 The diet of the fossa in the wild has been studied by analyzing their distinctive scats, which resemble gray cylinders with twisted ends and measure 10 – 14 cm (3.9 – 5.5 in) long by 1.5 – 2.5 cm (0.6 – 1.0 in) thick. Scat collected and analyzed from both Andohahela and Andringitra contained lemur matter and rodents. Eastern populations in Andringitra incorporate the widest recorded variety of prey, including both vertebrates and invertebrates. Vertebrates consumed ranged from reptiles to a wide variety of birds, including both understory and ground birds, and mammals, including insectivores, rodents, and lemurs. Invertebrates eaten by the fossa in the high mountain zone of Andringitra include insects and crabs. One study found that vertebrates comprised 94 % of the diet of fossas, with lemurs comprising over 50 %, followed by tenrecs (9 %), lizards (9 %), and birds (2 %). Seeds, which comprised 5 % of the diet, may have been in the stomachs of the lemurs eaten, or may have been consumed with fruit taken for water, as seeds were more common in the stomach in the dry season. The average prey size varies geographically; it is only 40 grams (1.4 oz) in the high mountains of Andringitra, in contrast to 480 grams (17 oz) in humid forests and over 1,000 grams (35 oz) in dry deciduous forests. In a study of fossa diet in the dry deciduous forest of western Madagascar, more than 90 % of prey items were vertebrates, and more than 50 % were lemurs. The primary diet consisted of approximately six lemur species and two or three spiny tenrec species, along with snakes and small mammals. Generally, the fossa preys upon larger lemurs and rodents in preference to smaller ones.
