Crickets sing their virtue[ 2012-07-06 ]
When it comes to finding that someone special to be their partner, it is most common to brag about one's suitability. Birds may show of their plumage, like the peacock, while other species may show off their new fancy car to attract a mate. A new study conducted by an international team of researchers led by the University of Bristol in the United Kingdom, has discovered how male crickets use similar tactics to attract a mate through the use of song. Presented in the journal PNAS, the study was funded in part by a Marie Curie Actions grant under the EU's Seventh Framework Programme (FP7).
Hearing the sound of crickets during the night of hot summer months is the norm. Male crickets basically advertise for mates through the use of loud repetitive songs. What they do is they rub their wings together, setting them into resonant vibration and creating a loud and intense sound. Not only does this sound allow the female crickets to locate the males but it allows them to discern the suitability of the males as a mate according to the sound they make.
Researchers believe female crickets tend to prefer large males as mates because they are somehow better at finding and using resources. So the sound a male makes reflects this. Larger males make lower pitched sounds and smaller ones have a higher pitch. What the females have to do is listen to discern the size of the male, something that researchers thought could never be faked. That is until they discovered that tree crickets, tiny, nearly transparent and highly unusual creatures, were able to change the pitch of their song with temperature. One species, Oecanthus henryi, sings at a squeaky high pitch of 3.6 kilohertz (kHz) when it is 27 degrees Celsius and at a deep bass 2.3 kHz when it is 18 Celsius. However, no one really knew how they managed this or even why they did it. This latest study succeeded in finding out how.
The scientists investigated these curious biomechanics using a sophisticated technique called microscanning laser Doppler vibrometry, which can pick up tiny vibrations. So sensitive is this technique that it can detect motion that is smaller than atomic bond lengths. While the tree cricket's wings vibrated more than that, the researchers found that the pattern of vibration 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.
Lead author Dr Natasha Mhatre 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.'
The researchers discovered that the creatures' song frequency is no longer related to their size but to how fast the tree cricket is able to move its wings. And because they are cold-blooded their activity is influenced by temperature. So when the temperature rises, tree crickets are more energetic and call faster and engage a higher frequency mode.
'Sometimes understanding how something works is crucial to understanding why it works that way,' Dr Mhatre said. '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.'
Data Source Provider: University of Bristol
Document Reference: Mhatre, N., et al. 'Changing resonator geometry to boost sound power decouples size and song frequency in a small insect'. PNAS Plus. doi: 10.1073/pnas.1200192109
Subject Index: Coordination, Cooperation; Life Sciences; Scientific Research