relations between overall intestine length (the vertical or y-axis) and total head and body length (the horizontal or x-axis). 'U' denotes members of the subfamily Ursinae, which includes grizzlies. There are several take-aways: first, that total intestine length tends to increase at a higher rate than does total body length; and, second, that ursines such as grizzlies have a comparatively longer total digestive tract compared to canids, felids, and even pandas. What does this mean? Probably that larger carnviores such as bears are comparatively better able to digest complex nutrients and, because of that, better adapted to an omnivorous lifestyle; and, paradoxically, that omnivorous ursine species are better able to digest their foods compared to the almost wholly herbivorous panda, which is consistent with the exceptionally low digestive rates exhibited by giant pandas (see Digestion).
The bottom graph at top left simply shows that the small intestine accounts for a progressively smaller fraction of the total intestine as body length increases among carnviores. Or, put another way, the large intestine, along with the capacity to digest starches and fiber, proportionally increases with size of the animal. Which is consistent with the tendency for smaller carnivores to be all the more dependent on meat compared to larger carnivores, exemplified by the bears.
The final figure at bottom left focuses solely on bears. The top graph shows that, among the ursids, total intestine length tends to increase in a near linear fashion with body mass (not body length, as per immediately above), which is a well-documented general trend among mammals. The bottom figure reiterates the broader pattern among carnivores illustrated in the top figure at left: the large intestine is increasingly large in comparison to the small intestine as total intestinal length increases. Again, the implication is that large bears (in this case, grizzly bears, represented by the brown dots) are better able to digest a starchy fibrous diet compared to smaller ursids such as black bears and giant pandas. Which also suggests increased capacity to support fiber-digesting bacteria (see below).
Gut microbiome
The figures above devote a lot of space to making a relatively simple point related to the composition of the microbiota of the bear gut. As highlighted by the two graphs in the figure top left, bears have a bacterial composition that is unique among all mammals that have been studied so far. Each gray dot in the graphs top left corresponds to a non-ursid mammal species; each burgundy diamond to a species of bear, including the giant panda. Species are plotted relative to a figurative space that is defined by the two highly abstracted dominant trends in composition of gut microfauna and flora, denoted by 'PCI' and 'PCII'. The graphs are adopted from compilations presented in seminal papers by Frederic Delsuc and Ruth Ley, co-authored with numerous colleagues. The basic point is that bears, barring perhaps the sloth bear (Ursus ursinus), occupy a unique portion of the microbiotic hyperspace. Bears have a gut biota that is unlike that of other mammals, and amazingly similar among bear species despite the fact that bears range from being almost wholly herbivorous (giant pandas) to almost wholly faunivorous (polar bears).
The figure above right begins to define the ways in which the microbiota of bears' guts are different from that of other mammals. The figure is adopted from a diagram in the paper by Frederic Delsuc that shows how various mammal species cluster based on the composition of their gut biota. Different colors correspond to the percent representation of different bacterial phyla in the microbiota of different species' digestive tracts. Again, bears cluster together as a subset of the carnivores. But bears are distinct from all other taxa in that their microbiota are comprised almost wholly of species of the phylum Firmicutes (shown in light blue) and, correspondingly, by a paucity of species belonging to the phyla Bacteroidetes and Tenericutes (in yellow and light purple; but see below). Firmucutes are gram positive organims that include both anaerobic (e.g., Clostridia) and aerobic (e.g., Bacilli) species; of which the anaerobic taxa have the potential to digest complex carbohydrates such as cellulose.
The figure at left presents some results of the rare research that has focused specifically on the gut microbes of bears, most of which was undertaken by Clarissa Schwab at the University of Alberta. Put succinctly, her research shows in greater detail the unique composition of bear gut bacteria, and also that some amount of fiber digestion can and does occur in bear guts, despite a lack of well-developed sites that are capable of supporting anaerobic fermentation.
Specific to this last point, the top graph shows the concentration of total short-chain fatty acids (SCFAs; also know as volatile fatty acids [VFA]) and, of these SCFAs, the concentration of lactate, all in feces collected from various mammal species, including bears. A key point here is that SCFAs, comprised principally of proprionic, butyric, and acetic acids, is diagnostic of bacterial fermentation of complex carbohydrates such as cellulose--a main constituent of 'fiber'. Lactate (a derivative of lactic acid) indicates consumption and fermentation of sugar-rich foods such as berries.
So, the basic point of the figure top left is that, even though the herbivorous species have higher concentrations of SCFAs, SCFAs do occur in bear guts. Which means that some amount of anaerobic fermentation can occur there. At the same time, lactate indicates a potentially sugar-rich diet, at least among the bears that were sampled.
The bottom graph in the figure immediately above focuses on some details regarding differences in bacterial taxa between the three featured bear species (polar, grizzly, and black bears) and the featured non-ruminant herbivores and omnivores (e.g., red panda, zebra, and elephant). The basic pattern distinguishing bears (and skunks) is that of (1) lower concentrations of a key group of anaerobic microbes (Clostridium cluster XIV); (2) lower concentrations of key lactic acid bacteria involved in fermentation of simple carbohydrates (e.g., sugars; Lactobacilli, Pediococci, and Leuconostoc sp.) which, unlike Clostridium, are all facultative anaerobes; and (3) higher concentrations of Enterobacteriaceae (and Enterococci), also facultative anaerobes. As a side note, Clostridim, Enterococci, Lactobacilli, Pediococci, and Leuconostoc sp., are all members of the phylum Firmicutes whereas the Enterobacteria are members of the gram negative phylum Protobacteria. But the more important point is that the facultative anerobes, of which bears have a greater comparative abundance, cannot ferment complex carbohydrates. A key conclusion made by Clarissa Schwab was that the abundance of especially Enterobacteriaceae and Enterococci in bear guts "indicate inefficient feed utilization." Yet, at the same time she concluded that "the presence of SCFAs in the feces of [bears] verified intestinal microbial activity."
The bottom line? Bears guts are, indeed, simple but, especially among the larger bears, endowed with a somewhat enlarged large intestine that engenders potential digestion of complex carbodydrates. The bacterial composition of bear guts is remarkably similar despite a wide diversity of diets among species, from carnivorous polar bears to herbivorous giant pandas. Some bacterial digestion does occur, most by facultative anaerobes that do well digesting simple carbohydrates. Yet some strict anaerobes are present, along with SCFAs that indicate a certain amount of fermentation of complex carbs, albeit much less than occurs in non-ruminant hindgut fermenters.
