University of Manitoba graduate student Angela Freeman has discovered that when male peacocks display their feathers they are also making deep rumbling sounds that are too low pitched for humans to hear.
Freeman, a biology student, is the first person to discover a bird that makes and perceives noises that are inaudible to humans. She reported her finding at the annual meeting of Animal Behavior Society on June 13.
When she played the newly discovered sounds for other peacocks, the females looked alert and the males were likely to shriek out.
Other animals, such as elephants, have long been known to produce such infrasound (sounds below 20 hertz), which humans can’t hear. And although a type of grouse called the capercaillie produces infrasound, it so far seems that those birds don’t communicate with it – they either can’t hear it themselves, or they just don’t care about it.
Not so for peacocks.
Freeman found that male peacocks were most likely to make their low sounds when they show off their magnificent train feathers. The two train displays that produced infrasound were what we Freeman and her colleague’s called the “shiver train” and the “pulse train”.
The shiver train is produced by vibration of the train feathers from the centre feathers to the outside of the array. It’s used by males when females are far away.
The pulse train is produced by a vibration emanating from the base of all the train feathers and it is used when the female is nearby.
Upon witnessing either of these, though, all a human would hear is a leaf-like rustling of the feathers.
Freeman was led to the project by her supervisor Professor James Hare. About decade ago, in a casual conversation, Hare’s colleague in the U of M’s biology department, associate professor Darren Gillis, said something to Hare that made him suspect peacocks produce infrasound. Inaudible sounds intrigue Hare, who, years ago with his student David Wilson, discovered that Richardson Ground Squirrels produce noises humans can’t hear.
Freeman received funding from the Natural Sciences and Engineering Research Council of Canada, the Manitoba Graduate Scholarship program, and through Sigma XI.