The bat is the only mammal truly capable of flight. Its wings are actually flexible membranes spread between its arms and hands.
The authors of the research article, published this week in Proceedings of the National Academy of Sciences, report that the tiny hairs spread across the a bat wing’s dorsal and ventral surfaces, act like the Pitot tubes on aircraft wings to help gauge speed and control flight.
Susanne Sterbing-D’Angelo, of the Institute for Systems Research at the University of Maryland, used a scanning electron microscope to map the distribution of the hairs in two species of bat: the big brown bat (an insect eater) and the short-tailed fruit bat.
They showed that the wing hairs were typically arranged in rows, with some minor differences between the two species of bats.
The researchers then demonstrated that stimulation of the wing hairs, with brief puffs of air from different directions, led to stimulation of the sensory nerve cells at the base of the hairs. This was distinct from tactile responses due to physical indentation of the skin in conditions of high level airflow.
They then used a common depilatory cream to show that the loss of hair removed the nerve cell response.
To evaluate the effect of hair loss on flight, the bats were trained to fly through an obstacle course and performance was measured before and after removing different sections of wing hair.
The authors found bats whose wing hairs had been removed flew faster and made wider turns. They believe nerve cell receptors at the base of these hairs detect turbulent air flow and likely help to stabilize flight.
This finding is in keeping with advice given to airline pilots to increase speed when recovering from engine stalls.
Bob Bullen, who runs an ecological consultancy in Western Australia known as Bat Call, said there are two aspects to aerodynamics: performance, which is energy management, and control, which is the ability to manoeuvre and be agile.
“What these guys have done, which is absolutely fabulous, is to prove in an experimental situation that the air patterns on the wings work in the way that we have been proposing,” said Bullen. “They give sensory feedback to the bat that enables them to fly right to the limits of performance and control.’
Bullen has been involved in research that has described the patterns of hair formation in the 37 species of bat found in Western Australia. He said the patterns are consistent with the bats different foraging strategies.
From bats that fly like helicopters to those that fly like butterflies, Bullen explains that each has evolved to optimize success in its habitat.
“We now have evidence that there’s a correlation between observed hair patterns and aerodynamic ability, which is reflected by the different foraging strategies,” he said.
Working out all the features and factors that affect flight patterns in different species of bat (of which there are approximately 1000 worldwide) will have wider implications than first envisaged.
“Militaries around the world are heavily into what they call micro-air vehicles or MAVs,” said Bullen.
He said understanding the biological basis for the breadth of diversity in bats’ wing hair patterns will add power to this type of technological research.