The brown bear (Ursus arctos) is one of the largest land carnivores on Earth. In North America, two subspecies—the Kodiak bear and the mainland Grizzly—are often compared for their size and power. Though both are members of the same species, their physical differences are significant. Examining their measurements, classification, and environments reveals the factors that differentiate these mammals.
The Definitive Size Comparison
The Kodiak bear is the largest subspecies of brown bear, consistently exceeding the average size of the mainland Grizzly. This disparity is most evident in the weight of mature males. A male mainland Grizzly (Ursus arctos horribilis) typically ranges from 400 to 800 pounds, while those in the contiguous United States often fall between 400 and 600 pounds. In contrast, a male Kodiak bear (Ursus arctos middendorffi) commonly weighs between 800 and 1,400 pounds, with some individuals surpassing 1,500 pounds before hibernation.
This difference in mass is also reflected in other physical dimensions. The average male Grizzly stands approximately 3.5 to 4.5 feet high at the shoulder, with a body length of 5.5 to 8 feet. The Kodiak bear averages a shoulder height of 4 to 5 feet and can reach an overall body length approaching 9 feet. The population average establishes the Kodiak bear as the heavier and taller subspecies. Female Kodiaks average 400 to 700 pounds, significantly larger than the 250 to 400-pound range typical for female Grizzlies.
Taxonomy and Geographic Range
The Kodiak bear and the Grizzly bear share a close genetic relationship, both classified as subspecies of the brown bear, Ursus arctos. The Kodiak bear is designated Ursus arctos middendorffi, recognized as a distinct population due to its geographic isolation. The mainland Grizzly is classified as Ursus arctos horribilis, representing the widely distributed interior brown bear populations across North America.
Geographic distribution is the primary factor leading to their distinct classifications. Kodiak bears are confined exclusively to the Kodiak Archipelago in southwestern Alaska. This isolation began approximately 12,000 years ago, following the last ice age, allowing the population to evolve separately from mainland relatives. The Grizzly has a much broader, continuous range stretching across the North American mainland, from Alaska and western Canada down into the contiguous United States.
Environmental Factors Driving Size
The size of the Kodiak bear is an outcome of the rich, isolated environment it inhabits, not just a unique genetic lineage. The Kodiak Archipelago provides an extremely reliable and concentrated food supply that is superior to the resources available to most mainland Grizzly populations. This abundance allows Kodiak bears to maximize their growth potential through phenotypic plasticity, where an organism’s size is heavily influenced by its environment and diet.
The most significant dietary component driving the Kodiak bear’s mass is the hyper-abundance of Pacific salmon. These high-protein, high-fat fish ascend the island’s many streams in massive, predictable runs. This offers the bears a caloric density rarely matched in mainland habitats. Kodiak bears can gorge on these salmon for weeks, allowing them to rapidly accumulate the fat reserves necessary to reach their pre-hibernation weights.
In contrast, the mainland Grizzly’s diet is more dispersed and often less calorically rich. While coastal Grizzly populations in Alaska also benefit from salmon, the interior Grizzly relies more heavily on vegetation, roots, berries, insects, and occasionally large prey like moose or elk. This leaner, more varied diet means the interior Grizzly cannot sustain the same rapid rate of growth or achieve the extreme body mass seen in the Kodiak population. The combination of geographic isolation and a consistently calorie-rich coastal environment allows the Kodiak bear to achieve its status as the largest living brown bear subspecies.