Discovery of bats remarkable navigation strategy revealed in new study

A long-standing mystery about how wild bats navigate complex environments in complete darkness with remarkable precision, has been solved in a new University of Bristol-led study. The findings are published today [21 January] in Proceedings of the Royal Society B.

While it is well known that bats hunting at night use biosonar (also known as echolocation) to map their surroundings, the question of how they process thousands of overlapping echoes in real time when navigating more complex habitats like forests has long remained a mystery. 

To uncover the mechanics behind this extraordinary ability, a team of aerospace engineers and biologists built a custom ‘Bat Accelerator Machine’ to test the theory that bats exploit ‘acoustic flow velocity’ to find their way in more challenging environments.

 Dr Athia Haron, the study’s lead author from Bristol’s School of Civil, Aerospace and Design Engineering, explained: “Bats have a remarkable sensory system, which allows them to interpret echoes from their own calls as they bounce off nearby objects, but how they manage to navigate complex habitats filled with many different obstacles and pinpoint prey with such precision has only now begun to be understood.

“A single bat call will return echoes from multiple objects in different directions and distances. For them to analyse each individual echo becomes too difficult, so they rely on alternative navigational strategies.”

Researchers believe bats use a concept called ‘acoustic flow velocity’ to navigate more challenging habitats.

Marc Holderied, Professor of Sensory Biology at Bristol’s School of Biological Sciences, added: “As bats fly and emit their calls, echoes return at slightly different rates depending on how close objects are and how fast the bat is flying. This creates a kind of sound flow. This concept is similar to how things seem to rush past your eyes faster when you pick up speed on a bike. By sensing changes in this sound flow, bats can map their surroundings and judge their speed, allowing them to move with remarkable precision.”

To test this theory, the team designed a field experiment with a custom-made bat accelerator machine comprising an eight metre flight corridor of revolving hedge-like panels lined with 8,000 acoustic reflectors (artificial leaves) designed to mimic the natural echoes of a hedge covered in real leaves.

One hundred and eighty-one pipistrelle bat flight trajectories were recorded over 3 nights. Of these, 104 bats that went through the bat machine for at least the full eight metre of the test section, were analysed.

During the experiment, the movement of these reflectors were manipulated to change the acoustic flow speed bats would normally experience during flight.  The team measured how bats adjusted their flight speed in response to acoustic-flow speed. When acoustic flow speed was increased by moving reflectors against the bats’ direction of travel, the bats flew significantly slower by up to 28% of the induced decrease. When the reflectors moved in the bats’ flight direction, the bats accelerated.

These adjustments indicate that bats are sensitive to changes in Doppler shift, a key feature of acoustic flow, and may rely on it to control their speed. The team’s discovery suggests that bats use Doppler-based acoustic flow for navigation, a principle that could inspire new methods for navigation in drone technology, potentially enabling drones and autonomous vehicles to navigate complex environments more efficiently.

Dr Shane Windsor, one of the study’s co-authors from Bristol’s School of Civil, Aerospace and Design Engineering, concluded: “We know bats fly swiftly, but we’ve shown that we can make them fly even faster with our corridor of ‘revolving hedges’— our bat accelerator. This experiment suggests that echolocating bats rely on ‘acoustic flow’ for speed control and provides evidence that bats may use this mechanism for navigation.”