When we hear crickets sing, we are given insight into a fierce competition for mates being fought around us. These singing insects are loudly proclaiming their quality to attract mates and to repel their rivals. Because cricket song carries such important information, it is vital that songs reach the right ears. For tree crickets (Oecanthus nigricornis), this might be tricky. This is firstly because lots of related species often sing in the same area and secondly because male song frequency rises with temperature. This means that for listeners, picking up the right song must be a bit like trying to tune into a radio station that keeps changing frequency. Natasha Mhatre and colleagues wanted to know how tree crickets stay tuned to this fluctuating frequency as temperatures change.
Mhatre and colleagues first focused on tree cricket tympanal ears. These membranes on the legs of crickets are forced to oscillate in the narrow frequency band that corresponds to the species’ song by motor cells. This amplifies the frequencies to which these ears are tuned and enables crickets to better hear songs of their own species. Mhatre and colleagues measured the frequency of these oscillations at different temperatures. They found that ear oscillation frequency rose by 112 Hz for each 1°C increase in temperature, closely mirroring the rise in male song frequency across this temperature range. This means that as male song frequency shifts with temperature, so does the frequency that listeners actively amplify.
To see how this change in oscillation frequency affects what crickets hear, Mhatre and colleagues measured neural responses to sound (or neural tuning) at the prothoracic ganglion where auditory information is first processed. They found that neuronal responsiveness at low temperatures was greatest around the frequency at which males sing in low temperatures. At warmer temperatures, neural tuning was greatest around the frequency at which warmer males sing. In other words, neural tuning also shifted up with temperature.
So it seems that tree crickets actively tune into their own song across a range of frequencies. But what effect does this have in the wild? To find out, Mhatre and colleagues looked at the responses of cold and warm crickets to sounds corresponding to the frequencies that male crickets sing at in both warm and cold temperatures. The team found that a song produced at a high temperature would have to be 18.5 dB louder to be heard by a low-temperature ear. This means that by actively tuning in to the correct frequencies, crickets can hear singers that are over three times further away. This is one of the first demonstrations of the consequences of adaptive auditory processes. In addition, these results show us that by changing the frequency that they actively amplify, female tree crickets may be better able to pick mates, while males can better defend their territories and decide when it is better not to pick a fight.