Bats’ biosonar inspires sensing technology research
By Deborah Blanchard
Bats can be found in all but the most remote places — only the polar icecaps and a few isolated oceanic islands are unoccupied by these remarkable nocturnal predators. Bats account for about one-fifth of all mammalian species; approximately 1,000 different species can be found worldwide.
Rolf Mueller, Virginia Tech associate professor of mechanical engineering, has traveled the world investigating the prolific species, using his fascination with the creatures to further a research program focusing on sensing technology for robots. Based at the Institute for Advanced Learning and Research (IALR) in Danville, Va., researchers at Mueller’s Bio-inspired Technology (BIT) Laboratory are conducting interdisciplinary research that bridges engineering and the disciplines of physics, computer science, and biology.
Based on few input signals, the bat’s brain is able to make the quick and reliable decisions that swift flight in confined spaces requires.
Although bats possess notoriously poor eyesight (no species is actually blind, despite that popular cliché), the mammals are able to shine in complicated, feature-rich natural environments through their use of both active and passive sonar. According to Mueller, “Bats excel at navigating these environments, while man-made sensory systems are prone to catastrophic failures in the same situations.”
The BIT lab director explains that the most common engineering approach to deal with such failures has been to equip autonomous vehicles with sensor systems like laser scanners and video cameras that deliver continuous streams of large amounts of two- and three-dimensional sensor data. Says Mueller, “These data streams often overwhelm the on-board computing resources of the autonomous systems, which means that achieving reliable real-time performance has remained an elusive goal.”
However, the Southside Virginia researcher believes that robotics can benefit from his study of the furry flying predators. “The split-second decisions that bats on the wing have to make will often be based only on a very small number of echoes. Based on these few input signals, the bat’s brain is able to make the quick and reliable decisions that swift flight in confined spaces requires,” says Mueller. Bio-inspired sonar technology might enhance the ability of autonomous vehicles to avoid collisions.
BIT researchers are investigating the hypothesis that bats can efficiently encode reliable sensory information at the interface between the animal and its environment. To explain this ability, the engineers are examining the elaborate baffle shapes comprising most bats’ ears. With features such as grooves, ridges, and flaps, these ear baffles range in size from relatively small to huge.
The scientists are also intrigued by “noseleaves,” thought to be sensitive to vibrations in the air. A leaf-life noseleaf is a flat skin outgrowth on the nose of some bats.
The ultrasonic waves that the bats emit and receive are diffracted on the surfaces of these shapes. The results of these diffraction processes depend upon the geometry of the shapes, the frequency of the ultrasonic wave, and the direction in which the sound wave is traveling.
Consequently, the pulses are either emitted into the environment or are received at the animal’s eardrum where they are filtered at the speed of sound — quite a feat without computers.
Across different species, bats are extremely versatile in their habitats — from desert to rainforest — and the type of food they pursue — insects; arthropods ranging from spiders to scorpions; and small vertebrates, such as fish, amphibians, reptiles, birds, and mammals (including other bats); as well as fruit, nectar, pollen, and blood. Each of these different combinations of environment and prey types defines a different set of sensory tasks that bats have mastered.
The bats’ noseleaves and ears determine which and how information is encoded into the received echo signals. Says Mueller, “A well-crafted, well-positioned selectivity filter will ensure that important environmental information is encoded reliably into the received signals. At the same time, the filter would ensure that irrelevant detail is suppressed and relevant information is encoded to be easily accessible.”
Mueller’s team believes that for applications in man-made sensor systems, it is important to understand both the spatial filtering strategies as well as the physical mechanisms that enable them.
Over the past several years, Mueller’s research has revealed intriguing physical mechanisms at work in bats. For example, in 2009 Mueller and his associates were credited with solving a mystery about “horseshoe bats” that has baffled the scientific community for over a half century. Mueller’s findings revealed that the Bourret’s horseshoe bat, which has a nose almost twice as long as other bats, employs the nose to create a highly focused sonar beam. Computer modeling of more than 120 species of bats suggests that they exploit the frequency-selective nature of resonance according to physical characteristics of their ears and noses.
All results to date imply that bats emit their biosonar pulses in a single ultrasonic beam. The effect is that the sonar beamwidth produces changes from narrow to wide depending upon the bats’ physical structure, Mueller says.
“This effect could be compared to technical band trap antennas that are used for electromagnetic waves. A band trap antenna contains a resonance circuit that shortens the length of the antenna at the resonance frequency.” In contrast, bats apparently experience incoming sounds as more complicated and varied patterns based on so-called “sidelobes” — secondary structures in the ear that point in directions away from the direction of maximum sensitivity. This situation could be compared to adding weak spotlights pointing to the side of a stronger spotlight.
In man-made sonar and radar systems, sidelobes are commonly regarded as nuisances that need to be suppressed. In bats, they seem to be prominent features that are enhanced by the shapes of the ears. For example, in the big brown bat, a projection on the ear called the tragus seems to be responsible for the creation of sidelobes. The tragus is a common element of the visible portion of the mammalian ear; in humans, it points rearward and is at the front edge of the outer ear. In bats, the tragus is often a comparatively large structure that ranges in shape from broad mushroom-like to long lancet-like versions. “The greater prominence of the tragus in bats may be seen as an indication of the importance of this structure and the sidelobes it produces to biosonar function,” says Mueller, adding, “We found that a small ridge on the lower inner wall of the outer ear makes a big difference.”
This was the case in the brown long-eared bat that is — as the name suggests — mostly known for its large ears. “Digital removal of the small ridge from this species resulted in the disappearance of an entire set of sidelobes; information-theoretic analysis showed that this set of frequency-swept sidelobes was responsible for about half of the information on target direction that was encoded by this bat ear,” Mueller says.
Bat sonar has major potential for technical applications based on the biodiversity in ear and noseleaf shapes that exists. Some of this diversity is likely to represent adaptations of the biosonar system to the specifics of the biosonar tasks that the respective bat species have mastered.
“Bats represent the outcome of a remarkable evolutionary success story,” says Mueller. “A detailed, quantitative understanding of the evolutionary trends that underlie their adaptations of the common functional principles of biosonar could be used in the design of customized technology.”