It's a noisy world when you're trying to find a mate and every crooning male around is doing his best to attract the ladies. So how do female crickets successfully select their own species' serenades against the background of everyone else's clamour?
Natasha Mhatre from the University of Bristol, UK, explains that most cricket females only hear their own males' calls. Their ears are cleverly tuned to the pitch of their males' chirrups. But insects' body temperatures are not constant, so what happens when the temperature rises? According to Mhatre, most crickets chirrup faster but at the same pitch. However, tiny tree crickets (Oecanthus henryi) raise the pitch of their song, leaving their mates with a problem: how to match their hearing to the male's constantly shifting stridulation. According to Mhatre, tree cricket females could use one of two approaches. They could either adjust their hearing as the temperature changes so that they selectively pick out their own males' chirrups at any pitch, or they could listen to every lothario simultaneously and identify their own species' calls by listening out for their own unique song motifs. So which strategy do they opt for?
Mhatre and her colleagues Rohini Balakrishnan from the Indian Institute of Science and Daniel Robert from the University of Bristol, decided to take a look at the female's ears (). Scanning a tiny beam of laser light across the insect's tympanum to measure its vibration, Mhatre played a tone rising from 0.5 kHz to 20 kHz and expected to see the delicate tympanum vibrate strongly like a drum skin at the frequency to which it was tuned. However, only a minute 200 μm long sliver of the tympanum vibrated and, instead of vibrating in response to a subset of frequencies, it responded almost equally to every frequency in the sound test. Wondering if there was a problem with the cricket's hearing, Mhatre tested another cricket's ear, but it was the same. After months of painstaking testing, the team finally accepted that instead of tuning their ears to track chirrups at any pitch, the females' ears appeared to be listening indiscriminately to everything.
But even if the female's tympanum could vibrate in response to every frequency from 0.5 to 20 kHz, that doesn't necessarily mean that she can respond to real male chirrups over that range of pitches. ‘We decided we needed to do the behaviour if we were going to round the story off,’ says Mhatre.
Back in Balakrishnan's Bangalore lab, Monisha Bhattacharya picked up the project to find out whether the females respond to chirrups across the entire pitch range. Synthesising male chirrups from a deep 1.5 kHz up to a high-pitched 8.5 kHz, Bhattacharya filmed female tree crickets' responses as she played the synthesised songs from one of two loud speakers, and found that the females only responded to chirrups between 2.5 and 4.5 kHz, which closely matches the natural range – 2.4–3.3 kHz – over which the males chirrup as temperatures increase from 18°C to 28°C.
So, instead of tracking the males' serenades as their pitch varies with temperature, female tree crickets listen out for every male, whether he's growling at 2.4 kHz or piping high at 3.3 kHz. However, this means that they must also be picking up background noise from every other amorous species in the neighbourhood, and the team is keen to find out how the females discriminate between their own species' songs and those of interlopers.
