The size of these second year cubs can be compared to the April 8th and 10th posting, which show first year cubs. Grizzly bear cubs generally stay with their mother for two years, although they will stay for three or four if the sow does not become pregnant in the fall of their second year. Pregnancy triggers a reaction in the sow through which she drives the cubs off and hibernates on her own in preparation for giving birth to new cubs the following spring. Cubs will often spend their first hibernation together, and three-year olds observed in frequent close proximity in the spring are most likely to be siblings who have denned together. Even second year cubs stay close to their mother especially when they are on a fishing river in the presence of other bears.