Osteometric Variation and Function of the Highspeed Wing of the Free-tailed Bat

Abstract

Osteometric variation of 12 bones in the high-speed wing of the Mexican free-tailed bat, Tadarida brasiliensis, was analyzed and found to be closely comparable to variation in the high-lift wing of Myotis. Coefficients of variation and correlation were obtained. We found high variation in phalanx one, digit II, and a marked increase of variation distally. Generally, we found coefficients of variation to fall between two and eight. Compared to Myotis, Tadarida has higher variation in phalanx two, digit V, and lower variation in the distal elements of digit III (emphasizing a probable functional significance of the wing tip). Overall variation is lower in Tadarida. There is strong correlation among distal bones of the third and fifth digits (important aerodynamically) and the humerus and radius in Tadarida. Similar bones often showed strong correlation. No marked preponderance of advanced characters was noted in either genus if specializations for maneuverable flight are given equal weight with those of rapid flight, and we question that Molossidae should be considered moreadvanced than Vespertilionidae. INTRODUCTION Structure and form of an organism are related to function, hence, obviously, to exploitation of an ecological niche. For example, in birds the form of the wing is related to mode of flight (Savile, 1957:212). Generally, the amount of variation of a structure in a population sample is apparently also related to function, in that variation is more extensive in nonfunctional or vestigial structures (Simpson, 1953; Bader and Hall, 1960:15). We attempt to analyze osteometric variation in the high-speed wing of the Mexican free-tailed bat, Tadarida brasiliensis, which ostensibly is capable of rapid and enduring flight (Vaughan, 1959, 1966; and others), in order to compare the nature and extent of this variation with that described by Bader and Hall (1960) for two species of Myotis, of slower, more maneuverable and probably less enduring flight. Comments are presented concerning some adaptations to flight of Myotis (Vespertilionidae) and Tadarida (Molossidae), although the primary aim is to obtain verification of the work by Bader and Hall (1960). Acknowledgments.-We are grateful to Mr. John Lynch and Mr. Macreay Landy for the bats collected by them in Veracruz, Mexico, and to Professor Hobart M. Smith for the loan of his comparator. We are especially indebted to Mr. James T. Wallace for his assistance in programming our problem for computer use at the University of Illinois Digital Computer Laboratory. MATERIALS AND METHODS Fifty bats utilized were obtained 26-28 July 1964 from a cave at Cofre de Perote, 2800 m elevation, Veracruz, Mexico, where all