The secret signal in a peacock’s tail

The peacock’s shimmering blue and green tail plumage is one of the most dazzling sights in the animal kingdom – and there’s more to the display than meets the eye.

Reporting in the journal Animal Behaviour in March, Angela Freeman and James Hare at the University of Manitoba, Canada, found that when peacocks fan their tail in a mating display, the rustling feathers generate a low-frequency noise called “infrasound” which is pitched so low humans can’t hear it, but peafowl can.

“It’s always fascinating to study animal senses that humans lack,” says Andy Bennett, an expert in animal behaviour and avian sexual selection at Deakin University in Geelong.

A visit to Winnipeg Zoo inspired the research. Hare noticed a lone peacock shaking his feathers and displaying his tail at a bare concrete wall. The peacock couldn’t see its own reflection, so what was the point? Could he be listening to the reflected sound?

So Hare – a biologist who had been researching communication between ground squirrels – returned with his recording equipment, and captured the rustling sounds of peacock tails. He discovered the splendid fan also emits infrasound – frequencies less than 20 hertz, which are below the threshold of human hearing.

The research progressed no further until Freeman joined the Manitoba faculty. Hare was trying to convince her to work with him on squirrels, but when he mentioned his interest in peacocks Freeman jumped at the idea. “At the time I was really into birds,” she says. “So several months later I was in Winnipeg doing recording and playbacks.”

Freeman and Hare found that playing back the low-frequency recordings made the peacocks and peahens more active and alert, compared to when they played only the higher frequency rustling sounds audible to humans that are also emitted from a displaying peacock’s tail.

Zoologists have known for years that animals such as elephants, whales and crocodiles can communicate by making low-frequency rumblings in their larynx. But evidence for infrasound communication in birds was patchy. Capercaillie grouse can produce it, and pigeons and chickens can perceive it. But peafowl are the first bird species shown to both make and hear infrasound.

So why, given the peacock’s flashy tail and loud high-pitched hoots, would they also need to communicate with infrasound? Freeman thinks of it as an accessory to the main show: “A peahen can only see a peacock if he’s in view. But acoustic signals – which travel quite far – are being used to signal to individuals out of view, we think.”

The peafowl’s natural habitat of dense, bushy jungle would frequently obscure peacocks, even when they are nearby. Low frequency sounds travel further than those of high frequencies. By pairing infrasound with its high-pitched calls, the peacock has all sonic bases covered. “A good sexual signal is one that works well at short and long distances,” Bennett says – whether the infrasound signal is perceived as “hello ladies” by peahens or as a territorial “go away” to fellow peacocks.

Bennett points out that larger tails can often attract more females, but “by making yourself gaudy or extravagant, whether that be in colour or size or sound, you also increase your risk of predation”. It may be that infrasound is the peacock’s secret call, a way of signalling its whereabouts to other peafowl without alerting predators such as big cats and stray dogs.

Could birds with similar tail displays, such as Australia’s lyrebird, also be luring their mates with stealthy sounds? Bennett doesn’t rule it out: “The fact that infrasound is produced by peacock tail feathers suggest that similar behaviours and effects might be found in other bird species.”

Exactly how peafowl perceive infrasound is still a mystery, whether it is through their ears or their legs, but it’s a question Freeman is looking into next – if she can get away from ground squirrel research. “It seems Jim convinced me it was a good idea after all!” she laughs.