The giant otter (Pteronura brasiliensis) is one of the world’s largest otters and lives in rivers, lakes, and wetlands of South America such as the Amazon and Orinoco. It has a long, streamlined body, webbed feet, and a broad, powerful tail that it uses to swim skillfully underwater, and it feeds mainly on fish. Unusually for the Mustelidae, it is highly social and lives in family groups centered on a breeding pair and their offspring. Group members communicate with a variety of vocalizations and cooperate to defend territory and young. The white throat markings differ between individuals and serve as identification cues. Once heavily hunted for its fur, it remains threatened by river development, deforestation, water pollution, and competition with fisheries. It is listed as Endangered (EN) on the IUCN Red List.