The Use of Bat Tail Membranes

The uropatagium, or tail membrane, is skin in between the legs and tail that appears in nearly all bats and pterosaurs. Its role in pterosaurs is debated, but bats we can at least do experiments with. Like everything else, there is no one ideal shape, but the tail membrane has adapted to its user’s lifestyle.

Bullen and McKenzie classify bat uropatagiums into four shapes, from that providing the most control and drag to the least. The first three characterize insectivores, the third is also of carnivores, and the fourth typical of fruit bats. Type 2 are the molossids which have a ring around their tail that allows them to retract the tail membrane. Type 3 provides the best lift to drag ratio at slower speeds – any surface beyond the widest part of a tail contributes only to drag. Some species use their tail membrane to capture bugs, and in those especially the urotapagium is much thicker.

Bullen and McKenzie assign urotapagium type 1 to M. schreibersii, type 2 to C. plicata, type 3 to H. ater, and type 4 to P. lombocensis. From McKenzie et. al. 1995.

Removing the tail membrane from a model of P. auritus (type 1?) had no effect on lift production – the angle of the legs modifying the camber of the wings had a much larger effect. Neither does it improve stability; in fact, Gardiner et. al. predicts the opposite. Though the tail membrane is thought to reduce drag by streamlining the body, a smaller membrane is associated with a faster flight speed. The feet, ankles and tail all seem to be the smallest size possible for their purpose, and the area of the uropatagium increases linearly with mass. What it is good for is altering the pitching moment, something the wings are poorly positioned to do. In short, the tail membrane is not much use in steady flight, but crucial for agile maneuvers.

Papers:

Hedenström, Anders, and L. Christoffer Johansson. “Bat flight: aerodynamics, kinematics and flight morphology.” Journal of Experimental Biology 218.5 (2015): 653-663.

Gardiner, James D., et al. “A potential role for bat tail membranes in flight control.” PloS one 6.3 (2011): e18214.

Bullen, R. D., and N. L. McKenzie. “Aerodynamic cleanliness in bats.” Australian Journal of Zoology 56.5 (2009): 281-296.