Deer and the Human Voice

26 February 2013

Dr David Reby, Senior Lecturer in Psychology at the University of Sussex, will be presenting a demonstration at The Voice on Friday 1 March at Wellcome Collection. In this post he explores the connections between the larynxes of deer and humans.

What can deer tell us about the evolutionary origins of our voice? We all know that, overall, men’s voices are lower-pitched than women’s, and most of the time we are able to recognise someone’s gender simply from listening to them, for example over the telephone.

In order to understand the basis of this difference, it is necessary to look at how the human voice is produced. According to the source-filter theory of voice production, we generate our voice in two stages. The first stage takes place inside the larynx (our “voice box”), where the vibrations of the vocal folds creates a sound wave characterised by its “fundamental frequency”. Men have lower pitched voices because they have much longer vocal folds that vibrate at a lower frequency.

Then, in the second stage, this source soundwave is filtered in the speaker’s vocal tract, whose resonance properties affect the timbre of the voice. In fact, changing the shape of our vocal tract to modulate its resonances enables us to produce different vowels when we articulate the sounds of speech.

Then, in the second stage, this source soundwave is filtered in the speaker’s vocal tract, whose resonance properties affect the timbre of the voice. In fact, changing the shape of our vocal tract to modulate its resonances enables us to produce different vowels when we articulate the sounds of speech.

But here too, because men have longer vocal tracts than women, their voice is characterised by lower resonances, giving them a more “baritone”, “deeper” quality, which is a key dimension of the gender of men’s voices.

Interestingly, men’s vocal tracts are on average 20% longer than women’s, giving them deeper voices than expected from the relatively small differences in body size between the two sexes. This suggests that over evolutionary time these differences may have been accentuated as a result of sexual selection. How can we investigate this hypothesis?

This is where deer can help us. Indeed, because their sexual calls are extraordinarily diverse, ranging from almost infrasonic low-pitched groans to extremely high-pitched bugles, deer provide an ideal model for understanding the evolution of mammal vocal signals.

For example, like human males, Scottish red deer stags have a longer vocal tract than females, and are even capable of extending it further when they roar (the arrows on the illustration point at the stag’s larynx or Adam’s apple):

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This enables them to produce extremely low resonances, making them sound much bigger than they actually are. Experimental research suggests that sexual selection may have favoured males that were capable of extending their vocal tract to sound more attractive to females and more threatening to rival males.

These observations are interesting because they may provide an explanation for why human males have a longer vocal tract, and therefore deeper voices than women. And indeed, recent work has shown that in humans men tend to rate deeper male voices as more physically and socially dominant. This suggests that in our species too, size exaggeration in the context of male competition may be at the origin of voice differences between males and females.

Finally, this size exaggeration hypothesis may also help in understanding why, unlike most mammals, deer and humans have a descended larynx, an adaptation that may ultimately have facilitated the evolution of human speech in our species.

The Voice takes place at Wellcome Collection on Friday 1 March. Find out more about David Reby’s work