Southern Pygmy Mouse
(Synonymy under subspecies)
Type.—Sitomys musculus Merriam, Proc. Biol. Soc. Washington, 7:170, September 29, 1892.
Range.—Southern Nayarit, Michoacán, México, Morelos, Puebla, and central Veracruz, southeastward to western Nicaragua, but unknown from southern Veracruz, Tabasco, and the Yucatán Peninsula (see [Figure 10]); occurs principally in the arid upper and lower divisions of the Tropical Life-zone.
Characters for ready recognition.—Unless otherwise noted, characters are usable only for the two age-categories of adult and old adult. Differs from B. taylori in: hind foot 16 millimeters or more; occipitonasal length, 19 millimeters [Pg 609] or more; zygomatic breadth, 10 millimeters or more; rostrum not deflected ventrally at frontoparietal suture but, instead, curving gradually toward anteriormost point of nasals; cingular ridges and secondary cusps on teeth more pronounced; basihyal having anterior pointed entoglossal process, shoulders of basihyal protruding anteriorly (characteristic of all age categories); baculum having broader shaft, spatulate to knob-shaped tip, wings at base projecting anteriorly; baculum more than 3 millimeters long; short process of incus knob-shaped rather than attenuate; muscular process of posterior crus of stapes prominent.
Characters of the species.—Size large (extremes in external measurements of adults; total length, 100-135; length of tail vertebrae, 33-56; length of hind foot, 14.1-17; length of ear, 9-12); upper parts dark reddish brown, or ochraceous-buff to nearly black; underparts pale pinkish buff to white or pale buffy.
Geographic variation.—Eight subspecies are here recognized (see [Figure 10]). Features that vary geographically are external size, color of pelage, certain cranial dimensions (occipitonasal length, zygomatic breadth, least interorbital breadth, length of rostrum, length of incisive foramina, depth and breadth of cranium, and alveolar length of upper molar tooth-row).
External and cranial size (except for B. m. handleyi) is less in the southernmost subspecies, B. m. pullus, B. m. grisescens, B. m. nigrescens, and more in the northernmost subspecies, B. m. musculus, B. m. brunneus, and B. m. infernatis. Increase in size from south to north is in keeping with Bergman's Rule that within a species, smaller individuals occur in warmer parts of its geographic range. Southern pygmy mice at high altitudes average larger than those from low elevations, except where the two species are sympatric. There the Southern Pygmy Mouse is uniformly larger, regardless of altitude.
Osgood (1909:257, 259) suggested that degree of relative humidity might in some way control color of pelage in both B. taylori and B. musculus. In B. musculus, the darker subspecies, B. m. brunneus, B. m. nigrescens, and B. m. pullus, occur in zones of rather constant high relative humidity, whereas the paler subspecies infernatis, musculus, handleyi, and to a less extent grisescens and pallidus, occur in zones of lower relative humidity. This is in keeping with Gloger's Rule, which states that melanins increase in the warm and humid parts of the range of a species, and reddish or yellowish-brown phaeomelanins prevail in arid climates. B. m. musculus ranges into areas where relative humidity is such that darker pelages might be expected, but this is in the area where the two species are sympatric, and color of pelage may be an important character of recognition.