This mother grizzly bear approached us, sat down, dug a small day bed in the rocks, and started nursing. Since she lives in Katmai National Park, where hunting is prohibited, she has learned to trust humans. Mother bears will defend their cubs aggressively if needed but, like most bear conflicts, she will avoid physical contact if possible. Most bears growl and posture at one another, but actual physical violence is rare. Standing on their hind legs is also rare as a defensive posture in grizzly bears. Mother bears are much more likely to stand on all fours between their cubs and a potential threat. They may also bluff charge a potentially threatening bear, once again being on all fours. Bears usually stand on hind legs to see over tall grass, or to get a better sniff of a scent that blew in. They also stand on hind legs to look for salmon in the water. Standing on their hind legs as an aggressive action is mostly seen in the movies, where grizzlies are made to look much more imposing and aggressive than they actually are in real life. My husband wrote a great article on this topic called Are Grizzlies Dangerous?
Grizzly Bear Blog 