Why do sheep bleat?​

Sheep research reveals reason behind mammals distinctive vibrato

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Research reveals reason behind vibrato voice in some mammals

Sheep, like many other mammals, transmit key identification information when they make vibrato bleats, new research says.

Sheep, like many other mammals, transmit key identification information when they make vibrato bleats, new research says.

WHY do sheep have that vibrato-like quality to their call?

New research might have the answer to the question of not just why sheep do it, but why evolution has tossed up so many operatic bleats, burrs and baahs right across the animal kingdom.

Many distantly related mammals, from giant pandas to fur seals and even capybaras  and some  lemurs make bleating noises with vibrato-like frequency modulation.

So what do they get out of the Pavarotti impersonation?

It is an effective way to communicate information. The  vibrating vocals are an effective way to carry acoustic signals of the bleater’s identity to the listener.

"Our results suggest that vibrato-like pitch modulation has evolved because it improves the perception of formants, key acoustic components of animal calls that encode important information about the caller's size and identity," said University College Dublin research fellow and study author Benjamin Charlton.

To test the theory, Mr Charlton got humans to listen to synthetic calls to see if they could detect subtle differences.

He found that vibrato significantly improved the ability of a listener to detect formant patterns - that means sound modulations produced in the vocal tract, as opposed to the vocal cords.

Air inside the vocal tract vibrates at different pitches depending on its size and shape of opening. The pitches produced in this way are called formants.  

The study included a phylogenetic analysis, or study of the evolutionary relationship between animals, of the calls of 92 mammalian species.

It showed that frequency modulations have evolved independently in six mammalian orders, including the Carnivora (cats and civets), Primates (humans, moneky, gorillas), Artiodactyla (cloven-hooved animals -sheep, cows, goats etc), Perissondactyla (odd-toed hooved animals - horses, rhinos), Rodentia (rats, mice), and Chiroptera (bats).

"We were very excited to find that distantly related mammal species appear to produce calls with vibrato-like pitch modulation in order to improve the perception of important information encoded by formants," Mr Charlton said.

The results demonstrate the importance to animals of transmitting information about their size and identity.

Future research will investigate the ability of non-human mammals to discriminate between different callers using re-synthesized, bleat-like calls with varying levels of frequency modulation.


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