Scientists studying a species of South American bush cricket with some of the smallest ears known have discovered it has hearing so sophisticated that it rivals our own. The study, published in Science, is the first to identify hearing organs in an insect that are evolutionary convergent to those of mammals.

The HFSP and BBSRC-funded research, led by the scientists at the University of Bristol, show how the bush cricket’s (Copiphora Gorgonensis) auditory system has evolved over millions of years to develop auditory mechanisms strikingly similar to those of humans, but using an entirely different machinery. 

In mammals, hearing relies on three canonical processing stages: an eardrum collecting sound, a middle ear impedance converter and a cochlear frequency analyser.  The bush cricket’s ears, which are found on its two front legs, can perform all three stages, using ears that, despite being much smaller, work like those of humans, but look very different.

After studying the bush cricket’s microscopic auditory system the researchers discovered how impedance conversion — the process of efficiently converting air-borne sounds into liquid-borne vibrations — takes place in these insects.  As a crucial stage of auditory processing in mammals, such process was unknown in insects and, in fact, was thought to only be an attribute of the ears of vertebrates. The bush cricket’s miniature solution to the problem of impedance conversion relies on a system of mechanical levers, a sort of microscopic see-saw formed by its eardrum that makes the link to the inner ear.

Collaborating with archaeologist Professor Kate Robson-Brown at the University of Bristol’s Department of Archaeology and Anthropology, the research team also unveiled the complex internal anatomy of the cricket’s ears. They found a new organ for insects which allows the animal to tell apart a wide range of frequencies.  By measuring nanoscale vibration using laser Doppler technology, the team went on to show that this system works just like the cochlea of mammals, yet about sixty times smaller.

Daniel Robert, one of the study’s lead authors and Professor of Bionanoscience at the University of Bristol’s School of Biological Sciences, said:  “Hearing is one of our most important senses as it enables us to perceive sounds with complex tonal structures. However, in other animals, hearing can often mean a matter of life or death, which explains why this insect has hearing that is so sophisticated. In the cacophony of their rain forest environment, it is crucial for these crickets to distinguish between a chorus of insect sounds and the ultrasounds of hunting bats.”  

Anticipating some of the outcomes of the study, Professor Robert adds: “This discovery that some insects possess such complex biophysical mechanisms for auditory processing is a break through; it will help us develop bio-inspired hearing devices that are smaller and more accurate than ever before, and with much-improved functionality.“

Dr Fernando Montealegre-Z, Senior Lecturer at the University of Lincoln and the study's other lead author, added: "We discovered a novel structure that constitutes the key element in hearing in these insects, which had not been considered in previous work. The organ is a fluid-filled vesicle, which we have named the ‘Auditory Vesicle’. This hearing organ mediates the process of conversion of acoustic energy (sound waves) to mechanical, hydraulic and electrochemical energy.  The integration laser Doppler vibrometry, and micro-CT scanning from the labs of Professors Robert and Robson-Brown allowed to identify the auditory vesicle and to conclude that the process relies on a tympanal lever system analogous to the mammalian ossicles, which serves to transmit air-borne sound to the fluid (the auditory vesicle), and also on the mechanoreceptors.

“Therefore the bushcricket ear performs the crucial stage of air to liquid impedance conversion and amplification just as in a mammal’s ear. These results were obtained on a single species, C. gorgonensis, which uses moderate ultrasonic frequencies of 23kHz. However, other species show similar ear mechanics. For instance, we have discovered a bush-cricket species in the rainforests of Colombia, which emits sounds at 150kHz. It is known how females detect distant calling males at such extreme frequencies. We do think this sensitivity comes, from the Auditory Vesicle. The University of Bristol in collaboration with my new institution, the University of Lincoln, will continue with this research, while also working with partners in other universities. These findings open way for designing ultrasensitive bio-inspired sensors, and change our views on insect hearing."

The study was funded by the Human Frontier Science Program and supported by the Royal Society of London, the UK-India Education and Research Initiative and the BBSRC. The paper, entitled ‘Convergent evolution between insect and mammalian audition’ by Fernando Montealegre-Z.^1* Thorin Jonsson¹, Kate A. Robson-Brown², Matthew Postles¹, Daniel Robert¹,  is published in Science.