The American black bear (Ursus americanus) is a medium-sized bear native to North America. It is the continent's smallest and most common bear species. Black bears are omnivores, with their diets varying greatly depending on season and location. They typically live in largely forested areas, but do leave forests in search of food. Sometimes they become attracted to human communities because of the immediate availability of food. The American black bear is listed by the IUCN as Least Concern, due to the species' widespread distribution and a large global population estimated to be twice that of all other bear species combined. American black bears often mark trees with their claws to show dominance in an area. Dominance is determined by the highest claw mark found on the tree. This behavior is common to many species of bears found in the United States and CanadaSows usually produce their first litter at the age of 3–5 years. Sows living in urban areas tend to get pregnant at younger ages. The breeding period usually occurs in the June–July period, though it can extend to August in the species' northern range. The breeding period lasts for 2–3 weeks. Sows tend to be short tempered with their mates after copulating. The gestation period lasts 235 days, and litters are usually born in late January to early February. Litters usually consist of two cubs, though litters of 6 have been recorded. At birth, cubs weigh 10–16 ounces (280–450 g), and measure 8 inches in length. They are born with fine, gray, downlike hair, and their hind quarters are underdeveloped. They typically open their eyes after 28–40 days, and begin walking after 5 weeks. Cubs are dependent on their mother's milk for 30 weeks, and will reach independence at 16–18 months. At the age of six weeks, they attain 2 lb, by 8 weeks they reach 5 lb and by the age of 6 months they weigh 40–60 lb. They reach sexual maturity at the age of three years, and attain their full growth at 5 years.