Beluga whales deliver their offspring in river mouths because the waters are warmer for their calves that lack fully developed blubber.
During the summer months of May through July, beluga whales are found at the mouths of rivers, where they feed, socialize, and deliver their offspring.
These waters are about ten degrees warmer than their usual arctic environments, at 18 to 20 degrees Celsius (64-68 degrees Fahrenheit), instead of 8 to 10 degrees Celsius (46-50 degrees Fahrenheit).
The warmer waters of the estuary benefit neonate calves, as they have a thinner blubber layer than full-grown adults and a strong dependence on their mothers. Females and their calves are especially tied to the estuary and are the first to return after a disturbance, such as boats or hunting.