Recent studies have advanced the understanding of how bats use echolocation for navigating complex environments and detecting stationary prey. One study led by the University of Bristol investigated the role of "acoustic flow velocity" in bat navigation and speed control, while another involving multiple universities explored how "steadiness of echoes" assists big-eared bats in finding insects on leaves. These findings contribute to biological knowledge and offer potential applications for autonomous technologies.
Acoustic Flow Velocity for Navigation and Speed Control
A study led by the University of Bristol, published on January 21 in Proceedings of the Royal Society B, identified a mechanism by which wild bats navigate complex environments in complete darkness. While bats are known to use biosonar (echolocation) to map their surroundings, the method for processing numerous overlapping echoes in real-time within complex habitats, such as forests, was previously unclear.
Researchers hypothesized that bats utilize "acoustic flow velocity" for orientation. Dr. Athia Haron, the study's lead author from Bristol's School of Civil, Aerospace and Design Engineering, suggested that analyzing individual echoes from multiple objects would be too challenging for bats, implying reliance on alternative strategies. Professor Marc Holderied, Professor of Sensory Biology at Bristol's School of Biological Sciences, explained that as bats fly and emit calls, echoes return at varying rates depending on object proximity and the bat's speed, generating a 'sound flow.' By detecting changes in this sound flow, bats can potentially map their environment and assess their speed.
To test this hypothesis, a team of aerospace engineers and biologists constructed a custom 'Bat Accelerator Machine.' This eight-meter flight corridor featured revolving hedge-like panels lined with 8,000 acoustic reflectors designed to simulate natural echoes. Over three nights, 181 pipistrelle bat flight trajectories were recorded, with 104 flights analyzed where bats traversed the full test section.
During the experiment, the movement of the reflectors was manipulated to alter the acoustic flow speed experienced by the bats. Researchers observed that bats adjusted their flight speed in response. When acoustic flow speed was increased by moving reflectors against the bats' direction of travel, bats flew up to 28% slower. Conversely, when reflectors moved in the bats' flight direction, the bats accelerated. These adjustments indicate that bats are sensitive to the Doppler shift, a component of acoustic flow, and may use it for speed control.
This discovery suggests that bats employ Doppler-based acoustic flow for navigation. Dr. Shane Windsor, a co-author of the study, noted that the experiment demonstrated the ability to influence bat flight speed using the 'revolving hedges,' providing evidence for bats' reliance on acoustic flow for speed control and navigation. This principle could inform the development of new navigation methods for drone technology and autonomous vehicles.
Detecting Prey on Leaves Using Echo Steadiness
Separately, scientists from the University of Cincinnati, the University of Antwerp, and the Smithsonian Tropical Research Institute conducted experiments to determine how big-eared bats detect insects on leaves in the dark. The research aimed to explain how tropical bats use echolocation to identify silent insect prey.
Previous behavioral studies had suggested that big-eared bats might use leaves as acoustic mirrors, where smooth, empty leaves reflect most echolocation sound away, producing minimal echo, while an insect on a leaf scatters sound, sending a detectable echo back to the bat. However, this theory posed a challenge: for bats to use this method, they would need to measure a leaf's orientation and position, a potentially time-consuming task.
The scientists, including lead author Dieter Vanderelst from the University of Cincinnati, concluded that bats may rely on the steadiness of echoes rather than requiring precise environmental mapping. This approach could allow bats to focus on promising targets more efficiently.
To test this hypothesis, a robot was constructed to imitate bat foraging. Equipped with ultrasonic sensors mimicking bat echolocation, the robot explored an array of cardboard leaves, one of which held a fake dragonfly, without measuring leaf size or orientation. The robot tracked stable targets as the echoes weakened. The robot identified prey-occupied leaves with 98% accuracy and falsely detected prey on empty leaves in 18% of trials. Analysis indicated that empty leaves produced echoes that varied sharply with the approach angle, whereas prey-bearing leaves generated consistent echoes regardless of the approach direction.
The experiment confirmed that bats can detect prey without constructing a detailed spatial model of their surroundings. This method, utilizing predictable acoustic patterns, may reduce cognitive load for bats navigating complex natural environments. The study suggests that evolutionary processes can favor efficient systems over complex calculations. Beyond biology, these findings could influence engineering, potentially inspiring new sonar-based technologies for applications such as detecting pests on crops or fruit within foliage.