Marine mammals such as seals, whales and sea lions have thick layers of blubber to protect them from the frigid water, but the sea otter lacks this insulation; instead, it must rely on its highly-adapted, dense coat of fur. Unfortunately, this coat nearly proved the sea otter's undoing, when, by 1910, the species reached the verge of extinction due to overhunting. The sea otter was extirpated in Canada, but a group from Alaska was reintroduced into British Columbia waters, and the sea otter is making a successful, if precarious, comeback. The sea otter possesses many appealing habits, such as a tendency to take daytime naps while floating on its back in a kelp bed, wrapping itself up in kelp fronds and covering its eyes with its paws. It is also known for its propensity for tool use, a tendency it shares with humans and chimpanzees. The sea otter will often dive deeply, then return to the surface with a sea urchin and a rock. Lying on its back, it will use the rock to crack the spiny test of the urchin.