[5] The giant otter shows a variety of adaptations suitable to an amphibious lifestyle, including exceptionally dense fur, a wing-like tail, and webbed feet.

The giant otter subsists almost exclusively on a diet of fish, particularly characins and catfish, but may also eat crabs, turtles, snakes and small caimans.

[1] P. b. brasiliensis is distributed across the north of the giant otter range, including the Orinoco, Amazon, and Guianas river systems; to the south, P. b. paraguensis has been suggested in Paraguay, Uruguay, southern Brazil, and northern Argentina,[14] although it may be extinct in the last three of these four.

[17] The earliest fossil evidence of the giant river otter dates to the Late Pleistocene of Argentina, and it was slightly larger than known modern specimens.

The smooth-coated otter (Lutrogale perspicillata) of Asia may be its closest extant relative; similar behaviour, vocalizations, and skull morphology have been noted.

[23][24] Early reports of skins and living animals suggested exceptionally large males of up to 2.4 m (7 ft 10+1⁄2 in); intensive hunting likely reduced the occurrence of such massive specimens.

The giant otter's highly sensitive whiskers (vibrissae) allow the animal to track changes in water pressure and currents, which aids in detecting prey.

Early travelers' reports describe noisy groups surrounding explorers' boats, but little scientific information was available on the species until Duplaix's groundbreaking work in the late 1970s.

[41] Duplaix suggests a division between "residents", who are established within groups and territories, and nomadic and solitary "transients"; the categories do not seem rigid, and both may be a normal part of the giant otter life cycle.

Defence against intruding animals appears to be cooperative: while adult males typically lead in aggressive encounters, cases of alpha females guarding groups have been reported.

[40] In another instance in Brazil, a carcass was found with clear indications of violent assault by other otters, including bites to the snout and genitals, an attack pattern similar to that exhibited by captive animals.

In Cantão State Park, otters dig their reproductive dens on the shores of oxbow lakes starting around July, when waters are already quite low.

They give birth between August and September, and the young pups emerge for the first time in October and November, which are the months of lowest water when fish concentrations in the dwindling lakes and channels are at their peak.

[5] At Tierpark Hagenbeck in Germany, long-term pair bonding and individualized mate selection were seen, with copulation most frequently taking place in water.

Insufficient land area proportions and unsuitable substrate conditions in zoos have historically been the primary cause of high cub mortality and physical and behavioral health problems among giant otters.

[53][54][55] In the wild, it has been suggested, although not systematically confirmed, that tourists cause similar stresses: disrupted lactation and denning, reduced hunting, and habitat abandonment are all risks.

Giant otters catch their own food and consume it immediately; they grasp the fish firmly between the forepaws and begin eating noisily at the head.

Giant otters adopt communal latrines beside campsites, and dig dens with a handful of entrances, typically under root systems or fallen trees.

Utreras[66] presumed habitat requirements and availability would differ dramatically in the rainy season: estimating range sizes of 1.98 to as much as 19.55 square kilometres (0.76 to 7.55 sq miles) for the groups.

[14] In 2021, conservationists at Fundación Rewilding spotted a wild giant otter swimming in the Bermejo River in Impenetrable National Park, located in the Chaco province of northeast Argentina.

[63] In addition, solitary animals and young may be vulnerable to attacks by the jaguar, cougar, and anaconda, but this is based on historical reports, not direct observation.

The implementation of CITES in 1973 finally brought about significant hunting reductions,[14] although demand did not disappear entirely: in the 1980s, pelt prices were as high as US$250 on the European market.

[79] Specific threats from human industry include unsustainable mahogany logging in parts of the giant otter range,[76] and concentrations of mercury in its diet of fish, a byproduct of gold mining.

Ecotourism also presents challenges: while it raises money and awareness for the animals, by its nature it also increases human effect on the species, both through associated development and direct disturbances in the field.

The IUCN lists Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Paraguay, Peru, Suriname, and Venezuela as current range countries.

[39] The species has likely been extirpated from southern Brazil, but in the west of the country, decreased hunting pressure in the critical Pantanal has led to very successful recolonization; an estimate suggests 1,000 or more animals in the region.

[2] A significant population lives in the wetlands of the central Araguaia River, and in particular within Cantão State Park, which, with its 843 oxbow lakes and extensive flooded forests and marshlands, is one of the best habitat patches for this species in Brazil.

[84] Duplaix returned to the country in 2000 and found the giant otter still present on the Kaburi Creek, a "jewel" of biodiversity, although increased human presence and land use suggests, sooner or later, the species may not be able to find suitable habitat for campsites.

A study of five indigenous communities in Colombia suggests native attitudes toward the animal are a threat: the otters are often viewed as a nuisance that interferes with fishing, and are sometimes killed.

They appear in a fish poisoning legend where they assist a man who has wasted his sexual energy, creating the anacondas of the world from his distressed and extended genitals.