Morphologic Variation among Rock Pocket Mice (Chaetodipus intermedius) from New Mexico Lava Fields

Abstract

-Intraspecific variation in external and cranial measurements of rock pocket mice (Chaetodipus intermedius) was evaluated using 312 adult specimens (145 males and 167 females). Males were larger than females for 8 of 16 characters examined, and significant geographic variation occurred in 15 characters for one or both sexes. Generally, largest means of male and female characters were from specimens representing the Carrizozo malpais; the smallest mice were from the Afton lava flows. Specimens from the Pedro Armendariz lava field and the nearby Fra Cristobal Range were the closest geographically and the most similar morphologically. Rock pocket mice (Chaetodipus intermedius) occur in northwestern Mexico and throughout much of Arizona and New Mexico (Hall, 1981). Of the 10 currently recognized subspecies of C. intermedius (Hall, 1981), three are confined to isolated black-lava outcrops, C. i. nigrimontis occurs on Black Mountain, Pima Co., Arizona; C. i. ater occupies the Carrizozo malpais, Lincoln and Otero Cos., New Mexico; and C. i. rupestris is found on the Afton lava flows, Dona Ana Co., New Mexico. In addition, an isolated population of C. intermedius inhabits the Pedro Armendariz lava field in Sierra and Socorro Cos., New Mexico (Weckerly, 1983). These geographic isolates differ markedly in coloration from populations of rock pocket mice in surrounding areas (Dice, 1929; Benson, 1932; Blossom, 1933; Weckerly, 1983). How much these populations differ morphologically from each other or from nearby populations has not been established. The restricted distribution and presumably intense natural selection for dark pelage coloration may also be associated with some selection for morphologic variation among the forms inhabiting lava fields. Best and James (1984) noted that some lava-dwelling rattlesnakes (Crotalus), in addition to being darker, were much smaller than other populations in New Mexico. Perhaps body size, as well as coloration, are affected by selection pressures on lava fields. However, Rogers and Schmidly (1981) found essentially no morphologic differentiation among similar populations of Neotoma albigula. To determine if populations of C. intermedius differed between lava fields and an adjacent rocky outcrop, we studied morphologic variation in specimens collected from three lava fields and one mountain range in south-central New Mexico. Our purposes were to investigate the degree of sexual dimorphism, amount and pattern of interlocality variation within each character, and phenetic relationships among populations. MATERIALS AND METHODS.-We analyzed 4 external and 12 cranial measurements (Fig. 1) for 312 adult Chaetodipus intermedius. Total length, tail length, hind foot length, and ear length were recorded to the nearest mm from the specimen tag. Nasal width and interparietal width were taken with an ocular micrometer in a stereo microscope at lOx, and the remaining cranial and mandible measurements were made to the nearest 0.1 mm using dial calipers. Specimens were NOVEMBER 27, 1985 THE SOUTHWESTERN NATURALIST 30(4):491-501 This content downloaded from 157.55.39.144 on Mon, 25 Jul 2016 04:07:31 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist