Bears can climb trees, although black bears are better tree-climbers than grizzly bears. Bears have excellent senses of smell and hearing, and better sight than many people believe. Bears are strong. They can tear cars apart looking for food. Intrusion into this space is considered a threat and may provide an attack.
Bears aggressively defend their food.
All female bears defend their cubs. If a female with cubs is surprised at close range or is separated from her cubs, she may attack. An aggressive response is the mother grizzly's natural defence against danger to her young. A female black bear's natural defence is to chase her cubs up a tree and defend them from the base. However, she is still dangerous and may become aggressive if provoked.