A reliable method for sexing giant otters (Pteronura brasiliensis) in the wild

Sexing of giant otters under field conditions is only possible when individuals are entirely out of the water, usually when basking or grooming. Body size is not indicative of gender, and the breeding pair does not necessarily consist of the largest animals in the group. Sexing is facilitated by the presence, in adult, parous females, of four permanently elongated teats (due to repeated and prolonged lactation) and, in mature males, of a visible scrotum. Identification of the breeding male is sometimes assisted his behaviour, specifically that of marking, and by his year-on-year permanence in the group. However, sexing is more difficult in adult females that have not lactated, or in sub-adults, juveniles, and cubs of both sexes. Here, we present a reliable method of sexing wild Giant otters of all ages and sexual status, tested with known sex individuals.