The giant otter (Pteronura brasiliensis) is a semi-aquatic mammal native to the freshwater systems of South America, primarily found across the Amazon, Orinoco, and La Plata river basins. This animal has evolved a powerful physique for its life as a top predator, earning it the local nickname, the “river wolf.” The giant otter is the longest species within the entire weasel family (Mustelidae), which includes weasels, badgers, and mink. Its dimensions clearly distinguish it from the twelve other otter species found worldwide.
Defining the Giant Otter’s Measurements
The giant otter earns its “giant” designation by surpassing all other otter species in total body length. Adult giant otters typically measure between 1.5 and 1.8 meters (5 to 6 feet) from nose to tail tip. Exceptional individuals have been documented reaching lengths up to 2.0 meters (6 feet, 7 inches).
This length is coupled with a robust, muscular build, resulting in substantial weight. Adult males generally weigh between 26 and 32 kilograms (57 to 71 pounds), while females are slightly lighter. The maximum recorded weight for the species is 34 kilograms (75 pounds). Although the giant otter is the longest mustelid, the sea otter may sometimes exceed it in mass.
The head and body length, excluding the tail, ranges between 1.0 and 1.5 meters (3.3 to 4.9 feet) for females and 1.5 to 1.7 meters (4.9 to 5.6 feet) for males. The powerful tail accounts for the remaining length, measuring up to 70 centimeters (28 inches).
Variation in Size
The measurements of the giant otter are not fixed numbers but rather a range influenced by biological and environmental factors. One key element of this variation is sexual dimorphism, where adult males are generally larger than adult females. Male body length and weight consistently occupy the upper end of the species’ size range.
The difference in size between the sexes is less dramatic than in many other mustelids. A male may weigh a few kilograms more than a female of similar length, but overall body size is not always a reliable indicator of gender in the field. The largest and heaviest individuals within any family group are almost always the dominant breeding pair.
Environmental factors also contribute to subtle size variations across the giant otter’s vast geographic range. Otters living in different parts of the Amazon or Orinoco systems may show slight differences in average size due to local prey availability and habitat conditions. The health and age of the individual also play a role, as older, well-established otters tend to achieve the maximum dimensions for the species.
Physical Features that Define Their Size
The giant otter’s dimensions are shaped by specific physical features that enable its aquatic lifestyle. A substantial portion of the otter’s overall length is attributed to its muscular tail, which is uniquely flattened. Unlike the rounded tails of many other otters, the giant otter’s tail is dorsoventrally flattened, particularly in the last two-thirds of its length. This paddle-like shape acts as a powerful propeller and rudder, driving the animal through the water and contributing to its elongated profile.
The animal’s head is also large and distinctive, described as globular or slightly flattened with a blunt muzzle. This structure features a pronounced sagittal crest, a ridge of bone along the top of the skull that provides a large anchor point for powerful jaw muscles. This strong musculature allows for a powerful bite, underscoring the otter’s status as an apex predator.
The feet are large and broad with extensive webbing. This inter-digital membrane stretches almost to the tips of the strong claws, transforming the paws into flipper-like appendages when swimming. These large, webbed hind feet provide maximum propulsion in its riverine habitat.