Taxonomist C.H. Merriam was first to recognize Kodiak bears as unique and the only subspecies of the brown bear, and he named the species "Ursus middendorffi" in honor of the celebrated Baltic naturalist Dr. A. Th. von Middendorff. Subsequent taxonomic work merged all North American brown bears into a single species (Ursus arctos). Genetic samples from bears on Kodiak have shown that they are related to brown bears on the Alaska Peninsula and Kamchatka, Russia, and all brown bears roughly north of the US. Kodiak bears have been genetically isolated since at least the last ice age (10,000 to 12,000 years ago) and very little genetic diversity exists within the population. Although the current population is healthy and productive, and has shown no overt adverse signs of inbreeding, it may be more susceptible to new diseases or parasites than other, more diverse brown bear populations.
Kodiak bears reach sexual maturity at age five, but most sows are over nine years old when they successfully wean their first litter. The average time between litters is four years. Sows continue to produce cubs throughout their lives, but their productivity diminishes after they are 20 years old. Mating season for Kodiak bears is during May and June. They are serially monogamous (having one partner at a time), staying together from two days to two weeks. As soon as the egg is fertilized and divides a few times, it enters a state of suspended animation until autumn when it finally implants on the uterine wall and begins to grow again. Cubs are born in the den during January or February. Weighing less than a pound (<450 g) at birth with little hair and closed eyes, they suckle for several months, emerging from the den in May or June, weighing 15–20 pounds (6.8–9.1 kilograms). Typical litter sizes on Kodiak are two or three cubs, with a long-term average of 2.4 cubs per litter. However, Kodiak bears have six functional nipples and litters up to six cubs have been reported. Sows are sometimes seen with five or six cubs in tow, probably due to adopting cubs from other litters. Most cubs stay with their mothers for three years. Almost half of the cubs die before they leave, with cannibalism by adult males being one of the major causes of death.
Kodiak bears that have recently left their mothers, at ages 3–5 years, have high mortality rates with only 56% of males and 89% of females surviving. Most young female bears stay within or near their mother’s home range, while most males move farther away. Most adult sows die of natural causes (56%), while most adult male bears are killed by hunters (91%). The oldest known boar in the wild was 27 years old, and the oldest sow was 35.