A purr is a continuous, low-frequency sound produced by a rhythmic muscular mechanism in the throat. While the domestic cat is the animal most famous for this soft rumble, they are not the only species capable of producing a purr-like vocalization. The purr is defined by its acoustic properties: a consistent, vibrating sound made during both the inhalation and exhalation phases of breathing. This continuous nature distinguishes the true purr from other rumbling noises made by various animals.
The Mechanics of Feline Purring
A domestic cat produces its distinctive purr through a specialized physiological process involving the larynx. The sound originates from the rapid oscillation of the vocal cords, controlled by the laryngeal muscles. A repetitive neural signal causes these muscles to contract and relax 20 to 30 times per second. This rapid action opens and closes the glottis, causing the air to vibrate as the cat breathes in and out, creating the continuous purr.
Recent research suggests that a unique pad of connective tissue embedded within the cat’s vocal folds may contribute to this ability. The purr’s constancy across the respiratory cycle sets it apart from most other vocalizations, which are limited to exhalation. This mechanism allows domestic cats and smaller felids to purr continuously for extended periods.
The continuous purr is often contrasted with the vocalizations of larger felids, such as lions, tigers, and jaguars, which can roar but cannot produce a true purr. This difference was traditionally attributed to the structure of the hyoid bone, which supports the larynx and tongue. In roaring cats, the hyoid is partially cartilaginous, allowing for a deep roar. In purring cats, the hyoid is fully ossified, or bony, which was thought to enable the purr. While the structural difference exists, the exact relationship between hyoid ossification and the ability to purr or roar remains debated.
Other Animals That Purr
While the domestic cat’s purr is the best-known example, similar low-frequency, rumbling vocalizations are found in a diverse group of animals. Distinguishing a “true” purr relies on the physiological mechanism used and whether the sound is continuous on both the inhale and exhale, as seen in domestic cats. Although many species in the family Felidae purr, the behavior is not exclusive to cats.
Other Felids
Most smaller wild cats, classified in the subfamily Felinae, can purr like house cats. These include the cheetah, cougar, bobcat, ocelot, and lynx. The cheetah, despite its size, is one of the largest felids that can purr, and its vocalization is sometimes described as a “whirr.” These smaller cats share the functional ability for the laryngeal mechanism of a continuous purr.
The ability to purr separates these smaller felids from the “roaring cats” of the genus Panthera. The vocalization serves similar functions to the domestic cat’s, often expressing contentment, non-aggression, or communication between a mother and her young.
Non-Felid Purrers
Beyond the cat family, various other mammals produce acoustically similar vocalizations, though the mechanism is typically different. The family Viverridae, which includes civets and genets, exhibits a true purr. Other animals whose rumbling sounds are described as purring include raccoons, guinea pigs, and certain species of mongoose.
Rabbits make a soft, purr-like sound called a “tooth purr,” produced by gently grinding their teeth when content. Gorillas, ring-tailed lemurs, and bears have also been observed making low, rumbling noises, often while eating. These analogous sounds are functionally similar, signaling a relaxed state or non-aggression, but they do not rely on the rapid laryngeal oscillation that defines the domestic cat’s mechanism.
The Biological Reasons for Purring
The function of purring is complex, extending far beyond a simple expression of happiness or contentment. This low-frequency vocalization serves several functional roles for communication and self-maintenance across different species.
The primary function is communication, particularly between a mother and her young. Since kittens are born blind and deaf, the mother cat’s purr provides a low-vibration signal they can feel, guiding them toward her for nursing and warmth. Adult cats also use purring to signal non-aggression, functioning as a greeting or an invitation for interaction. The “solicitation purr,” which incorporates a high-frequency meow-like sound, is used by domestic cats to prompt humans for food or attention.
Another element is self-soothing, as purring is frequently observed when cats are injured, stressed, or in pain. The act of purring releases endorphins, which are natural pain-relieving chemicals, helping the animal calm itself and manage discomfort. This low-energy vocalization allows a cat to self-medicate and stabilize its breathing during times of distress.
A third major hypothesis focuses on the therapeutic frequency of the purr’s vibrations. Domestic cat purrs fall within a frequency range of 25 to 150 Hertz (Hz). Frequencies in this range are known to promote tissue and bone healing in other medical contexts. The theory suggests that the low-frequency vibrations stimulate bone density, promote muscle repair, and accelerate wound healing, serving as a natural form of internal vibration therapy. This self-healing mechanism is thought to be an evolutionary adaptation, helping small animals recover from injury and maintain bone health.