Calomys callosus, commonly known as the big-eared mouse, is a rodent species found primarily in South America. It inhabits a variety of environments, including grasslands, forests, and agricultural areas. This species is characterized by its large ears, long tail, and soft fur, which can vary in color from brown to gray. These mice are primarily nocturnal and are known for their adaptability to different habitats. They feed on seeds, fruits, and insects, playing a role in their ecosystem as both prey and seed dispersers. Calomys callosus is also of interest in scientific research due to its potential as a model organism in studies of disease and ecology.