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 (Jw) versus body mass (mtot), wing span (b) and wing area (S) were as expected for geometrically similar animals of different size. The exponent for Jwversus body mass in bats deviates from that found for birds, while the exponent for Jw versus wing span does not. A multiple regression was used to show that Jw may be estimated by:
Jw = 4.49 × 10−3mtot0.53b2.15S0.65.
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 (mw) was plotted against body mass for the eight bat species, which gives:
mw = 0.112mtot1 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.