For a majestic animal that looks equally at home as the monarch of the glen or astride the great American Plains, the wapiti's call is somewhat disconcerting. Sounding more like the shrieking cries of a Ringwraith from the Lord of the Rings, their haunting high-pitched screeches can carry great distances. ‘Larger animals tend to have deeper resonances and lower voices’, says David Reby, from the University of Sussex, UK, explaining how the pitch of an animal's voice tends to be a good indicator of its size: animals from larger species have larger larynges that vibrate at a lower pitch than animals from smaller species. However, the wapiti's extraordinary bugle calls would be more at home coming from the body of an animal the fraction of the size of these impressive red deer. The paradox had puzzled scientists for decades, but when Megan Wyman returned from a trip recording deer bellows in New Zealand, Reby knew that they might have a chance to finally lay the mystery to rest.
Visualising the spectrum of the eerie shrieks, Reby could clearly see the unnatural sounding high-pitched shriek at frequencies up to 4000 Hz. However, there was another band of lower pitched sound around 150 Hz, the frequency at which Reby would expect the male deer's vocal folds to vibrate. And when Reby and Daniella Passilongo investigated a series of the calls, they realised that the high- and low-pitched sounds shifted independently: sometimes the high-pitched wail rose and fell while the tone of the lower pitched roar remained constant. So the vocal folds were vibrating and producing a call that matched the animal's size while the deer simultaneously produced a high-pitched, high-volume, wraith-like cry by whistling.
Having realised that the deer were using two mechanisms to produce their eerie calls, Reby and colleague Ben Charlton wanted to confirm that the lower pitched vocalisation was produced by the vibrating vocal folds. So, when Yann Locatelli contacted Reby to let him know that one of the wapiti males in the herd at France's Muséum National d'Histoire Naturelle reserve had died, this was Reby's opportunity to take a close look at the structure of the deer's throat to learn more about their sound production. After CT-scanning the head and throat of the animal in the position that they assume when calling, Reby could see that the vocal folds were ∼3.5 cm long, the ideal length for producing the low-pitched component of the call. And when Roland Frey investigated the structures in the deer's throat, he noticed that there were two possible routes for the animals to produce the high-pitched shriek. In the first scenario, the wapiti could whistle through their nostrils and adjust the wailing pitch by flaring and contracting their nostrils. However, he noticed that the deer's soft palate – known as the velum – descended far down in the animal's throat, separating the nasal tract from the oral cavity, possibly allowing the deer to blow air into the nasal cavity through the glottis. And when physicist Joel Gilbert calculated how the air might vibrate in the oral cavity, Gilbert realised that the jet of air from the glottis could hit the velum in much the same way that air in a flute vibrates, to produce a whistle that matched the wapiti's call at frequencies around 2600 Hz.
So the wapiti are being honest about their size. However, the low-pitched roar that accurately reflects their stature is drowned out over longer distances by their strident whistle, and Reby is keen to discover what other messages are communicated by the wraith-like shrieks.