Diet Meat Effects
Meat is handy shorthand for denoting a food comprised mostly of protein and fat. As I describe on the page devoted to digestion, protein and fat are the most digestible of all nutrients and, because of that, the essential ingredients of an energy-rich diet. Put a slightly different way, meat is the most concentrated form of protein and fat commonly eaten by bears. Given that we can only rarely directly measure the nutrient composition of bear diets, it is useful at times to use the fraction of meat (or berries, for that matter, to denote little protein) as a proxy and, from that, get a general sense of how a protein- and energy-dense diet affects outcomes such as body size and female reproduction...which is what I address below.
But before I get into the details of meat effects, it is worth noting that not all meat is equal. Fractions of water, protein, and fat vary with the season and from prey animal to prey animal. Emblematic of this variability, at left I've created a summary of variation in the mass of edibles--including body fat--and metabolizable energy for a representative source of meat: female mule deer.
As you can see, the amount of edibles increases (obviously) with animal age, up to around age 4 in the case of mule deer; the fraction of body fat varies with the season; and, with all of this, the amount of metabolizable energy available to a carnivore such as bears varies substantially seasonally and with age of the prey animal.
The figures at right show data that I assembled from North American study areas reporting both the body mass of grizzly bears captured for research purposes as well as information sufficient to determine the percent of the bear diet that was comprised of meat. On the meat front, Garth Mowat was the primary source, based on either direct measures or estimates derived from interpolation of one form or another. I differentiate adult males from adult females, and areas where bears had access to abundant spawning salmon from those where the primary source of meat was land-dwelling herbivores (for details, follow this link and then this one). I also denote the Yellowstone area--a region of particular interest to me--with a white-centered dot.
The top two graphs immediately to the right show trends towards larger size among populations of grizzlies that consume more meat, but moreso for males than for females. The trend among females that eat terrestrial meat is barely discernable, and that of salmon-eating bears is substantially greater than the trend among populations of grizzlies that eat terrestrial herbivores.
The graph at bottom right shows an increase in the ratio of male body mass to female body mass as the percentage of meat in the collective bear diet increases. In other words, size dimorphism of the sexes is greater where bears eat more meat--especially terrestrial meat (the burgundy dots). This is just a different way of representing the lesser response of females versus males to increasing amounts of dietary meat, noting, again, that the trend is greater among interior versus coastal salmon-eating populations. The dampened trend for all populations, including coastal ones, is shown by the solid line whereas the trend for interior populations alone is shown by the dashed line.
The different responses of males versus females and of coastal versus interior populations to increased dietary meat beg for explanation. I can only speculate, but I suspect that differences between the sexes have to do with differences in diet within any given population, as well as differences in sex-linked physiological responses to dietary protein--all driven, in turn, by differences in the reproductive strategies of male versus female grizzlies.
More specifically, although responses of the two sexes to dietary protein have not been explicitly studied (or at least reported) for bears, we do know a fair amount about this phenomenon in two other omnivores: swine and humans. In both Sus scrofa and Homo sapiens, males and females fed the same amount of protein differ in the efficiency with which they accumulate lean body mass. Not surprisingly, males are more efficient than females. So if such were true for bears as well, this phenomenon could plausibly contribute to the greater population-level response of male versus female grizzly bears to elevated dietary meat.
Moreover, study after study has shown that, on average, male grizzlies eat more meat compared to female grizzlies in any given population. This could partly be a result of motivation and resulting dietary preferences. It could also partly be the consequence of on-average larger males being better able to dominate a concentrated food source, as is typical of packages of meat. So differences in diet between males and females could also explain some differences in population-averaged body size between the sexes.
But what about the differences between coastal and interior ecosystems?...between areas with spawning salmon compared to areas without? As I describe on the page devoted to effects of diet protein, bears fed as much salmon as they can eat can grow to a much larger size (albeit at a slower rate) compared to bears fed deer. As the research on that page also shows, a diet exceedingly rich in protein (as would be the case with salmon) actually leads to less efficient accretion of body mass. But this lesser efficiency (as perhaps manifest in the lesser rate of gain among salmon-fed bears) can be more than offset by access to large volumes of food. As would certainly be the case--and for sustained periods of time--in most of the Pacific coastal areas with multiple runs of spawning salmon (see this page on salmon-dominated diets). So, the point here is that the greater response of population-averaged body mass to increasing dietary meat among grizzlies with access to salmon may simply be a consequence of these bears having access to a veritable glut of energy-rich food; perhaps more than it has anything to do with protein content of salmon, as such.
The figures at left use somewhat different data to recapitulate the relationship of female body mass to dietary meat, but with the addition of two graphs, at top, showing the relationship of population-averaged litter sizes to female body mass and diet meat. I extracted these data from a 1999 paper authored by Grant Hilderbrand, although the trend lines are ones that I fit.
The three data points farthest to the right in the bottom graph all represent populations having access to abundant spawning salmon. Without differentiating these three populations, as I did above, you get a sinuous fitted trend line which (as I speculate above) probably has as much to do with the shear abundance of salmon as it does with quantities of meat in the diet, as such.
But the added value of these graphs is the relationship between size of litters (the y axis) and both female body mass and amount of dietary meat (the x axis) in the top graphs. Population-averaged litter sizes increase with both female body mass and the amount of meat consumed. Maybe.
At this point in time, I am a skeptic. There are only 8 populations represented in the graphs at top, and one of the 3 associated with the largest female body mass and greatest amounts of ingested meat has amongst the smallest average litter size. Population density effects? Possibly. But the point is that litter size is probably affected by multiple factors in addition to population-averaged consumption of meat, which, is itself, merely a surrogate for density of digestible energy and nutrients.
In keeping with this last point, there is ample evidence that bears more rapidly gain body fat on an energy-rich but protein-poor diet--as would be the case with berries or any fatty food. And, on top of this, there is additional evidence that the reproductive success of adult females depends more on reserves of body fat than on lean body mass, as such, especially when adolescent females have attained a minimum body size (for more on this see Birth & Death Rates and Growth.
