Landmark morphometric assessment of the genus Baiomys (Mammalia: Rodentia): Geographic variation, secondary sexual dimorphism, and character displacement in sister species

The purpose of this research was to evaluate geographic variation and secondary sexual dimorphism within the two recognized species of Baiomys (Rodentia: Muridae), and to determine whether (and to what extent) character displacement occurs within either or both of these species where they are found in sympatry. The genus Baiomys is composed of two extant species, B. taylori and B. musculus, which are quite similar in appearance. The two species are distinguishable by a limited number of cranial features (occipitonasal length, zygomatic breadth, and curvature of rostrum), hyoid, and baculum. Landmark point coordinates were collected from dorsal and ventral views of the crania and mandibles of 513 specimens representing the geographic distribution of both species. Geometric morphometric techniques were used to examine sexual dimorphism and geographic variation within each species, and to determine whether character displacement was present within the area of sympatry. A discriminant function analysis of Baiomys taylori showed a significant difference between the two sexes (p=0.03), whereas B. musculus did not (p=0.15). Additionally, a discriminant analysis showed significant differences (p=0.05) among four pre-designated groups (B. taylori allopatric and sympatric, and B. musculus allopatric and sympatric). The only significant pair-wise difference occurred between B. taylori sympatric and allopatric populations, whereas the sympatric B. taylori and B. musculus populations were quite similar to each other. A correlation coefficient (r) of 0.45 between morphometric variation and latitude was found for both species and sexes combined, and the slopes of the two species were not significantly different from each other (p=0.46). Furthermore, it was discovered that interpoint distances (¡Ö size) in the cranium and mandible increased in lower latitudes in both species, so that the largest B. taylori and smallest B. musculus were found in the zone of sympatry. This high correlation between morphological score and latitude across both species, together with the results of the discriminant analysis, indicate that character displacement does not occur in cranial and mandibular characters in Baiomys species. These findings are contrary to early speculations that character displacement was present in the genus.