Feline researchers have long believed that purring is caused by voluntary muscle contractions, but a new report indicates that this vibration in the larynx of cats can be explained by the myoelastic aerodynamic theory of phonation.
Research into the complex action that produces a unique vibration in the larynx of cats – known to most of us as purring – has taken a major turn. It turns out that the biomechanics of the sounds that domestic cats emit when they feel comfortable or stressed are closer to a snore than to a voluntary muscle spasm.
New research published in Current biology suggests that connective tissue masses are embedded in the vocal folds of the larynx of domestic cats. This allows cats to produce self-sustained low-frequency oscillations without neural input or muscle contractions. The studies showed that anatomical adaptations – the ‘pads’ of tissue in the vocal fold – respond to air entering the lungs.
What exactly is spinning?
Voluntary muscle contractions were thought to produce the vibratory component of spinning. 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 that tightens the fibers and produces movement.
The authors of the new study suggest that purring may instead result from cats’ laryngeal pads. This is consistent with the myoelastic aerodynamic theory, which states that vocal fold oscillation is produced as a result of asymmetric force functions over closing and opening portions of the glottal cycle. The team claims that the flow of air entering and leaving the lungs activates the vibrations of the vocal cords, creating sounds similar to those of a human voice and characteristic sounds in animals. To test this, the scientists experimented with eight larynges removed from domestic cats (all had been humanely euthanized when diagnosed with terminal diseases). Their phonetic systems were housed in vertical tubes that supplied warm, moist air, similar to the air that enters the body when breathing. The researchers were able to induce the low-frequency phonation characteristic of spiders without neural stimulation.
The research does not rule out the possibility that muscle contractions play a role in spinning, but the team claims there is insufficient evidence to conclude that this is the sole cause of spinning. Instead, the research indicates that air dynamics can activate the vibration mechanism.
Why do cats purr?
Cats purr their entire lives, from the time they are kittens. Science has not yet fully understood why they purr under all conditions, but biologists, veterinarians and animal scientists have come to some general conclusions:
- Kittens purr so their mothers can find them
- Spinning promotes the healing of wounds
- Spinning 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 conclusions have caused some controversy. Biomechanical engineers interviewed by Science claim that the experiment was limited to verifying the functioning of the larynx in isolation, without taking into account the complex systems of a living cat, which they argue represents a significant error. For example, scientist David Rice likened the research to removing the mouthpiece of a wind instrument and analyzing the sound it produces independently of that instrument’s context.
This article was originally published by WIRED in Spanish.