Otters are the true and only amphibian ferrets, and although all otters are very similar in appearance, different species have distinctly different behavior habits. Otters forage in the water most of the time, but also spend a lot of time resting on land, with the exception of the sea otter, which has never come to shore in its entire life.
Otters have a very dense layer of fluff, which can reach about 70,000 per square centimeter; there is also a long layer of bristles that can act as an air barrier when they are in the water. Since the otters that often dive do not have a fat layer like seals and sea lions, they need to regularly enter the water to rinse their fur so that it maintains the quality of the water without allowing the water to penetrate into the skin.
Otters have a relatively long body, soft and easy to bend, and are very suitable for swimming in the water for a long time. Most species of otters have short limbs and webbing on their palms. Most otters have hairy tails with thick roots and sharp tail ends, while some species of otters have flattened tails laterally. There are many tentacles around the nose and mouth, as well as on the elbows, which can be sensitive to determining the position of prey. Otters have small, rounded ears; when underwater, the valves of the ears and nostrils close automatically so that water does not enter. Most species of otters have claws (here the claws refer to the sharp, curved nails on the toes, and the claws mentioned below refer to this), but some species have small claws that are almost completely degenerate, such as the African small claw otter and the otter of the genus Otter. The claws of these otters are small but sensitive, especially useful when searching for prey in the rocks or picking up the remains of crabs, at which point long toes can be extended.
Otters feed mainly on fish, but most species of otters also eat frogs, crawfish, crabs, and some birds and small mammals. The fish they prey on are mainly some species that live underwater and are slow to move, such as eels, and sometimes also prey on some fish that swim quickly but stay in one place for a while.
Otters have an extremely high metabolic rate, which may be to adapt to life in the water. Water is a rapidly conductive carrier, and the heat in the otter's body is quickly carried away by the water, so the otter needs to be constantly active to maintain body temperature, so the metabolic rate is high. Otters living in Europe and Asia eat large amounts of fish every day, weighing up to 15% of their body weight, in order to maintain their body temperature in the water. Sea otters require a larger amount of food, but it is difficult to estimate the exact number. In order to catch prey with high energy, otters need to take a lot of risks, and the predation strategy they adopt may require a lot of physical energy. This also makes the otter have to constantly adjust according to the amount of food available when preying. For example, in water with a temperature of 10 °C, otters are in danger of life if they cannot catch at least 100 grams of fish in 1 hour, and to avoid this danger, they have to change the way they prey. Otters typically go into the water for 3 to 5 hours a day, and the female otter that feeds her cubs may take up to 8 hours, while the sea otters hardly leave the water. Most otters fish alone or prey on other prey, but some species of otters, such as large otters, slippery otters, small claw otters, North American otters, and spot-necked otters, may hunt cooperatively in groups.
Common otters in Europe and Asia are the most widely distributed species, and before the extinction of the Japanese otter in the 1970s, there were otters from Japan to Ireland, from Siberia to southwest to Sri Lanka in the Indian Ocean, and further on, the distribution range may be even wider. This common otter is generally not found in the same place as the hairy-nosed otter, but will occasionally find it in the same river as the slippery otter and the small clawed otter. Similarly, some species of the American otter genus in the New World generally do not appear in the same place, only the North American otter and sea otter sometimes have a common habitat, and the long-tailed otter and the large otter sometimes appear in the same river. In Africa, the two species of small-clawed otters (the African small-clawed otter and the Congolese small-clawed otter) do not appear in the same place, but the two may appear in the same place as the spot-necked otter.