August 14, 2011 : Giant Otter

The giant otter (Pteronura brasiliensis) is a South American carnivorous mammal. It is the longest member of the Mustelidae, or weasel family, a globally successful group of predators. Unusually for a mustelid, the giant otter is a social species, with family groups typically supporting three to eight members. The groups are centered on a dominant breeding pair and are extremely cohesive and cooperative. Although generally peaceful, the species is territorial and aggression has been observed between groups. The giant otter is diurnal, being active exclusively during daylight hours. It is the noisiest otter species and distinct vocalizations have been documented that indicate alarm, aggressiveness, and reassurance. The giant otter ranges across north-central South America. The giant otter lives mostly in and along the Amazon River and in the Pantanal.

The giant otter is an apex predator and its population status reflects the overall health of riverine ecosystems. It feeds mainly on fish, including cichlids, characins (such as piranha), and catfish. One full year study of giant otter scats in Amazonian Brazil found fish present in all fecal samples. Fish from the order Perciformes, particularly cichlids, were seen in 97% of scats, and Characiformes, such as characins, in 86%. Fish remains were of medium-sized species that seem to prefer relatively shallow water, to the advantage of the probably visually oriented giant otter. Prey species found were also sedentary, generally swimming only short distances, which may aid the giant otter in predation. The giant otter seems to be opportunistic, taking whatever species are most locally abundant. If fish are unavailable it will also take crabs, snakes, and even small caimans and anacondas.

The species can hunt singly, in pairs, and in groups, relying on its sharp eyesight to locate prey. In some cases, supposed cooperative hunting may be incidental, a result of group members fishing individually in close proximity; truly coordinated hunting may only occur where the prey cannot be taken by a single giant otter, such as with anacondas and the Black Caiman. The giant otter seems to prefer prey fish that are generally immobile on river bottoms in clear water. Prey chase is rapid and tumultuous, with lunges and twists through the shallows and few missed targets. The otter can attack from both above and below, swiveling at the last instant to clamp the prey in its jaws. 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. Carter and Rosas have found that captive adult animals consume around 10% of their body weight daily—about 3 kilograms (7 lb), in keeping with findings in the wild.

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