Mammals
Giant Otter
Pteronura brasiliensis · also called giant otter, giant river otter, river wolf
The giant otter is the Amazon’s “river wolf” — the longest weasel on Earth, and the only otter that lives in large, loud, cooperative families that patrol a river like a pack. A group of giant otters hunting a lagoon or mobbing a caiman is one of the loudest and most coordinated spectacles in the Neotropical wetlands.
Overview
Giant otters live in the slow rivers, oxbow lakes, and seasonally flooded forest of the Amazon, Orinoco, and Pantanal systems. They need clear, fish-rich, calm water with sloping banks where a family can clear communal campsites — trampled areas where the group rests, grooms, and marks a shared latrine. Each family is a breeding pair plus their offspring from several years, all of whom help feed and guard the youngest pups.
The longest mustelid
At up to 1.8 metres, the giant otter is the longest member of the weasel family — longer than a wolverine or a European badger, though the stockier sea otter outweighs it. It has enormous webbed feet, a broad, muscular, wing-shaped tail, and short, dense, water-shedding fur. Every otter has a unique cream throat marking that identifies it.
River wolves
The “river wolf” name fits their behaviour. Families hunt cooperatively, herding fish and working a pool together, and eat 3–4 kilograms of fish each per day. They are fiercely territorial, patrolling boundaries and driving off rival groups, and they will collectively mob a caiman — or harass a jaguar — that threatens the pups. They are also exceptionally vocal, with a repertoire of around 15–22 calls from soft coos to an explosive alarm snort.
Status
The giant otter is Endangered. It was hunted almost to extinction for its pelt through the mid-1900s, with hundreds of thousands of skins traded before protection came in the 1970s. It has rebounded in some reserves but now faces forest clearance, gold-mining mercury pollution (which builds up in the fish it eats), overfishing, dams, and tourism disturbance. It is extinct in Argentina and reduced to remnants across much of its former range. Perhaps 1,000–5,000 remain.
Giant Otter: common questions
How big is a giant otter compared to other otters?
A giant otter can reach 1.8 metres from nose to tail tip, making it the longest species in the entire weasel family (Mustelidae), which also includes weasels, badgers, wolverines, and minks. It is not the heaviest otter — the sea otter is shorter but much more thickset and can weigh 45 kilograms against the giant otter's 30 — but no mustelid is longer. Giant otters also have proportionally huge webbed feet, a flattened, wing-like tail, and dense velvety fur.
Why are giant otters called "river wolves"?
The Spanish name lobo del río, "river wolf," reflects how they live and hunt. Like wolves they are highly social pack animals built around a dominant breeding pair, they hunt cooperatively — a family will herd fish or work a lagoon together — and they aggressively defend a group territory, patrolling its boundaries and driving off intruders. A family of giant otters will also gang up on a caiman that comes too close to the pups, mobbing and biting it until it leaves, and groups have been recorded killing small caimans and even harassing jaguars.
Why is the giant otter endangered?
The species was hunted relentlessly for its exceptionally dense, valuable pelt through the mid-20th century — hundreds of thousands of skins were exported — and was eliminated from large parts of its range before hunting was banned in the 1970s. It has recovered in some protected areas but now faces newer threats — clearing and flooding of riverine forest, gold mining that fills rivers with mercury (which concentrates in the fish the otters eat), overfishing that removes their food, dams, and disturbance from boats and tourism. Its need for large, clean, fish-rich, undisturbed river systems makes it very sensitive to change.
Are giant otters dangerous to people?
Not normally. They are curious and will approach boats, and a family defending pups or a kill can be intimidating — surfacing together, snorting, and periscoping to look at an intruder — but attacks on humans are extremely rare and usually involve a cornered animal or a habituated one being fed. The reverse is the real story — people have been by far the greater danger to giant otters. In captivity a large male's bite is powerful enough to be taken seriously by keepers.