A Hind-Wing in Bats Figure 4. Takeoff trajectory and wing versus tail-membrane motions. Digitized tracings of left wing tip (blue) and tail tip (red) for three takeoff trajectories observed in little brown myotis (Myotis lucifugus). Angle of flight trajectories are represented by tilt of each cylinder. Data are provided below each cylinder indicating least degree of left wing stroke angle to body plane (b) and maximal tail-membrane stroke angles above the body plane (h = +angle) and below the body plane (h9 = 2angle) for each individual adjusted to the horizontal for comparison. Hinge indicates a pivot-point presumably at the sacral/caudal vertebral joint. Body plane is indicated by the dotted arrow and U = velocity. doi:10.1371/journal.pone.0032074.g004 the tail-membrane relative to body mass. Although neither regression line is significant due to high variation (upstroke: R2 = 0.33, R = 0.57, P = 0.08; downstroke: R2 = 0.09, R = 0.29, P = 0.41), the consistent switchover pattern underscores a potential change in kinematics depending on the body mass of an individual, an area of future research. Species-specific patterns of tail-membrane employment We calculated the degrees of tail-stroke fanning sweeps for 44 individuals of the four vespertilionid species with the highest sample sizes and that varied in wing loading, flight ecology, and ecomorphology (Figure 7). The average extent to which the tail- membrane was used during horizontal takeoffs was greatest for M. lucifugus, known to be an aerial-pursuit forager in open habitats and having the highest comparative wing loading of the group we tested. The ground foraging pallid bat (A. pallidus) has the lowest wing-loading of the group and showed the least use of the tail- membrane during takeoffs. However, because this species hunts large terrestrial prey such as scorpions, ground beetles, and centipedes, the use of the tail-membrane may be prompted more when launching from the ground with a prey item in its mouth large enough to significantly increase wing loading. The two other myotis species analyzed showed intermediate use of the tail- membrane between these two extremes. Post hoc Tukey-Kramer Multiple-Comparison Test (Table 1) denoted significant differenc- es between M. lucifugus and the two species with lowest wing loadings (A. pallidus and long-eared myotis, M. evotis); whereas the fringed myotis (M. thysanodes) that has intermediate wing loading and is adapted for aerial-pursuit of insects in cluttered habitats, used its tail-membrane in an intermediate fashion that was not significantly different from either M. lucifugus and M. evotis (DF = 35, Critical Value = 3.814, Alpha = 0.05, Table 1). Discussion Herein we report on extensive use of the tail-membrane as a potentially important mechanical contributor to flight mechanics in bats during very slow take-off flights from a horizontal platform. The way and the extent to which the tail-membrane in vespertilionid bats is used have not been observed in any other PLoS ONE | www.plosone.org 5 February 2012 | Volume 7 | Issue 2 | e32074