The white-ankled mouse (Peromyscus pectoralis) is a species of rodent in the family Cricetidae. It is found in Mexico and in New Mexico, Oklahoma and Texas in the United States. The white-ankled mouse is commonly found in coexistence with the Brush mouse and Texas mouse (P. botlii and P. attaweri). Often, the sympatric overlap in characteristics between these species makes it difficult to identify a specific species. The most distinguishing feature of the white-ankled mouse, and the one most used to identify the species, is the baculum of males (Hooper 1958). The tip of the white-ankled mouse’s baculum is long and cartilaginous, whereas the tip of the Brush and Texas mouse is short and rounded (Clark 1952 and Hooper 1958).