A Hind-Wing in Bats Figure 5. Tail-membrane extension relative to body mass. Model two regression analysis showed a significantly positive relationship between (LogN) dorsal extension of the tail-membrane above the body plane and (LogN) mass (g) of individual little brown myotis (M. lucifugus), indicating that degree of dorsal extension of the tail-membrane is mechanically adjusted relative to mass to be lifted. doi:10.1371/journal.pone.0032074.g005 flying vertebrate. We show that the tail-membrane can be incorporated in an extensive, complex, and flexible manner (Video S5) as a tail-wing that can be called upon to produce rearward thrust at slower flight speeds. Offsets in timing by lagging the downstroke of the tail-membrane behind the wings appears to result in rearward thrust during moments of the wing upstroke when minimal lift is being produced [18,19]. The full importance and adaptive nature of this form of flight will require further investigation such as interactions between wings and tail- membrane vortices, but herein we supply the foundation of adaptive and controlled use of the tail-membrane in vespertilionid bats, the extent to which had not been previously documented or calibrated. Evidence indicates that the tail-membrane is used in well-coordinated flight mechanics and shows plasticity by individuals in its use depending upon immediate needs for flight support. In addition, use of the tail-membrane during horizontal takeoffs matches predictions based on wing-loading and ecomor- phology of insectivorous bats. We term this mode of flight, ‘Tail- Assisted-Flight-Thrust’ (TAFT) that is initiated by fanning motions of the tail-membrane in producing rearward thrust. Figure 6. Change in oscillation offsets relative to body mass A. Histogram showing extent of oscillation offsets between the left wing and tail tip during the upstroke (blue bars) and downstroke (red bars) for the little brown myotis (M. lucifugus), plotted against individuals that varied in body mass. B. Regression plot of the offset data shows the switch-over point of regression lines where downstroke offsets (red line) become more pronounced than upstroke offsets (blue line) in relation to body mass. Although neither regression line is statistically significant due to high variation in the sample, the consistent trend towards greater offsets occuring during downstrokes as body mass increases suggests meaningful shifting of kinematics with mass. doi:10.1371/journal.pone.0032074.g006 PLoS ONE | www.plosone.org 6 February 2012 | Volume 7 | Issue 2 | e32074