They are also adapted to hear infrasound frequencies below 20 hertz. Swimming speeds of up to 20mph have been recorded, but normally manatees prefer to cruise at speeds of at 2-6mph. They will typically surface to breathe through their nostrils every three to five minutes. When they are using a great deal of energy, however, manatees may surface for air as often as every 30 seconds. They can stay submerged for up to 15-20 minutes.
Manatees have a slow reproductive rate.
More normally, one calf is born every 2-5 years. At birth, the calf is around 3-4 feet long. Guided by the mother, it swims to the surface to breathe. The manatee gestation period is over a year, usually around 13 months. Mothers nurse their young for a long period.