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Courtship in the cricket world

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1 May 2012

Everyone wants to present themselves in the best light - especially when it comes to finding a partner. While some rely on supplying honest information about their attributes, others exaggerate for good effect. A new study by researchers at the University of Bristol, published in PNAS, has discovered how male crickets may embellish the truth to attract a mate. With funding from BBSRC to understand more about the fundamental mechanisms that these insects use to communicate, and some sophisticated technology, scientists delved into the biomechanics of cricket courtship.

An image of a tree cricket (Oecanthus henryi). Dr Natasha Mhatre, University of Bristol.
An image of a tree cricket (Oecanthus henryi). Image: Dr Natasha Mhatre, University of Bristol.

Male crickets advertise for mates by singing loud repetitive songs at night. They rub their wings together, setting them into resonant vibration, making a loud and intense sound. Females use this sound to find a mate and to establish which are the most desirable. Many cues gauge desirability, including size, but could a cricket be faking?

Female crickets tend to prefer large males; possibly because they may be better at finding and using resources, and hence their size reflects their advantageous genes. Males that are larger make lower pitched sounds, and smaller ones have a higher pitch. Females can simply listen and gauge the size of the male. It was previously thought that the males could do nothing about it and always sang at one frequency, because of the inflexible mechanics of singing.

It was then discovered that tree crickets change the pitch of their song with temperature. One species Oecanthus henryi sings at a high pitch of 3.6 kHz at around 27°C and at a deeper 2.3 kHz when it's 18°C. However, it was not know how this happened.

In a collaborative study, scientists from the University of Bristol and the Indian Institute of Science began to investigate the biomechanics behind the behaviour. They used a sophisticated technique called microscanning laser Doppler vibrometry, which can pick up tiny vibrations. The technique is so sensitive that it can detect motion that is smaller than atomic bond lengths. The researchers found that the pattern of vibration of the cricket's wings was unusual. The whole wing vibrated, instead of just a small part, and instead of having a single sharp vibratory peak near song frequency, there were two fused peaks.

Dr Natasha Mhatre, lead author on the study and an expert in the biomechanics of singing and hearing in insects at Bristol's School of Biological Sciences, said: "The unusual long shape of their wings has always intrigued us. Using a method called finite element modelling, borrowed from engineering, we were able to show that geometry is key. As wings go from short to long, different vibratory modes start coming closer in frequency and amplitude and start merging with each other. "

When a tree cricket is singing, its wings are not locked into a single vibrational mode or frequency and so they vibrate at a range of frequencies. Because they are cold-blooded, the activity of insects is highly influenced by temperature. Hence, when the temperature rises, tree crickets are more energetic and call faster and so higher frequencies are produced. For these crickets, size is no longer related to their song frequency. Instead, how fast the tree cricket is able to move its wings becomes an important factor. This opens up many possibilities for these unique crickets, including using song to disguise their true size.

The researchers concluded that the most probable reason for changing the geometry of their wings is to increase the amount of sound they can make.

Dr Mhatre explained: "Sometimes understanding how something works is crucial to understanding why it works that way. Understanding mechanics lead the way to understanding the evolution of tree cricket song. By studying the mechanics, we have shown that variable frequency song is a by-product of increasing sound power and not a desired feature in itself."

While the rules of cricket courtship have been rewritten, the mystery of what exactly is coded in the pitch of the tree cricket song remains to be unravelled.

The study was funded by the UK India Research and Education Initiative (UKIERI), an EU Marie Curie fellowship grant and a Biotechnology and Biological Sciences Research Council (BBSRC) grant.


Notes to editors

Paper: 'Changing resonator geometry to boost sound power decouples size and song frequency in a small insect' by Natasha Mhatre, Fernando Montealegre-Z, Rohini Balakrishnan, Daniel Robert in Proc. Natl. Acad. Sci..

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