During the summer, when the whales gorged on krill to build up fat reserves, they sang mostly during the night.

This forces the body to get energy from fat reserves and muscle, and leads to rapid weight loss.

After gorging themselves through the summer to build fat reserves, they retreat to a den where their heart rate plummets.

To remain in stasis for long periods, these animals build up brown fat reserves and slow all body functions.

Males typically enter hibernation earlier than females, and adults earlier than young-of-the-year, which need more time to build up fat reserves.