what we are observing in vepsertilionid bats where tail fanning motions are consistent in generating rearward flight thrust. Is the use of the tail membrane in bats novel? We argue that the use of TAFT locmotion by incorporating fanning of the tail-membrane to produce rearward thrust, thereby acting as a tail-wing in bats, represents a novel form of vertebrate flight that mimics thrust production produced by tail motions in some aquatic organisms. Although some of the hind limb motions we observed in vespertilionid bats may act to adjust camber of the wings, the extent to which they use the tail-membrane far exceeds what would be required for that task. Our comparison of horizontal takeoff between the short-tailed fruit bat (C. perspicillata) that has only a partial tail-membrane and our vespertilionid species underscores how extensively the latter uses the tail- membrane for generating rearward thrust during takeoff. In addition, contraction and expansion of the tail-membrane during the stroke cycles clearly conform to predicted power-stroke kinematics. In vespertilionid bats, only the tail-sweep involving dorsal extension of the hind-wing correlated with body mass and thus it appears that dorsal component of the sweep is primarily used to produce rearward thrust. Curiously, all individuals filmed swept the tail-membrane through the ventral flexion component below the body-axis regardless of how much dorsal extension of the tail was used. Thus, it appears that there may be a duel-function of the tail-wing motion relative to its position above and below the body axis. Similar to birds, there appears to be an effort during horizontal takeoffs to either minimize drag when the tail- membrane is moved below the body axis or perhaps bats are using the ventral component of tail-membrane sweep to enhance lift [11] by curving the flight surface to increase wind-speed across the dorsum relative to airflow across the ventrum (Figure 10). This effect would be simiar to an airplane putting down its flaps during takeoff to generate the lift necessary during slow flight speed. A Hind-Wing in Bats Indeed, we have just begun to uncover the myriad ways in which the tail-membrane may be employed to facilitate navigation of complex terrains and habitats as well as horizontal launches from rock-crevices, buildings, and ground roosts. As is, the tail- membrane kinematics used during platform takeoff is variable among individuals within species, showing high plasiticity in its phasing and integration with the wings. Further, the ease by which individuals use the tail-wing indicates a well-integrated kinematic orchestration among the four limbs, tail vertebrae, and tail- membrane of vespertilionid bats. As with birds [26], there appears be a neurological decoupling of muscle control between the wings and tail-membrane in bats, but this will require EMG data to confirm. That the extensive use of the tail-membrane corroborates aspects of flight morphology such as wing loading and aspect ratio and ecomorpholgical predictions, shows an evolutionary under- pinning for use of a tail-wing for thrust and lift production. Future investigations into this phenomenon should include species of bats from other families as well as investigations into the interactions of vortices produced by the wings and tail-membrane and under what ecomorpholgical parameters the tail-wing is employed. Analysis of the skeletal elements that support fanning motion of the tail-wing will greatly further understanding of the complex nature of tail-membrane adaptations for flight in bats as well as the evolution of flight and species diversity. As with all analyses of flight kinematics, interpretations of, and explanations for potential force production and its effects on flight dynamics requires considerable caution. Herein we put forth our interpretation of how the tail-membrane appears to be used as a supportive tail- wing to produce thrust and lift during takeoff flight from a horizontal platform. Much more sophisticated analyses will be required to test this model. Lastly, casual observations of high-speed videos provided by other researchers of individuals from other families of bats engaged in skimming water to drink and in nearly vertical flight into caves show clear evidence of TAFT locomotion. Although not yet quantified, we anticipate that most species of bats with full tail Figure 10. Illustration of hypothetical model of how thrust and lift may be generated. Using the tail-membrane fanning motions (black arrow indicates direction of travel) of a long-eared myotis (Myotis evotis), we illustrate a hypothetical duel-use of the tail-wing wherein dorsal extension and downstoke above the body plane delivers thrust, whereas ventral flexion generates thrust and also lift when held in a curved position below the body axis. This positioning would be analogous to an airplane extending its flaps to increase air-speed above a flight surface. doi:10.1371/journal.pone.0032074.g010 PLoS ONE | www.plosone.org 9 February 2012 | Volume 7 | Issue 2 | e32074