The polar bear owes its scientific name 'Ursus maritimus' to the swimming ability it has developed to reach packs of floating ice up to hundreds of kilometers from the mainland. Its elongated neck gives it a more hydrodynamic silhouette than the other bear species yet it has neither the otter's webbed feet nor the beaver's flattened rudder tail. Only its 1.9 to 3.9in (5-10cm) thick layer of fat helps it to float and protects it from icy water. It is a slow but enduring swimmer with a top speed of 4.9mi/h (8km/h).