A Hind-Wing in Bats Figure 7. Use of the tail-membrane across species. Integration of ecomorphology with use of the tail-membrane showed significant differences among species (P = 0.01) most broadly separated ecomorpholgically. Mean tail-stroke values (degrees of arc across the body-axis) were highest for the little brown myotis (MYLU) that has the highest wing loading and a preference for open-aerial flight as compared to the other three species. The ground foraging pallid bat (Antrozous pallidus, ANPA) has the lowest wing loading of the group and had the shortest tail sweep on average. Post-hoc pair-wise comparisons showed that MYLU was signficantly different from the long-eared myotis (M. evotis, MYEV) and ANPA, whereas the fringed myotis (M. thysanodes, MYTH) was not significantly different from any other species . ** = significant difference at the P = 0.01. Connecting lines show which groups were signficant different. See Table 1 for details. doi:10.1371/journal.pone.0032074.g007 Hypothetical model for contributions of the tail- membrane during takeoff flight Comparative motions of the tail-membrane between the long- eared myotis (Vespertilionidae: Myotis evotis) and the short-tailed fruit bat (Phyllostomidae: Carollia perspicillata) allowed for insightful comparisons during horizontal takeoffs (Figure 8). We filmed five short-tailed fruit bats multiple times and none showed dorsal extension of the hind-limbs or caudal vertebrae during horizontal launches as was clearly evident in vespertilionid bats. In C. perspicillata, there were some lateral and medial motions of the hind-limbs, but this pattern was inconsistent and in some individuals was not evident. There is some evidence using a simplified bat model based on the long-eared bat, Plecotus auritus [11], that positioning the tail-membrane during wind tunnel flights at speeds near 6 m s-1 significantly influences flight control. Thus Table 1. Post hoc Tukey-Kramer Multiple-Comparison Test showed significant differences (DF = 35, Critical Value = 3.814, Alpha = 0.05) among vespertilionid species in degree of tail- membrane sweep during takeoff from a horizontal platform. Group MYLU MYEV MYTH ANPA Count 14 12 7 11 Mean 118.90 96.82 103.83 92.39 Significantly Different Groups ANPA, MYEV MYLU NONE MYLU MYLU = Myotis lucifugus, MYEV = M. evotis, MYTH = M. thysanodes, ANPA = Antrozous pallidus. doi:10.1371/journal.pone.0032074.t001 for C. perspicillata, leg motions observed during takeoff launches in some individuals may be to adjust the camber of the wings. Use of the tail-membrane in vespertilionid bats in many ways mimics the fanning motions of the flukes in swimming mammals like dolphins [21,22,23,24] and, in particular, the manatee [20,25]. Paddle-formed tails in fish and manatees have low aspect-ratios, function around a single hinge (similarly to a rounded, paddle-formed human hand fan), and are efficient in producing forward thrust at low-speeds [17,25]. The motion of the bat’s tail-membrane mimics this fanning motion, however, unlike that observed in aquatic and semi-aquatic animals, the bat’s tail- membrane is unfolded only during the downstroke. The fact that bats fold up the tail-membrane during the upstroke phase apparently to reduce negative lift forces and expand it during the downstroke phase to apparently accentuate rearward air displacement agrees with expected airfoil kinematics . In addition, tail-membrane fanning motions in bats are controlled by three pivot points, the hip-joints and the sacro-caudal joint of the tail that must be coordinated, and this differs from the single, central- axis control of the flukes in dolphins and manatees [21,23,25]. That the tail-membrane displaces air in a direction that would produce rearward thrust is consistent with the physics of fanning motion [17](Figure 9) and it is difficult to conceive of alternative reasons that vespertilionid bats would place so much effort in tail- membrane fanning if the resulting contribution was not significant. Our video of C. perspicillata shows that this species typically flapped directly off the horizontal platform rather than performing a push- off as observed in the vespertilionid bats and C. perspicillata showed no evidence of attempted tail-mebrane flapping. In vespertilionid bats, as the tip of the tail moves along a sinusoidal path through each stroke cycle, the position of the tail- membrane changes relative to that sinusoidal path. This why we predict that the downstroke provides a thrust-force similar to that PLoS ONE | www.plosone.org 7 February 2012 | Volume 7 | Issue 2 | e32074