Zygodontomys brevicauda ranges in head-body length from 95 to 155 mm, and its tail ranges from 35 to 130 mm in length. Tail length is typically three-quarters the length of an individual's head-body length. Adults weigh 60 to 80 g, and captive-born pups weigh 3 to 4 g at birth. Zygodontomys brevicauda has grizzled brown pelage with hues of yellow and red. The venter ranges from grayish-white to darker shades of gray. Pelage color varies with population and habitat type. Island residents are more richly pigmented than those found in arid, mainland regions.
Members of Zygodontomys can be distinguished from most sigmodontinids by their shorter tails and short hind feet, and can be distinguished from Akodon (i.e., grass mice) by the short nails on their forefeet. Zygodontomys brevicauda can be distinguished from its closest extant relative, Zygodontomys brunneus, by its smaller size and deeper zygomatic notches. Sexual dimorphism has not been reported in this species.
Average mass: 80 g.
Range length: 95 to 155 mm.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry