Bacular Variation in Kangaroo Rats (Genus Dipodomys) of the heermanni Group in Baja California, Mexico

Abstract

-Variation in skin and skeletal measurements of kangaroo rats (genus Dipodomys) was evaluated using 265 specimens from 11 localities in Baja California, Mexico. Cluster analyses were utilized to verify field identification at the two localities where D. gravipes and D. agilis were sympatric and to examine each of the other nine collecting sites for possible sympatric forms. Sexual dimorphism occurred in two external, three skull, and nine postcranial measurements. Males were significantly larger than females in all of these characteristics. Significant interOTU variation occurred in 41 of the 42 characters analyzed. Using correlation analyses the 19 least correlated characters were selected from the original 42. Dipodomys gravipes was readily separable from the other taxa. The data indicate that there are only two species of kangaroo rats (D. agilis and D. gravipes) of the heermanni group in Baja California. Kangaroo rats (genus Dipodomys) of the heermanni group occupy most of Baja California from the mountains of the north through the desert areas that cover most of the peninsula (Huey, 1951). Kangaroo rats in Baja California have been studied by Villa R. (1941), Alvarez (1960), Huey (for example, 1925, 1927, 1951, 1962, 1964), Stock (1974), and Best and Schnell (1974). Lidicker (1960) discussed D. merriami and D. insularis, and Lackey (1967) examined kangaroo rats of the heermanni group, though he was primarily interested in D. stephensi and D. cascus in southern California. In addition I have analyzed bacular variation of D. agilis and D. gravipes as well as relationships between morphological and ecological variation of the populations of D. agilis discussed herein (Best, manuscripts submitted for publication). Using univariate and multivariate statistical techniques, I have identified some patterns of variation in morphologic variables associated with kangaroo rats in Baja California. My purposes were to investigate the following: (1) degree of sexual dimorphism; (2) amount and pattern of interlocality variation within each character; (3) phenetic relationships between the populations; (4) taxonomic relationships of the populations. MATERIALS AND METHODS Kangaroo rats were collected at 11 localities in Baja California during June and July 1972 (Table 1). Specimens were divided into 13 Operational Taxonomic Units (OTUs), because at two of the collecting sites, two taxa of kangaroo rats of the heermanni group occur sympatrically--D. gravipes and D. agilis (Table 1, OTUs 2, 3, and 5, 6). Prior to this report the taxonomic designations (from range maps in Hall and Kelson, 1959, unless otherwise indicated) of the OTUs were as follows: (OTU 1) D. agilis martirensis; (2) D. agilis simulans; (3) D. gravipes; (4) D. agilis simulans; (5) D. agilis plectilis; (6) D. gravipes; (7) D. peninsularis pedionomus; (8) D. paralius; (9) D. antiquarius (Huey, 1962); (10) D. peninsularis eremoecus; (11 and 12) D. peninsularis peninsularis; (13) D. peninsularis australis. For the present analyses I have treated these as populations because their systematics are not fully understood (Lackey, 1967; Stock, 1974; Best, manuscript submitted for publication). Specimens were aged according to the criteria of Best and Schnell (1974). The following external measurements, accurate to the nearest mm, were taken on freshly collected specimens before they were prepared as standard museum study skins and skeletons: total length; tail length; hind foot length; ear length from notch; body length was calculated by subtracting tail length from total length. The 37 skeletal measurements (Fig. 1) were taken to the nearest 0.1 mm with the same dial calipers. All specimens are deposited in the collection of Recent mammals at the Stovall Museum of Science and History, University of Oklahoma.