The comforting purr of a cat has long been a source of wonder and delight for many. However, the mechanics behind this unique feline sound have eluded understanding for years. Recent research offers new insights and challenges some long-held beliefs.
Fibrous pads: The key to the cat's purr?
At the heart of the latest discovery are so-called “pads” embedded in the cat’s vocal cords. According to a study published in Current Biology, these pads consist of an extra layer of fatty tissue that allows the vocal cords to produce low-frequency vibrations. Remarkably, this purring process doesn’t require any input from the brain.
Historically, the field of purring explanations has been marred by ambiguity and untested hypotheses. As Bonnie Beaver, a renowned veterinary scientist at Texas A&M University, noted, “The world of scientific discourse has long lacked an in-depth exploration of the subject. This new research brings us one step closer.”
One mystery surrounding cat purring has been the discrepancy in size and frequency. House cats, which often weigh around 4.5 kilograms, produce purrs between 20 and 30 hertz (Hz). Such low frequencies are generally associated with much larger animals, such as elephants. While the mighty roars of big cats are well documented, domestic cats have limited themselves to low frequency purrs.
It’s important to understand that most mammalian vocalisations, including a cat’s meow or hiss, result from a process called flow-induced self-sustained oscillation. This is when the brain sends a signal that causes the vocal cords to squeeze together, producing sound through the rapid movement of the cords as air passes by. Once initiated, the cords can vibrate without further input from the brain.
However, a long-held belief from the 1970s suggested that the purring of domestic cats was different. Called the ‘active muscle contraction hypothesis’, it suggested that cats controlled their laryngeal muscles to produce the purr. Although popular, this theory has now been put to the test.
To test it, scientists carried out a complicated experiment on eight domestic cats. After obtaining consent from the owners, the larynxes of these euthanised cats were isolated. Then, under controlled conditions, air was passed through the vocal cords to ensure there were no muscle contractions or brain input. The result was revelatory: all eight isolated larynges were able to produce the purring sound without any active muscle involvement.
On closer inspection, the scientists discovered previously overlooked masses of fibrous tissue in the cat’s vocal cords. These ‘pads’ could be responsible for increasing the density of the vocal cords, allowing the cats to produce the low frequency sounds. This is somewhat reminiscent of the “vocal fry” phenomenon in human speech.
The ongoing debate: what really drives purring?
The study suggests that purring, like meowing or hissing, is a passive phenomenon that becomes automatic once the initial signal is provided by the cat’s brain. Karen McComb, from the University of Sussex, says this fits better with existing knowledge of vertebrate vocalisations.
But the scientific community remains divided. David Rice of Tulane University argues that an isolated larynx may not accurately represent the vocal behaviour of a living cat. Therefore, drawing conclusions based solely on excised larynxes may be overreaching.
Christian Herbst, lead author of the study, postulates that purring is likely to result from a mixture of neural control and self-sustained vibration. The true nature of a cat’s purr may remain elusive due to the challenge of studying purring in an undisturbed environment, as a cat typically purrs only when at rest.
In conclusion, while the latest findings provide fascinating insights, the mystery of the cat’s purr may remain unsolved for some time to come. Science often thrives on uncertainty, and in this case the feline mystery continues to captivate the curious.
Prepared by Mary Clair