Results Bats captured and filmed We filmed 95 individuals of five species of vespertilionid bats as they launched from a horizontal platform after being held stationary by hand. From these, 48 videos met the criteria for analysis (minimal pitch, roll, and yaw during takeoff, ability to track wing tip, wrist, and tail-membrane tip motions throughout beat cycles, as well as smooth, unfettered takeoffs from start- points). Numbers of videos used per bat species were: 12 long- eared myotis, M. evotis; 14 little brown myotis, M. lucifugus; 7 fringed myotis, M. thysanodes; 4 Townsend’s big-eared bats, Corynorhinus townsendii, and 11 pallid bats, Antrozous pallidus. All myotis species and Townsend’s big-eared bats were wild-caught and filmed in the field. Pallid bats were filmed at the University of Wyoming, Laramie, USA, where a free-flying, captive colony is held. Takeoff sequencing The push-off kinematics and mechanics of horizontal takeoffs have been described for the ground-dwelling common vampire bat (Phylostomidae: Desmodus rotundus) and, although push-off mechan- ics was not described in a study of the dog-faced fruit bat (Pteropodidae: Cynopterus brachyotis), calculation of power produc- tion by the wings during horizontal takeoffs were made [15,16]. For vespertilionid bats used in our study, horizontal takeoffs involved a similar push-off to that described for D. rotundus, A Hind-Wing in Bats wherein the wrist and thumb acted as the primary launch points. However, whereas D. rotundus launched with a head-up body posture, vespertilionid bats typically rocked forward onto their wrists, resulting in a head-down posture with the body-axis tilted at times 30–45 degrees below the horizontal (Figure 1A). Subsequent rotation of the body to a horizontal plane occurred quickly within the first wing and tail-membrane beat cycles. In some cases, however, myotis bats did launch with a head-up posture similar to D. rotundus, but this was relatively rare (n = 3). Curiously, the pallid bat (Antrozous pallidus), a ground foraging species, did typically launch itself with a head-up posture similar to D. rotundus, or used a level body-axis orientation. Townsend’s big-eared bat (C. town- sendii, N = 4) used a very steep ascent during takeoff nearing 60u to the horizontal before leveling off in flight (Figure 1B), whereas myotis species typically showed a less dramatic ascent angle (,30u). The pallid bat (A. pallidus, N = 11) also at times used a very steep angle of ascent with one individual launching at an angle of 69u. Complex orchestration of tail-membrane flapping mechanics By filming bats head-on as they launched from the horizontal platform, we revealed that the tail-membrane complex (hind limbs, tail, and tail-membrane) undergoes intricate and coordi- nated motions, as do the wings, to minimize downward forces during the upstroke and maximize air-displacement throughout the downstroke (Figure 1A, Videos S1 and S2). By adducting the Figure 1. Orchestrating a tail-membrane flap. A) 1–2. Fringed myotis (Myotis thysanodes) illustrating adduction of its rear limbs to collapse the tail-membrane during the upstroke. 3–4. Abduction of the hind-limbs at the top of the upstroke occurs in preparation for the tail-membrane downstroke, thereby maximizing surface area and air displacement leading to rearward thrust. 5. Collapsing of the tail-membrane at the bottom of the downstroke in preparation for the next upstroke. B) Sequence drawings illustrating motion and timing between wings and tail-membrane motions during initial phase of a platform takeoff by Townsend’s big-eared bat (Corynorhinus townsendii). doi:10.1371/journal.pone.0032074.g001 PLoS ONE | www.plosone.org 2 February 2012 | Volume 7 | Issue 2 | e32074