Granitic Isla Partida, about 2 square miles in size, lies just south of Angel de la Guardia. Because of its lack of vegetation , bats are the only resident mammals. At sundown ,this desolate island comes alive with the sounds of thousands of Fishing Bats (Pizonyx vivesi)which crawl from their subterranean refuges and take flight together in search of food. They swoop down over the water of the Gulf gathering small fish and crustaceans. During the day, these bats are found beneath rock crevices on the talus slopes of Partida. The are not found elsewhere in the world