Peromyscus pectoralis
White-ankled Mouse
Peromyscus pectoralis and P. boylii are closely related morphologically and seem to occupy the same habitat. In the Sierra Madre Oriental, according to the field notes of the collector (Heinrich, June 6 to August 5, 1953), individuals of P. pectoralis had a pinkish coloration on the mouth and forefeet produced by the juice of the "nopal" cactus fruit, on which obviously the mice feed, whereas only a few specimens of boylii were thus discolored. It was noted that boylii was feeding on acorns. Furthermore, the two species may differ in time of breeding; in August, males of pectoralis had the testes well developed when those organs were small in boylii collected at the same locality.
A specimen from 53 kilometers north of El Limón, was shot at a height of 10 feet on a concrete underpass. Other specimens were taken in a trap line that yielded Peromyscus boylii, P. leucopus and Liomys irroratus.
Two subspecies of P. pectoralis occur in Tamaulipas: P. p. collinus is widely distributed in the central and western parts of the state and P. p. eremicoides occurs only in the western "corner" of the state.
Peromyscus pectoralis collinus Hooper
1952. Peromyscus pectoralis collinus Hooper, Jour. Mamm., 33:372, August 19, type from San José, 2000 ft., Sierra San Carlos, 12 mi. NW San Carlos, Tamaulipas.
Distribution in Tamaulipas.—Along the central and western mountains.
A female obtained on January 21 at a place 53 kilometers north of El Limón, contained three embryos. A lactating female was taken on August 2 in the Sierra Madre Oriental. Males, as previously noted, had well-developed testes in August. The weights of 17 males and 20 females from the Sierra de Tamaulipas were, respectively, 26.6 (24-33), and 25.6 (21-31) grams.
Measurements of specimens from different localities in Tamaulipas averaged about the same, except that those of specimens from Palmillas, averaged smaller. The small size suggests intergradation between the subspecies collinus and eremicoides. The latter occurs to the west and differs from collinus in smaller size, more grayish coloration, completely white tarsal joint and relatively longer tail. Hooper (1952:374) reported specimens from Jaumave as intergrades between the two subspecies before mentioned and Osgood (1909:164) identified two specimens from there as eremicoides. In the present account, individuals from Palmillas and Jaumave are referred to collinus.