Research with animals we can’t see and sounds we can’t hear

The field site at Wittenham Clumps is on the Northmoor Trust Estate with fabulous views over the Thames in one direction and Didcot power station in the other. The plan was to set up equipment in Little Wittenham Woods to detect the ultrasonic calls of the speckled bush cricket and to estimate the height in the pine trees from which they were singing.

David Robinson The speckled bush cricket

Quentin Cooper would then visit for the Material World radio programme. The insects sing at dusk and after dark – but only in dry weather. The weather in August was almost continually wet. The first possible week for recording was a wash-out.

In the following week the weather forecast for Tuesday was light showers so as the weather was due to get worse later in the week, we took a risk and happened to catch a gap in the rain showers, although it was a bit cold and there were only a few animals singing.

So, in darkness we set up the ultrasound recording equipment and Quentin interviewed us about the project in general. He then followed us as we made some estimates of how high some of the individuals were. The animals are only about two centimetres long, but were singing from over ten metres up in the fir trees.

Copyrighted Image David Robinson The size of the problem

In darkness, of course, things are not quite so simple and Quentin commented that he had not interviewed somebody before where all he could see of them was their head-torch! Interestingly, he was unaware of the bats flying low over his head, illuminated in the head torch beam.

David Robinson The equipment

The technique we use is to set up two ultrasound detectors at the end of a 12m long base line. Each detector is quite directional so we align each to a singing animal and measure the angle of inclination and the bearing. From these measurements we can work out the elevation of the animal, using trigonometry.

Copyrighted Image David Robinson Using trigonometry to position the creature
Using trigonometry to position the creature. [Image: David Robinson]

The speckled bush cricket is notable for the brief and extremely fast communication between males and females. In a typical exchange, which is used to bring the sexes together, the male calls with a series of around five short bursts of ultrasound each only one thousandth of a second long, with about the same time between each burst.

The female replies very rapidly – as fast as twenty-five thousandths of a second – with one or two burst of sound around a thousandth of a second long. The male uses her reply to locate her.

For the female to reply so rapidly, her nervous system must have very few interconnections, and certainly there is no time for her brain to process the information. Her reply is a reflex. The sound of these insects is the fastest love song in the insect world.

So imagine the scene in the dark high up in the trees – and we have just measured one at a height of 14 metres – where the male is calling to the female and then using her short ultrasonic reply to locate her and to work out his path through the trees towards her in the darkness.

And, to add a final twist, the male recognises the female only if her reply falls within a small time window - twenty-five to thirty-five thousandths of a second after his call. He can compensate for the speed of sound through air, if she is distant from him, but if she does not reply within his time window he ignores her.

So, we got the data we needed and Quentin and Deborah got their recordings. It rained for the rest of the week!

Want to know what the cricket sounded like? Download the mp3.

David Robinson was recording speckled bush crickets for BBC Radio 4's Material World programme.

Find out more

How do we listen to nature?
What sounds do humans miss?
Why is nature making so much noise?

Wittenham Clumps - managed by the Northmoor Trust

Want to know more about insects? Try the Amateur Entomologists Society

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