Flapping Tail Membrane in Bats Produces Potentially Important Thrust during Horizontal Takeoffs and Very Slow Flight Rick A. Adams*, Emily R. Snode, Jason B. Shaw School of Biological Sciences, University of Northern Colorado, Greeley, Colorado, United States of America Abstract Historically, studies concerning bat flight have focused primarily on the wings. By analyzing high-speed video taken on 48 individuals of five species of vespertilionid bats, we show that the capacity to flap the tail-membrane (uropatagium) in order to generate thrust and lift during takeoffs and minimal-speed flight (,1 m s21) was largely underestimated. Indeed, bats flapped the tail-membrane by extensive dorso-ventral fanning motions covering as much as 135 degrees of arc consistent with thrust generation by air displacement. The degree of dorsal extension of the tail-membrane, and thus the potential amount of thrust generated during platform launches, was significantly correlated with body mass (P = 0.02). Adduction of the hind limbs during upstrokes collapsed the tail-membrane thereby reducing its surface area and minimizing negative lift forces. Abduction of the hind limbs during the downstroke fully expanded the tail-membrane as it was swept ventrally. The flapping kinematics of the tail-membrane is thus consistent with expectations for an airfoil. Timing offsets between the wings and tail-membrane during downstrokes was as much as 50%, suggesting that the tail-membrane was providing thrust and perhaps lift when the wings were retracting through the upstoke phase of the wing-beat cycle. The extent to which the tail-membrane was used during takeoffs differed significantly among four vespertilionid species (P = 0.01) and aligned with predictions derived from bat ecomorphology. The extensive fanning motion of the tail membrane by vespertilionid bats has not been reported for other flying vertebrates. Citation: Adams RA, Snode ER, Shaw JB (2012) Flapping Tail Membrane in Bats Produces Potentially Important Thrust during Horizontal Takeoffs and Very Slow Flight. PLoS ONE 7(2): e32074. doi:10.1371/journal.pone.0032074 Editor: Brock Fenton, University of Western Ontario, Canada Received August 22, 2011; Accepted January 20, 2012; Published February 29, 2012 Copyright: 2012 Adams et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Research was funded by the Provost Research Fund at the University of Northern Colorado. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: rick.adams@unco.edu Introduction The evolution of flight in mammals occurred more than 50 million years ago and led to today’s diversity of bats that compose nearly 24% of all living mammalian species [1]. The dawn of bat flight was evolutionarily independent from events that gave rise to birds and pterosaurs, the only other groups of vertebrates known to have evolved flight. Accordingly, the form and function of bat wings differ considerably from birds and pterosaurs and the unique nature of bat flight has stimulated much interest in habitat use and foraging ecology of various species [2]. Although studies of flight using wind tunnels have been pervasive, increasingly research has focused on free-flight kinematics in bats which may deliver a more realistic interpretation of how bats use their wings in nature [3–10]. Undoubtedly, the nearly singular emphasis on the wings (hand- + arm-wings) of bats as the major adaptive character for flight is warranted and expected. However, such focused research has somewhat diverted investigations of other potentially important attributes and dimensions of flight. For example, an aspect of bat flight not adequately investigated is the use of the tail-membrane (uropatagium) as a contributor to flight performance. Although some insectivorous bats are known for using their tail-membrane to catch insects in flight and also in some elements of flight control [11–12], the extent to which bats invest their tail-membrane to augment flight performance has rarely been considered. Two investigators [10,13,14] have speculated that the tail-membrane may produce lift during flight and one [10] has noted slight upward and downward motions of the tail-membrane in synchrony with the wingbeat cycle during wind tunnel flights in the brown long-eared bat (Plecotus auritus, Vespertilionidae). Herein we provide the first evidence that vespertilionid bats are capable of extensively flapping their tail-membrane in coordina- tion with the wings and offer a testable model wherein the mechanics and kinematics of the tail-membrane are consistent with expectations for the production of thrust and lift. We hypothesize that the tail-membrane in vespertilionid bats has a significant role in providing thrust and lift during takeoff events at very low flight speeds (,1 m s21) particularly at moments when the wings offer minimal contributions to flight. We predict that the tail-membrane motions will be consistent with expectations surrounding maximizing positive force production and minimizing production of negative lift forces. We also predict that the degree (extent) of tail-membrane flapping will be correlated with the body mass of individuals of similar wing dimensions because greater thrust is required to lift greater mass from a stationary position. We tested our hypothesis and predictions by filming individuals from five species of vespertilionid bats launching from a horizontal platform expected induce significant flight stress during takeoff. PLoS ONE | www.plosone.org 1 February 2012 | Volume 7 | Issue 2 | e32074