Peacock tails are one of the most beautiful sights in the animal kingdom. But they're not just pretty to look at. The birds also use their huge tails to make a loud noise – which humans are utterly incapable of hearing.
When a peacock shakes his tail, it produces a sound that is so low-pitched, humans can't hear it. That's despite the noise being about as loud as a car going past a few metres away.
Angela Freeman and James Hare of the University of Manitoba in Winnipeg, Canada have shown that peacocks can make these "infrasound" noises with their tails. The birds can also sense and respond to the noise.
"Humans don't often think about infrasound as a potential signal, because we don't perceive it," says Freeman. "I think it's more common than we think."
The pair's results are published in the journal Animal Behaviour.
Hare got the idea when he visited a local zoo and saw a peacock shaking its tail at a concrete wall. Initially baffled, he realised that the huge tail looked a bit like a satellite dish, and wondered if the bird was listening to itself.
Freeman and Hare recorded the sounds made by 46 displaying peacocks. "They shake the feathers," says Freeman. "It just kinda looks like they're vibrating."
To human ears, it sounds like "rustling grass in the wind". But the recordings revealed that the shaking tail feathers also made noises too low-pitched for humans to hear. This infrasound was loud, between 70 and 108 decibels.
When Freeman and Hare played the recordings back to other peafowl, both males and females became more alert and spent more time walking or running. Also, "the males would call in response to these signals," says Freeman.
It's not clear how the peacocks detect the infrasound. "We know that some birds are able to perceive vibration through their legs," says Freeman. But the sound might not have been intense enough for that.
"The other option is they're hearing it through their ears," says Freeman. The inner ears of peafowl do seem to be the right shape to pick up infrasound.
For the males, making infrasound might serve two purposes.
"We think it's probably something to do with territory maintenance," says Freeman. A male holding a territory might hear the infrasound produced by an approaching rival, and call out to warn him off. "If he hears another male it would make sense for him to respond in some way."
The infrasound could also be a way to attract females.
A peacock's large tail is a signal of strength and good genes, essentially telling females "I'm in such good condition, I can afford to carry this enormous heavy tail behind me".
The infrasound might reinforce that message, says Freeman. "If you don't have a tail you can't produce the sound," she says.
Producing low-frequency sound might seem pointless, but in peafowls' natural habitat it makes sense. They live in scrubby woodland, so high-pitched calls get muffled even over quite short distances.
"Oftentimes they can't see each other from where they're displaying," says Freeman. "We think that possibly these signals could signal to females and males who are out of sight behind a bush or over the hill."