Buzz
In the past, scientists believed that the sound of a cat’s purring came from blood rushing through the inferior vena cava, a major vein that carries blood from the lower body to the heart. However, more recent research indicates that purring is actually generated by the contraction of the laryngeal muscles, which are responsible for controlling the vocal cords and the glottis (the space between the cords).
A brain mechanism called a 'neural oscillator' triggers rhythmic contractions of these muscles, causing the glottis to open and close quickly. As the cat breathes, air passes over these vibrating muscles and the glottis, generating bursts of sound at a rate of 25 times per second, which produces the recognizable purring noise. Interestingly, the cat does not appear to consciously control this process, making the purr more of a reflexive muscle movement rather than a deliberate vocalization.
Purring and the Hyoid Bone
While the process of purring may seem straightforward, it's curious that not all cats are able to produce the sound. Initially, it was believed that only domestic cats could purr, but research has shown that other members of the Felinae subfamily, such as bobcats, cheetahs, lynxes, and pumas, can also purr. In contrast, members of the Pantherinae subfamily, like lions, tigers, leopards, and jaguars, are unable to purr. A key difference may lie in the structure of the hyoid bone, which supports the tongue and larynx. Cats that can purr tend to have more rigid hyoid bones, whereas larger cats possess more flexible hyoids that help them roar but might prevent them from purring.
It could be more complex than it seems. The additional tissue pads that enhance the roars of larger cats may make it harder, or even impossible, for them to purr. Some big cats might have the capability to purr but simply don’t, or perhaps it hasn’t been observed. In some cases, larger cats have been known to produce purr-like sounds, but only during exhalation, whereas smaller cats can purr throughout their entire breathing cycle. These purr-like noises have yet to undergo acoustic analysis to determine if they truly qualify as purring.
