Body Mass Effects
The relationship shown above is an interesting and compelling introduction to the effects of body mass on foraging efficiencies and weight gain of bears. Each dot represents the results of a controlled experiment involving one bear allowed free access to different kinds of foods. All of these data, again, thanks to research by Dr. Charles Robbins and his graduate students at Washington State University.
At one level, the figure above shows something that might be considered self-evident: larger bears tend to ingest greater volumes when given unlimited access to food. But, then, they would need to given their greater energetic needs, even accounting for the lower rate at which metabolic needs increase relative to each increment of body mass (at roughly the 0.75 power). Energy consumption, even at rest, increases nonetheless.
But the interesting thing is that the rate at which intake increases varies substantially among foods, but especially for berries in contrast to everything else. You might expect intake of protein-rich foods such as deer or salmon to increase at a lesser rate as body mass increases. Each gram of such a food delivers a subtantial amount of digested energy entailing a comparatively slow rate of passage through the gut, with gut length and passage rate scaling at less than a 1:1 rate with body size (see Digestion and Gut morphology). And intake of deer is probably less than intake of salmon at any given body mass because of the comparatively greater effort required to process a gram of tissue from a deer compared to a more easily ingested and chewed gram of fish.
But what about foliage and berries? It would be reasonable to expect that intake of foliage would increase at roughly the same rate as intake of berries given that both of these are vegetal foods. So why is the intake of foliage so depressed compared to berries for large bears? For the answer, see the page of Foraging Efficiencies. But, in short, when bears graze, ingestion is partly limited by the time it takes to chew a mouthful of fibrous foliage, which means that bite rate drops as a function of bite size. Which means that bears need large standing volumes of tall-statured foliage to increase total ingested volumes in defiance of the limits imposed by chewing time--which rarely happens.
Insofar as berries are concerned, again, remember that the figure above pertains to bears offered unlimited access to food. And the probable explanation for ingestion of such large volumes as body mass increases? Unlike with foliage, processing time for berries, once ingested, is not of great consequence. Moreover, bears are highly motivated to ingest large volumes of protein-deficient berries (see Digestion) in an effort to meet their protein requirements, but with a resulting glut of digested energy which is either expended as heat or stored as body fat (see the page on Protein and Energy Effects for more details on this).
But where things get really interesting is when you look at per day rates of gain in body mass as a function of bear size--especially under circumstances where the involved bears had access to unlimited amounts of different kinds of food...or not.
The graphs at left summarize this relationship for a number of individual bears that ate fruit (A), grazed foliage (B), and consumed meat from either deer or salmon (C). The open circles denote the results for captive bears; the brown dots, results for grizzly bears in the wild; and the gray squares, for black bears in the wild. Importantly, the captive bears had access to unlimited amounts of the various foods, whereas the wild bears typically did not.
It perhaps make sense to go over these graphs in reverse order, from bottom to top, starting with meat. Notice that, unlike with foliage and fruit, the trend lines for captive (dotted) and wild (solid) bears increase without reaching a plateau or peak. Which means that bears are able to grow increasingly large even as their body mass increases and, when fed unlimited amounts of meat, these gains can be phenomenal--upwards of 4 kg per day for a 300 kg bear. Moreover, no bear lost body mass eating a diet comprised mostly of meat. And, it is worth emphasizing that these gains were mostly as lean body mass, and despite the depressed per kg energetic efficiencies of a high protein diet (see Protein and energy effects).
Effects on rate of weight gain
With foliage, it is perhaps not surprising that a number of wild bears seemed to lose body mass when subsisting almost wholly on such a food. Foliage is not very nutritious nor digestible (see Digestion). No wild bear but one gained much weight. But, the important trend is the one documented for captive bears with access to unlimited amounts of foliage (the dashed line). These captives were able to gain weight up until around 125 kg of body mass, after which gains declined, even into negative territory around 250 kg. In other words, the modest increases in ingested volumes of foliage that occurred at large body masses (see above) could not offset the increased energetic needs of a large bear, even given a less than 1:1 scaling of unit volume metabolic rate with body mass.
Something very similar was evident for wild bears subsisting on berries. Gains increased, peaking for bears of around 100 kg mass, and thereafter declined, even into negative territory for one c. 150 kg grizzly. To explain this pattern requires additional explanation of graph A. The ascending dotted red and blue lines denote the theoretical maximum rates of gain for bears eating unlimited quantities of serviceberries and huckleberries; that is, for captive bears. But this theoretical maximum is obviously not achieved, primarily because of the limits imposed by foraging inefficiencies under field conditions; for example bite sizes and bite rates. These limits are described by the descending gray lines--the farthest left associated with a bite size of 1.7 berries and a bite rate of 55 per minute, the farthest right with a bite size of 2.5 berries and a rate of 90 per minute. The point being that foraging efficiencies take a major toll on what bears can realize from eating berries, with major implications for even modestly-sized bears. Notably, though, at peak, daily gains for wild bears eating berries were over three times greater than gains for wild bears eating foliage: roughly 0.5 versus 0.15 kg per day.
There are some important implications of all this: Most important, if you are a bear weighing much in excess of 100 kg, weight gains are going to be greatest on a meat diet, not a berry or foliage diet. And much of these gains will be in lean body mass (see Protein and energy effects). So, given the consistently greater size of adult males compared to other bears--typically in excess of 150 kg (see Variation in size)--you would expect adult males to eat disproportionate amounts of meat. Which is, indeed, the case. And, adopting such a strategy, you would expect males to grow ever larger over time, which seems to be the case as well (see Growth). But all of this with an important proviso: If you are female weighing somewhere between 100-150 kg, rates of body fat accumulation will probably be greatest on an energy-rich but protein-poor berry diet, as long as you can eat large volumes, closer to 2.5 berries per bite, at 90 bites per minute.
One important additional proviso: None of the graphs above deal with foods rich in fat, but containing adequate protein. More concretely, we don't have a clear picture of the nutritional benefits entailed for bears eating army cutworm moths and whitebark pine seeds: two of the most important bear foods in the Yellowstone ecosystem. Although, it is not too hard to imagine that these foods are among the best bear foods of all.
Bears in context as grazers
As a final note on body mass effects, it is worth placing bears--grizzlies specifically--in context of other species. Given the paucity of comparable information on frugivores and meat specialists, most of the relevant information pertains to grazers.
The two graphs at right show how maximum rates of ingestion, rates of processing ("chews"), and chews per gram of ingested volume vary for eight grazers as a function of body mass--including the omnivorous grizzly bear. One important point: as the size of an herbivore increases, the size and mass of the grinding apparatus increases--which is to say, the jaw and masseter and temporalis muscles. Which predictably slows the rate at which the jaw and its associated muscles work (because of the entailed intertia), whether reckoned as chews per minute or chews per gram of ingested food.
And such is the case, as clearly shown in figure B. Specifically for grizzlies: observed chew rates don't vary much from what you would expect by the overall relationship with body mass. If anything, grizzlies tend to chew a little faster, but a little less given the size of the ingested bite, than you would expect by their size.
Graph A, above, distills all of the mechanisms associated with grazing into one outcome: the maximum potential rate at which an herbivore processes grazed herbage. More to the point, the potential rate of processing increases proportionately more rapidly than does body mass of the involved species; at a rate greater than 1:1 (opposite of the rapidity of jaw action, as such). The largest grazers ingest and process proportionately more volumes than do the smallest grazers. But, then, the digestive strategy of megaherbivores is often to pass large volumes through the gut while digesting less of the throughput compared to small or moderate-sized herbivores, most of which are ruminants.
And notice grizzly bears. They, along with one other species, exhibit the greatest positive deviation of all from the relationship expected from body mass alone, albeit based on only eight species. Which is to say, the pattern above is consistent with a grazing strategy on the part of grizzlies that entails passing a comparatively large volume of comparatively under-processed foliage through the gut in a relatively short period of time. Which is, in fact, consistent with rates of transit for vegetal foods that have been observed for bears (see Digestion).