OSTEOMETRIC VARIATION AND FUNCTION IN BATS

Since the most extensive and direct evidence of macroevolution, the fossil record of the vertebrates, consists almost entirely of skeletal and dental remains, quantitative studies of these systems are of special interest to the evolutionist. Students of continuous variation of the skeleton have amassed an imposing quantity of data over the past several decades, much of which has been, of necessity, of a purely descriptive nature. Although a general synthesis based on studies of osteometric variation and correlation appears to lie some distance in the future, certain generalizations have emerged which consistently lead to verifiable predictions. Furthermore, there has been an ever increasing tendency to relate numerical values to relationships of genetic and functional significance. The studies of Kurten (1953, 1957) and Olson and Miller (1958) are recent examples. One of the most fundamental and readily verified generalizations in osteometrics is the circumscribed nature of the phenotypic variation within a species population in spite of the presumed great potential variability of the population. In terms of the coefficient of variation, values for groups with a definitive adult size (e.g., most mammals and birds) regularly fall within the approximate limits of 2 and 8. The concepts of developmental and genetic homeostasis as developed by Lerner (1954) and empirical data derived from wild and inbred rodent populations (Bader, 1956) suggest that groups with quite different degrees of genetic homogeneity may nonetheless manifest very similar amounts of phenotypic variation. Thus, inferences as to the relative extent of genetic heterogeneity based solely on phenotypic variation must be made with considerable caution. Consistent differences in observed variation can be demonstrated, however, both between different regions of the skeleton and between taxa of higher rank. It is with such disparities that the present study is concerned. One of the most persistent differences in magnitude of variation between higher categories of vertebrates is that which pertains to birds and mammals. In general birds exhibit a lower variation in osteometric characteristics than that shown by homologous traits of mammals. A greater intensity of centripetal selection on flyers suggests itself as a possible interpretation. With respect to differences related to position in the skeleton, several avian studies have revealed a trend of increased variation particularly in the wing, from the proximal to the distal portion of the appendage. This trend appears to be either lacking, or much less regular, in terrestrial mammals. In this context an osteometric analysis of bats should be of considerable interest. Will they exhibit magnitudes and trends of variation typical of terrestrial mammals or of birds, or, perhaps, partake of characteristics of both? Will the two appendages, which differ strikingly in morphology and function, reflect this difference in terms of quantitative variation and correlation ? The behavior of the first digit of the wing should be of particular significance. Although it is borne on the forelimb, it does not aid in support of the flight membrane (in the species studied) and resembles very closely the digits of the foot in size, shape, and function (fig. 1). No extensive osteometric study has previously been carried out in the bats.