Lessons in echolocation from bats and dolphins
Dolphins can readily distinguish between fish and similarly shaped decoys, or detect fish hidden under sediment
18 March 2011 by John Rees
Most of us use sight to find out about our environment, but echolocating animals - like bats, dolphins, oilbirds and cave swiftlets - use sound. Now scientists are trying to apply the techniques animals use, to help us in many fields of human endeavour. John Rees from the British Geological Survey tells us more.
Echolocating creatures 'see' using echoes, judging an object's distance and size from the reflections of their calls. And they can identify and measure objects with much greater resolution than we can using sight. It's an amazing ability. Take dolphins, for instance. They can detect objects smaller than a centimetre from a distance of more than 100m. And they do so using acoustic wavelengths that are far larger than the targets they are identifying - so they can locate something no bigger than a millimetre using wavelengths of several centimetres.
We designed a tiny, lightweight recorder that could be carried in a backpack strapped to adult Egyptian fruit bats.
Perhaps most intriguing is their ability to sense the physical properties of objects just using their calls. Dolphins can readily distinguish between fish and similarly shaped decoys, or detect fish hidden under sediment. Even more incredibly, they do all this in highly cluttered acoustic environments - where there's not just a lot of noise from their own calls reflecting off neighbouring objects or organisms but where many other animals are also communicating and echolocating. These skills are not unique to cetaceans, like dolphins and whales; other echolocating animals have evolved very similar capabilities over millions of years.
We've made some headway towards understanding animal acoustic systems, and some aspects have already been applied successfully in engineering, for example in the development of sonar systems. But these look exceedingly primitive compared to the complexity of biological echolocation. Clearly we need to know more about how these animals function.
The Basic Technology Programme for Biologically Inspired Acoustic Systems (BIAS) was created to address that need. It involved scientists and engineers from the British Geological Survey and the universities of Edinburgh, Leeds, Leicester, Southampton and Strathclyde. And it combined many diverse disciplines: animal acousticians, mathematicians, signal processors, acoustical engineers, and geological and medical physicists, who all came together to work out how the techniques of echolocating animals could inspire technological solutions to human problems.
An Egyptian fruit bat with a dorsally mounted sensor which is attached using velcro
One aspect we looked at in detail was echolocator calls, particularly how the animals vary the sounds they emit to help them navigate and catch prey. Bats use a wide and complex array of signal types - pulses of sound at fixed and fluctuating frequencies, many typically lasting only a quarter of a millisecond. The BIAS team captured and analysed these calls, and have stored them in an extensive call library.
Something we wanted to do early on was record bat echoes as they're received by the animals themselves, as opposed to the signals recorded by microphones as the bats fly past. So we designed a tiny, lightweight recorder that could be carried in a backpack strapped to adult Egyptian fruit bats.
Capturing the sounds the bats themselves hear while flying has revealed some of the tricks they use when catching prey - for example they use both higher and lower frequencies than we'd previously thought.
Through experiments like these we realised we needed to pay particular attention to subtle shifts within sound waves as they change over time. Analysing such shifts within animal signals isn't straightforward, so we designed our own signals and new tools to enable us to process them. It's proved to be a highly successful strategy. The new signals are more than ten times more accurate than the ones previously used for distance measurement and, most significantly, they have a similar performance to those emitted by the animals themselves.
We used these bio-inspired signals to explore the physical characterisation of materials too, and it's produced some notable successes, in diverse fields. For example, the transmission and reception of bat- and dolphin-like signals in human tissues, using high-frequency sound, has helped in the measurement of moving heart tissue in cardiac imaging. And we've used lower frequency signals to determine the characteristics of solid and granular geological materials.
One of the immediate benefits to stem from the project has been the development of a range of novel transducers - instruments that emit and receive bio-inspired signals. These possess improved sensitivity and bandwidth when compared with conventional counterparts.
The project has led to significant advances in the resolution with which we can sense objects and identify what they are. This new knowledge is already being put to practical use, for example in non-destructive evaluation (NDE) of materials - such as screening the content of cargoes to see if they contain what they're supposed to. There's lots of potential here too, such as helping position robotic vehicles working in cluttered or noisy environments, like nuclear reactors; and research on object recognition could help hearing-impaired people through a new generation of hearing aids or cochlear implants. And the BIAS team is actively exploring several environmental science applications, including the characterisation of underwater sediments.
These advances are exciting, but the project has brought home to us just how far we still have to go to come anywhere near the sophistication of echolocating animals. As Carl Sagan said, "It is of interest to note that while some dolphins are reported to have learned English - up to 50 words used in correct context - no human being has been reported to have learned dolphinese." That remains true, but BIAS has made a start.
Professor John Rees is NERC's natural hazards theme leader and represented BGS in the BIAS project.