In the world of North American bears, there are considerable advantages to being big. The biggest males generally mate with more females than medium size or small males do. For example, one study found that three large male black bears encountered more than twice the number of females in the breeding season as several smaller males did, and a much higher proportion of these encounters were with receptive females. As a result, the three big males fathered ninety-one percent of the cubs. Being big led to winning more face-offs and fights with other males and perhaps also to being favored by females. Big males are also able to dominate smaller bears and gain almost exclusive access to important food resources in many situations.
Being big also has pay-offs for females. They too are more likely to win threatening encounters with other bears (when they can’t be avoided). Moreover, big females are likely to produce more cubs than smaller females. Research has shown that fat females produce more surviving cubs than less-fat females, because they have more energy for producing milk to feed their new cubs, born during hibernation. Although both small and large females can be fat, large females have better access to food resources, because they can dominate smaller bears, and they can carry more fat on their large frames.
Here in Southeast, researchers suggest that access to spawning runs of salmon in late summer and fall allows bears to become both bigger and fatter than bears that don’t have access to salmon. Eating meat, especially salmon, seems to allow bears to grow extremely big. However, some bears in Southeast don’t come to the salmon runs, staying instead in the alpine zone. Apparently they give a higher priority to avoiding the risks of encountering dominant bears that rule the salmon streams, and they probably have lower reproductive success. Spawning runs of other fishes offer foraging advantages to hungry bears too: It would be interesting to learn if the grizzly bears feeding on the spawning runs of broad whitefish in the Mackenzie River in the Northwest Territories get bigger and fatter than those without access to the runs.
Being big has its advantages, certainly, but there is also a ‘down side’ to large size. Big bears can’t run as fast as smaller ones, and researchers suggest that they are more likely to hunt by ambushing prey rather than pursuing it. Big bears can climb trees but they are much less agile in doing so than smaller bears. Trees offer refuge, especially to smaller bears, from other, larger bears. Tree climbing also gives agile bears access to food in some cases. For example, in spring black bears forage on cottonwood flowering catkins and young seed pods; near the Visitor Center they sometimes strip the trees of most of their branches in order to reach the catkins and pods. A really big black bear would have trouble clambering up many of the middle-sized cottonwoods up by the Visitor Center to gather the edible catkins, but the smaller bears do so with apparent ease. Later, in the summer, both black and brown bears in Southeast climb wild crabapple trees to get the fruit; outside of Southeast, bears climb (or did so before they were exterminated in many states) many kinds of trees to reach the fruit.
Big body size also makes it difficult to gain weight in preparation for hibernation by eating vegetation alone. Putting on fat is necessary for survival during the long months of hibernation and for females to produce milk to feed their cubs. Although our bears commonly eat a lot of green plant food, they can’t digest plant fibers. So apparently big bears just can’t get enough nutritious plant material to put on the necessary weight. Very big bears probably also have difficulty gaining weight on a diet of berries, except perhaps in really good ‘berry years.’ It could be argued that the bigger the bear, the more meat it needs to eat; and conversely, meat eating is necessary to achieve large size in the first place.
Being big has another major disadvantage: Hunters often take pride in killing large animals, be they sheep or goats or bears. So trophy hunting imposes a risk on large body size. The consequences of removing large, dominant individuals from a population are well understood (but commonly ignored): loss of the large individuals upsets to social organization and probably increases the risk of infanticide by previously subordinate male bears (killing cubs tends to bring the female back into breeding readiness). More mating by smaller bears eventually results in a population of smaller bears, because the genes for large size become less common in the population.