Skeletal indicators of ecological specialization in pika (Mammalia, Ochotonidae)

Pika species generally fall into two ecotypes, meadow-dwelling (burrowing) or talus-dwelling, a classification that distinguishes a suite of different ecological, behavioral, and life history traits. Despite these differences, little morphological variation has previously been documented to distinguish among ecotypes. The aim of this study was to test whether postcranial features related to burrowing are present in meadow-dwelling species and whether talus-dwelling species exhibit postcranial modifications related to frequent leaping between rocks. To test this, the scapula, humerus, ulna, radius, innominate, femur, tibia, and calcaneus of 15 species were studied and measured. Twenty-three measurements were taken on 199 skeletons, and 19 indices were constructed from these measurements. Indices were compared between the two ecotypes using Student's t-test. Comparisons among ecotypes, species, and subgenera were made using one-way ANOVA with the Tukey honest significant difference post hoc test. Multivariate results were generated using principal components analyses. Thirteen forelimb and hind limb indices proved significant in distinguishing the meadow-dwelling, talus-dwelling, and intermediate forms. A number of these indices are associated with burrowing or leaping in other mammals, providing some support for the hypothesis that postcranial modifications in pika are related to locomotor differences. This evidence of morphological responses to ecological specialization will be useful for reconstructing the paleobiology of extinct taxa, assessing the behavioral variability of extant species, and improving our understanding of the evolutionary history of pikas.