rear appendages during the upstroke, individuals fully collapsed the tail-membrane, thereby minimizing its area and downward air pressure across the surface (Figure 1A, panels 1 and 2). When the tail-membrane reached the top of the upstroke (Figure 1A, panel 2), the legs were abducted to fully extend the tail-membrane (Figures 1A, panel 3) as it was swept through the downstroke. Fanning motion of the tail-membrane (Figure 1A, panels 3 and 4; Figure 1B, Video S3) would displace air perpendicular to the sweeping motion thereby generating thrust in a rearward direction [17]. The first tail-membrane cycle ended with adduction of the legs at the bottom of the downstroke in preparation for the next upstroke (Fig. 1A, panel 5). Another important component of the tail-membrane in flight was the ability to alter stroke timing (exemplified in Figure 1A, panel 4), wherein the timing of the tail-membrane downstroke lagged behind the downstroke of the wings by nearly 50%. Lagging the downstroke of the tail-membrane behind that of the wings may serve to provide timely lift and thrust during takeoffs at very slow flight speeds (,1 m s21) [18,19]. Although the formation of closed-loop ring vortices have been shown to contribute to lift during the upstroke in wind tunnel flight speeds between 5.060.1 m s21and 6.760.4 m s21 in C. brachyotis [20], tests done on the greater horseshoe bat (Rhinolophidae: Rhinolophus ferrume- quinum) and the long-eared bat (Vespertilionidae: Plecotus auritus) flying at speeds below 3 m s21, showed ‘little to no useful lift’ produced by the wings during the upstroke phase [18,19]. In addition, at speeds below 3 m s21 in the nectar-feeding bat (Phyllostomidae: Glossophaga soricina), unstable air flow dynamics result in unfavorable (negative) thrust and lift production during the upstroke. However, there was some indication that at the top of the upstroke, inverting the camber of the hand-wing tips by supination potentially produced some thrust [19]. Integrating our data for vespertilionid bats, extensive investment of fanning motions of the tail-membrane during very slow flight from horizontal launch to cruising speed indicates supportive thrust and/or lift generation. A Hind-Wing in Bats Degrees of synchrony between wings and tail-membrane motions Temporal offset patterns, particularly between downstroke timings of the wings and the tail-membrane were quite typical among the species tested. Upon takeoff, both wings moved through the upstroke in tandem to their top positions. However, there was clear variation of synchronized motions between the wings and tail-membrane during initial downstrokes among individuals and the degree of synchrony varied even across wing cycles within the same launch event. Commonly, but not always, the first downstrokes initiated an offset timing (we term these ‘oscillation offsets’) between wings and the tail-membrane. In most cases, the opening downstroke asynchrony was driven by a time- lag in downward motion of the tail-membrane relative to the downward motion of the wings leading to variation in cycling times. For the little brown bat (M. lucifugus, N = 14) the mean wing and tail-membrane overall cycle times (top of upstroke through bottom of downstroke to top of upstroke) were not significantly different (Xwing = 0.07 seconds, Xtail-membrane = 0.069 seconds; T- value = 1.33, P = 0.89). However, standard deviations of the tail- membrane cycles were twice those measured for the wings (SDwings = 0.0079, SDtail-membrane = 0.017) indicating greater rela- tive variability in spatio-temporal use of the tail-membrane. At slowest cycling speeds, the tail-membrane was flapped at only 72% of the wing cycle speed (wing-minimal-cycle-time = 0.084 - seconds, tail-membrane-minimal-cycle-time = 0.117 seconds), whereas during the fastest cycle speeds, the tail-membrane flap was 39% faster than the fastest wing cycling speed (wing-maximal- cycling-speed = 0.054 seconds, tail-membrane-maximal-cycling- speed = 0.033 seconds). Thus, the tail-membrane was employed over a wider range of cycling speeds than observed for the wings and in most cases an individual’s kinematic integration of the tail- membrane appeared dampened (reduced fanning motion) as they accelerated during takeoff. As mentioned, relative cycle timings between the wings and tail-membrane did vary among wing-beats within an individual takeoff event. For example, in the little brown Figure 2. Stroke initiation offsets. Stroke offsets between the left wing and tail-membrane are illustrated by plotting the digitized motions of the left wrist (blue line) and tip of the tail (red line) for the little brown myotis (Myotis lucifugus) through three wingbeat cycles. In this example, the individual used intitiation offsets of the upstroke and downstroke timings in order to produce asynchronous flapping between the wings and the tail- membrane. Black lines indicate degree of divergence in timing of downstroke motions (oscillation offsets). doi:10.1371/journal.pone.0032074.g002 PLoS ONE | www.plosone.org 3 February 2012 | Volume 7 | Issue 2 | e32074