Bats Use Acoustic Flow Velocity to Navigate Complex Environments in Complete Darkness

A long-standing mystery about how bats move through dense, cluttered environments at night has finally been cracked. A new study led by researchers from the University of Bristol reveals that bats rely on a clever sound-based strategy known as acoustic flow velocity to navigate with remarkable precision in complete darkness. The findings, published in Proceedings of the Royal Society B, shed light on how bats manage the overwhelming flood of echoes produced by their own echolocation calls when flying through complex habitats like forests and hedgerows.

Why Bat Navigation Has Always Been a Puzzle

It has been known for decades that bats use echolocation, also called biosonar, to orient themselves and hunt prey at night. They emit high-frequency calls and listen to the returning echoes to understand what is around them. This system works well in open spaces, but scientists have long struggled to explain how bats cope in highly cluttered environments, where a single call can generate thousands of overlapping echoes from leaves, branches, and other obstacles.

Processing each individual echo in real time would be computationally overwhelming, even for a bat’s highly specialized sensory system. This raised a critical question: how do bats simplify the problem without crashing into obstacles?

The Concept of Acoustic Flow Velocity

The Bristol research team proposes that bats solve this problem by focusing not on individual echoes, but on the overall pattern of sound movement, known as acoustic flow velocity. This concept is closely related to optic flow in vision, where moving objects appear to pass faster across your field of view as you speed up, such as when cycling down a narrow path.

In bats, the same idea applies to sound. As a bat flies and emits calls, echoes return at different rates depending on the bat’s flight speed and the distance to surrounding objects. These changes create a continuous “flow” of sound information. By sensing variations in this flow, bats can judge how fast they are moving and how close obstacles are, without needing to analyze every echo separately.

Building the Bat Accelerator Machine

To test this idea, the research team combined expertise from aerospace engineering and sensory biology to design a unique field experiment. They built a custom device called the Bat Accelerator Machine, an eight-meter-long flight corridor lined with 8,000 acoustic reflectors designed to mimic the echoes of real leaves in a dense hedge.

These reflectors were mounted on revolving, hedge-like panels that could be moved either with or against the bats’ direction of flight. By controlling the motion of the reflectors, the researchers could artificially increase or decrease the bats’ perceived acoustic flow velocity without changing the actual physical layout of the environment.

This setup allowed the team to directly test whether bats adjust their flight behavior in response to changes in acoustic flow.

Observing Real Bats in Flight

The experiment focused on Pipistrellus pipistrellus, commonly known as the common pipistrelle bat. Over three nights, researchers recorded 181 flight trajectories of wild bats passing through the corridor. Out of these, 104 flights in which bats completed the full eight-meter test section were selected for detailed analysis.

The key variable measured was flight speed, which served as a clear indicator of how bats responded to manipulated acoustic flow conditions.

What the Results Revealed

The results were striking and consistent. When the reflectors were moved against the bats’ direction of travel, the acoustic flow speed increased. In response, bats slowed down, reducing their flight speed by up to 28 percent of the induced change. When the reflectors moved in the same direction as the bats, reducing the acoustic flow speed, the bats accelerated.

These adjustments show that bats are highly sensitive to changes in Doppler shift, a key feature of acoustic flow where the frequency of echoes changes depending on relative motion. Rather than relying solely on discrete echo timing, bats appear to use Doppler-based flow information to regulate their speed and maintain safe navigation through complex spaces.

Why This Matters for Understanding Echolocation

This study provides some of the strongest experimental evidence to date that bats use acoustic flow as a core navigational tool. It explains how bats avoid sensory overload in cluttered environments and helps resolve a long-standing debate in sensory biology about how echolocating animals simplify complex auditory scenes.

Importantly, the findings suggest that speed control and navigation are tightly linked in bats. By maintaining a preferred acoustic flow velocity, bats can dynamically adjust their flight behavior to suit their surroundings, whether flying through open air or dense vegetation.

Broader Implications Beyond Bats

The discovery has implications far beyond bat biology. Engineers working on drones, autonomous vehicles, and robotic navigation systems are increasingly interested in bio-inspired solutions for operating in environments where GPS and vision are unreliable.

Acoustic flow-based navigation could offer a new approach for machines to move safely through dark, dusty, smoky, or visually cluttered spaces, using sound instead of cameras. The bat accelerator experiment demonstrates that relatively simple sensory rules can produce highly effective navigation strategies.

A Closer Look at Pipistrelle Bats

The common pipistrelle is one of the smallest and most widespread bat species in Europe. Despite its small size, it is an exceptionally agile flier, capable of hunting insects in dense vegetation. Its success makes it an ideal subject for studying advanced echolocation strategies.

Pipistrelles emit rapid sequences of ultrasonic calls while flying, adjusting call rate and frequency depending on their environment. This flexibility likely works hand-in-hand with acoustic flow sensing, allowing them to fine-tune both where they fly and how fast they move.

How Acoustic Flow Differs from Traditional Echolocation

Traditional explanations of echolocation focus on distance measurement, where bats calculate how far away an object is based on echo delay. Acoustic flow adds another layer by emphasizing relative motion and speed rather than precise object mapping.

This approach reduces cognitive load and allows for real-time decision-making, which is critical during fast flight. Instead of building a detailed 3D map of their surroundings, bats rely on dynamic patterns of sound change to guide their movements.

A Major Step Forward in Sensory Biology

By demonstrating that free-ranging bats actively adjust their flight speed based on manipulated acoustic flow, this study moves beyond laboratory theory and into real-world behavior. It shows that acoustic flow velocity is not just a theoretical concept, but a functional, biologically relevant mechanism used by bats in natural conditions.

As researchers continue to explore how animals solve complex sensory problems, studies like this highlight how evolution has produced elegant solutions that still outperform many human-designed systems.

Research paper:
https://doi.org/10.1098/rspb.2025.2481