Diffraction of Life - biosonar camouflage, cloaking and concealment

Lead Research Organisation: University of Bristol
Department Name: Biological Sciences

Abstract

Invisibility cloaks are fantastic devices in popular culture from Harry Potter to Star Wars. The science behind cloaking has been advanced to a level that brings a future real-life invisibility cloak within our reach. In fact, several cloaks with partial functionality have already been realised with so-called metamaterials - assemblies of multiple elements engineered to have properties not yet found in nature.
An even more promising field for the development of a functional cloaking device is not for light, but for sound - acoustic cloaks. Because the wavelengths of sound are longer than those of light waves, it is easier to design and build acoustic metamaterials and hence effective cloaks. Indeed, the most advanced acoustic cloak can now completely hide an object on a surface - a so-called carpet cloak. As a metamaterial it consists of partly overlapping perforated plates, arranged much like roof tiles.
While we know of no metamaterials for light in nature, is this also true for acoustic metamaterials? Which organism would need such a device to hide itself acoustically? We propose the answer lies in the 65MY old arms race between echolocating bats and their moth prey. A 'biosonar cloak' against bats would reduce predation pressure on the moths and therefore offer substantial evolutionary benefits. Interestingly, the layers of scales on a moth's body surfaces bear remarkable structural resemblance to an acoustic carpet cloak.
We hypothesise that moth wings are an acoustic metamaterial engineered by nature. We will investigate whether the scales on the moths have acoustic properties that hide the moth from an echolocating bat.
In our pilot study, we have developed a 'biosonar visualizer' that creates acoustic images revealing the reflective nature of body parts. This technique is closely related to medical ultrasound imaging (tomography). We also use a laser scanner to measure how the layer of scales vibrates in response to ultrasound. From these preliminary data we find a surprising range of interesting adaptations: First, scales on a (dead and dried) moth wing change wing reflectivity by a factor of four. In another very exciting discovery, we find that the long tails of Luna moths reflect strong echoes such that they attract the bat's attention and attack away from the moth's body. We also find that the eye spots, used in a visual display to startle an approaching predator, also stand out acoustically. Finally, we have evidence that moths choose the places to rest and adjust their wing position to reduce contrast and blend into the substrate acoustically. This pilot data make clear that there is a wide and promising unstudied field of echoacoustic adaptations.

In analogy to visual camouflage, we introduce an entirely novel field of research - biosonar camouflage.

We identify several possible strategies for camouflage. One strategy is for the moth to reflect very little ultrasound (cloaking) when in flight, thus reducing the distance over which a bat can detect it. Another strategy involves mimicking the echoes of a resting surface. In this scenario, the moth resting on the bark of a tree is acoustically blending with its environment.
Our research will establish what acoustic properties and sound processing mechanisms have evolved in moth scales in response to bat biosonar.
Since the industrial revolution, the world has become a noisy place where man-made sounds are pervasive throughout our living habitats. Acoustic pollution is a source of discomfort and stress for humans and animals. We will use our understanding of moth wings to 3D print scaled prototypes with acoustic metamaterial properties at audible frequencies. Thereby we contribute new bio-inspired solutions to current noise control challenges at the low frequencies so important to human speech and comfort.

Technical Summary

In the 65MY old arms race between echolocating bats and moths the strength of the moth's echo determines the distance over which bats can detect it. Our pilot data now make clear that -in analogy to visual camouflage- there is a wide unstudied field of echoacoustic adaptations, which we introduce here as 'biosonar camouflage'.
Our central hypothesis is that the layers of scales on the moths' body act as acoustically active metamaterials with echo-reflective properties that reduce predation risk by biosonar.
One camouflage strategy is efficient absorption of ultrasound (cloaking) in flight to reduce the bat's detection range. Another involves matching absorption to that of the resting substrate to acoustically blend with it.
Our pilot data show that fresh scales affect ultrasound absorbance, revealing biosonar 'cloaking', 'matching to background', 'shadow elimination', 'deceptive false targets' and 'eye spot startling signals'. None of these mechanisms have been studied, but they only work at frequencies used by bats, which is direct evidence for a role against biosonar.
Our innovative set of approaches, mainly developed and exclusively available in Bristol, offers a unique opportunity to discover novel near-field and boundary layer mechanisms of sound manipulation.
We hypothesize that the actual mechanism is a combination of static absorptive, dissipative, refractive and active mechanical interactions between structure and ultrasound. Using acoustomechanical characterisation and mathematical modelling we will test five different absorptive processes. We will manufacture and test plastic replicas of wing structures scaled up to frequencies relevant to humans. Humans do not use or are sensitive to ultrasonic biosonar, yet our research will develop novel ways to visualise and understand this elusive part of the natural world, and aim to inspire man-made radar and sonar camouflage and ultra-thin absorptive structures for architectural acoustics and noise control.

Planned Impact

The planned research will benefit researchers in the field of bioacoustics, including the sensory ecology and biophysics of moths and bats. Establishing the echo acoustics of moths will further inform the constraints set upon bat echolocation strategies, and therefore the evolution of hearing in bats. Several other disciplines related to the studies of environmental acoustics, architectural acoustics, noise control and abatement will also be interested by our approach and research rationale. In particular, the identification and characterization of structurally complicated metamaterials -the scaled wings of moths- at the length scale of centimetres to micrometers, including nanoscale mechanical responses will spark the interest of designers and developers of acoustic foams and panels. In addition, our finite element modeling work will constitute a novel basis for the "in silico" design of adapted, hybrid alternatives to conventional alveolar foams.
Perhaps starting at the times of the industrial revolution, the world has become a noisy place. Machinery, vehicle and human-generated sounds are pervasive throughout our living habitats, and the living habitat. Our research is expected to impact on our fundamental understanding of acoustic ecology, and provide further tools to mitigate the transmission of sounds where they are unwanted. Acoustic pollution is a source of discomfort for humans and animals. In effect, sound pollution affects not only hearing, but is a source of stress, annoyance and affects sleep and communication. Noise at the work place, home and learning environments has been shown to affect cardiovascular health, attention and academic performance.
Research outcomes have direct impact on how technology will contribute to improve the quality of the human and natural environment. Our work is therefore of direct relevance to national and global human health and human comfort.
In terms of public engagement, the research planned will appeal to a broad cross-section of the public as part of an increasing awareness of the role of noise in everyone's life and the importance of quality of life in the urban environment. Our findings will contribute to current efforts to improve noise control, especially at low frequencies (around 200 Hz), which are notoriously difficult to control. Such contribution will be achieved by direct engagement and collaboration with noise control professionals and manufacturers.
As detailed in our Pathways to Impact document, we will also directly engage with the media, science festivals, environmental agencies and other public organisations, particularly through our interactive biosonar imaging exhibit (tomograph). The public will thus actively benefit from our activities through the electronic media (web pages, twitter, YouTube channel), but also through activities in science festivals, contributions to the general press, and television and radio interviews.
Finally, in conducting this research programme, the team (PDRAs) will gain and benefit from further training and experience in project and personnel management, as well as developing strong communication skills through public engagement and industry and policy-driven knowledge exchange activities. Importantly, we will ensure that training is delivered to the entire team, and that of volunteers, enhancing the educational value of impact, and generating increased opportunities for science to engage with the public and policy makers, teachers, school children, industrial partners and fellow academic researchers.

Publications

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Shen Z (2018) Biomechanics of a moth scale at ultrasonic frequencies. in Proceedings of the National Academy of Sciences of the United States of America

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Holderied MW (2018) Ultrasound avoidance by flying antlions (Myrmeleontidae). in The Journal of experimental biology

 
Description We found that butterflies have stronger echoes than moths, which means moths are less visible to bat biosonar compared to a butterfly of the same size flying at night. We discovered that this is due to a number of adaptations on the moth body surface. First, the body hair on moths but not butterflies act as very effective ultrasound absorbers, creating stealth protection against detection by bat biosonar. The same is true for a furry flap called tegula, which covers and therefore stealth camouflages the insect's wing joints. Similarly, The microstructure of Lepidopteran scales differ systematically between diurnal and nocturnal taxa. The individual scales have resonances in the bat ultrasonic range and their assembly on the wings creates a joinet behaviour qualifying it as the first example of a biological acoustic metamaterial.
Exploitation Route Biological meatmaterials will inspire the very active field of cloaking, metamaterials and transformation acoustics. These have great promise for sonar and radar applcaitions as well as building acoustics.
Sectors Aerospace, Defence and Marine,Construction

 
Description Leverhulme Research Fellowship
Amount £41,000 (GBP)
Organisation The Leverhulme Trust 
Sector Academic/University
Country United Kingdom
Start 09/2017 
End 06/2018
 
Description University Research Fellowship
Amount £10,000 (GBP)
Organisation University of Bristol 
Sector Academic/University
Country United Kingdom
Start 08/2017 
End 07/2018
 
Title Finite element modelling - Multiphysics modelling 
Description This method is standard within the engineering and physics community. It is increasingly used in the life sciences, as software is becoming available and amenable to the complexity of biological systems. Also, a large portion of accessibility is due to the power of modern desktop computers. 
Type Of Material Model of mechanisms or symptoms - non-mammalian in vivo 
Provided To Others? Yes  
Impact The method has impacted on our capacity to model the complex interaction of organisms with their physical environment. This aspect - that we call Physical Ecology - is expanding and is poised to touch many realms of life Sciences. For us the impact has been significant as the model predictions have allowed us to better understand the sensory ecology of the organisms we study, mostly insects, but also plants. 
URL https://uk.comsol.com/
 
Description Bristol City Museum 
Organisation Bristol City Council
Country United Kingdom 
Sector Public 
PI Contribution Joint research using the museum insect collection
Collaborator Contribution Provide logistical support and access to their collection of insect specimens.
Impact ongoing
Start Year 2016
 
Description DIAMOND light source 
Organisation Diamond Light Source
Country United Kingdom 
Sector Academic/University 
PI Contribution Providing samples, receiving training and conducting measurements on site.
Collaborator Contribution Provide training, support and access to imaging infrastructure. Support with analysis and interpretation.
Impact Data produced are currently analysised for future publication.
Start Year 2017
 
Description Natural History Museum London 
Organisation Natural History Museum
Country United Kingdom 
Sector Public 
PI Contribution Imaging equipment, analysis tools and manpower for data collection.
Collaborator Contribution Logistical support, consulting, and access to their collection and research infrastructure.
Impact Data collection so far
Start Year 2017
 
Description BBC Four 'Sound Waves: The Symphony of Physics' filming of bats and moths 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Developing contribution with BBC team and filming at my field site at the University's Fenswood Farm.
Now features as part of the BBC four series https://www.bbc.co.uk/programmes/b08h5gk8 which is regualrly rebroadcast.
Year(s) Of Engagement Activity 2017
URL https://www.bbc.co.uk/programmes/b08h5gk8
 
Description Bat Research Conference (Durban) talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Gave presentation at the biggest and most influential topical international research conference.
Year(s) Of Engagement Activity 2016
 
Description Bristol Botanic Garden Bee and Pollination Festival 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Presentation on biosonar sensing to the visitors of the festival.
Year(s) Of Engagement Activity 2017
URL http://www.bristol.ac.uk/botanic-garden/events/2017/bee-and-pollination-festival-2017.html
 
Description Bristol Natural Histaory Consortiium Festival of Nature interactive display - BATtleships 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact An interactive display at the 2017 Festival of Nature in Bristol. The title is BATtleships and it shows how we can use ultrasound to detect and localize objects in the absence of light.
Year(s) Of Engagement Activity 2017
URL http://www.bnhc.org.uk/?post_type=festival-of-nature&p=7594
 
Description Hack the Senses 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Professional Practitioners
Results and Impact Participated in the hack the senses hackathon in London.
Year(s) Of Engagement Activity 2016
URL https://www.hackthesenses.com/
 
Description Natural History Museum research seminar 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact Research seminar in the NHM on aocustic camouflage in moths.
Year(s) Of Engagement Activity 2018
 
Description Nature Roulette at Festival of Nature 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Series of individual presentations to small groups of visitors.
Year(s) Of Engagement Activity 2017
URL http://www.bnhc.org.uk/nature-roulette-will-meet/
 
Description Pint of Science talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Presentation as part of the Pint of Science initiative of public outreach.
Year(s) Of Engagement Activity 2017
URL http://pintofscience.com/
 
Description Presentation at Bristol Moth Group meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Study participants or study members
Results and Impact Bristol Moth Group led by Ray Barnett from Bristol City Museum is a stakeholder in the project through providing logistical support in acquiring specimens and developing the research focus. Members of the research team provided feedback at the annual meeting of the moth group.
Year(s) Of Engagement Activity 2017
 
Description Presenting at the Innovate UK Knowledge Transfer Network meeting on Industrial Application of Metamaterials 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact We were invited to present our project at the 2017 Knowledge Transfer Network event 'Industrial Applications of Metamaterials - a UK strategy' and contributed to the event.
Year(s) Of Engagement Activity 2017
URL https://www.ktn-uk.co.uk/events/industrial-application-of-metamaterials-a-uk-strategy
 
Description Wildlife Photographer of the Year - BATtleships interactive display for visitors 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Interactive BATleships display used at the opening of the WIldlife Photographer of the year exhibition at Bristol's MShed.
Year(s) Of Engagement Activity 2018