The moments of inertia of the wings about the shoulder joint and about the roll axis were estimated in eight species of bats, using strip analysis. The moment of inertia of the bat's trunk about the roll axis was estimated by assuming the body and head to be ellipsoids. The slopes of the regressions of moment of inertia of one wing about the shoulder joint ( J w ) versus body mass ( m tot ), wing span ( b ) and wing area (S) were as expected for geometrically similar animals of different size. The exponent for J w versus body mass in bats deviates from that found for birds, while the exponent for J w versus wing span does not. A multiple regression was used to show that J w may be estimated by: J w = 4.49 × 10 −3 m tot 0.53 b 2.15 S 0.65 . The mean value of the moment of inertia originating from the trunk is 7 % of the bat's total moment of inertia (of wings and body combined) about the roll axis. The mass of one wing ( m w ) was plotted against body mass for the eight bat species, which gives: m w = 0.112 m tot 1 11 . The slope for our bats, 1.11, is similar to that obtained for birds, 1.10. Adaptations to reduce the moments of inertia may be more important for increasing a bat's flight agility (roll acceleration) than for decreasing the total mechanical power required to fly. The influences of wing moment of inertia and wing shape on manoeuvrability and agility are discussed.
Two manoeuvres in bats are described: rolls through 180°, in Nyctalus noctula , and a series of sideslips in Otomops martiensseni . These manoeuvres cause a rapid loss of height. They are initiated by pronation of one wing and supination of the other. After the roll, when the bat is in an upside down position, the lift force of the wings is directed downwards, causing a tight turn downwards (apparently for insect catching). During sideslip the body drag of the bat is increased. This reduces the total lift/drag ratio, thus steepening the equilibrium gliding angle.