Feline researchers have long believed that purring is produced by voluntary muscle contractions, but a new report indicates that this vibration in the larynx of cats could be explained by the myoelastic aerodynamic theory of phonation.
Studies of the complex action that produces a unique vibration in the larynx of cats – known to most of us as purring – have taken an important turn. It turns out that the biomechanics of the sounds domestic cats make when they feel comfortable or stressed may be closer to snoring than a voluntary muscle spasm.
New research published in Current biology suggests that masses of connective tissue are embedded in the vocal folds of the larynxes of domestic cats. These may allow felines to produce self-sustaining low-frequency oscillations without neural input or muscle contractions. Research has found that anatomical adaptations – the “pads” of tissue in the vocal cord – respond to air entering the lungs.
What is a purr, really?
Voluntary muscle contractions were thought to cause the vibrational component of purring. A contraction is initiated when the nervous system generates a signal that travels through a motor neuron to a neuromuscular junction. Once there, it releases a chemical message which tenses the fibers and triggers movement.
The authors of the new study suggest that purring instead results from cats’ laryngeal pads. This is in accordance with myoelastic aerodynamic theory, which indicates that vocal fold oscillation is produced as a result of asymmetric forcing functions on the closing and opening portions of the glottal cycle. The team argues that the flow of air in and out of the lungs activates vibrations of the vocal cords, producing sounds similar to those of the human voice and characteristic sounds in animals. To test this, the scientists performed experiments on eight larynges taken from domestic cats (all of which had been humanely euthanized when diagnosed with a terminal illness). Their phonetic systems were housed in vertical tubes that provided warm, moist air similar to the air that enters the body when breathing. The researchers were able to discover the low-frequency phonation characteristic of purring without neuronal stimulation.
The study does not rule out the possibility that muscle contractions play a role in purring, but the team says there is not enough evidence to conclude that it is the sole cause of purring. Instead, research indicates that air dynamics can trigger the vibration mechanism.
Why do cats purr?
Cats purr all their lives, from the time they are kittens. Science has not yet fully understood why they purr in all circumstances, but biologists, veterinarians and zoologists have come to some general conclusions:
- Kittens purr so their mothers can find them
- Purring promotes wound healing
- Purring produces serotonin, which is why it is often compared to human smiles.
- Domestic cats purr not only when they are happy, but also when they are stressed.
The article’s findings caused some controversy. Biomechanical engineers interviewed by Science claim that the experiment was limited to checking the functioning of the larynx in isolation, without taking into account the complex systems of a living cat, which they say represents a significant oversight. Scientist David Ricefor example, compared the research to removing the mouthpiece of a wind instrument and then analyzing the noise it produces independent of the context of that instrument.
This article was originally published by WIRED in Spanish.
