Hair Sensors Assist Bat Flight

Bat wing membranes are not sheets of naked skin. Instead, they are covered with tiny hairs. In some species, such as Tadarida condylura (Molossidae), the hairs aid in streamlining the airfoil (Figure 1), thereby reducing drag. In others the wing membranes have fewer hairs, but a new study shows that tiny hairs on the wing membrane serve a very different function in some microchiropteran bats (Sterbing-D’Angelo et al., 2011).

Tadarida
Figure 1. A drawing of the cross section of the wing in the molossid bat Tadarida condylura, showing how the hairs on the posterior wing membrane create a more streamlined airfoil. (Adapted from Wilson and Gardner 1980)

Researchers from the University of Maryland and Ohio University used big brown bats (
Eptesicus fuscus) and short-tailed fruit bats (Carollia perspicillata) to test the hypothesis that the fine hairs on the wing membranes act as sensory receptors to provide information that aids in flight control. First, the wing hairs were mapped and anatomically described using scanning electron micrographs (Figure 2). The hairs occur over the entire wing surface, but come in tow distinct types; long hairs that are found closer to the body and forearm, and tiny hairs (100-600 microns long) that are found in rows along the trailing edge of the wing.
sensory_hairs
Figure 2. Scanning electron micrographs of the sensory hairs on the wing of a bat before (A) and after depilation (B and C) and the position on the wing where the hairs in the SEM photos were located. (From Sterbing-D’Angelo et al., 2011)

The researchers then subjected the shorter hairs to tiny puffs of air lasting only 40 milliseconds and recorded from neurons in the bats brain that respond to tactile stimuli from the wing membrane. Specifically, they recorded from electrodes placed in the bat’s primary somatosensory cortex. The air puffs were gentle enough to deflect the hairs but not the surrounding wing membrane. The researchers then removed all the hairs from the bat’s wing membranes using a depilatory solution and recorded the neuronal responses again.

The short sensory hairs elicited a neuronal response in the somatosensory cortex when exposed to air puffs, but the response disappeared when the hairs were removed (Figure 3). Furthermore, the response was directional and depended on the location of the hairs on the wing membrane. Interestingly, the hairs on the trailing edge of the wing were most sensitive to air puffs coming from the rear (posterior to anterior). This suggests that these tiny hairs may serve as turbulence sensors, warning the bat that airflow has been disrupted along the trailing edge of the wing. Such turbulence is more likely when the bats are flying at slow speeds or making rapid, sharp turns. Thus, the hair sensors may serve as stall detectors.

sensory_hairs2
Figure 3. Neural responses of somatosensory neurons to air puffs before and after depilation of the sensory hairs on the bat’s wing. Colors indicate position on the wing. Solid lines on the graphs denote post-delipatory responses and the open areas under the upper lines represent pre-depilatory responses. (From Sterbing-D’Angelo et al., 2011)

Bats were also trained to fly through a maze of obstacles in a flight room. The researchers compared their flight performance before and after removal of the tiny sensory hairs. Without sensory hairs, bats made wider turns, flew further away from obstacles, and increased their flight speed. These results indicate that the sensory hairs along the trailing edge may serve as velocity sensors – in the absence of velocity information, the bats responded by increasing flight speed.

Thus, it appears that these tactile hairs provide bats with real-time feedback on airflow and velocity as the animal flies in a cluttered environment.



References

Sterbing-D'Angelo, S., Chadha, M., Chiu, C., Falk, B., Xian, W., Barcelo, J., Zook, J., & Moss, C. (2011). Bat wing sensors support flight control Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1018740108

Wilson, D. and A. Gardner. (1980)
Proceedings of the Fifth International Bat Research Conference. Texas Tech Press, Lubbock, TX.