Pitch Purrfect: How Do Cats Produce Purring Vocalizations?

Picture the scene: you’re lying on your sofa after a long day, with a cozy blanket, a cup of tea, and a little kitten purring like a motorbike in your lap. Bliss for cat lovers! But have you ever wondered how exactly your feline friend is able to produce that rumbling purring noise? Because if you have, you are not alone.

The puzzling aspect of purring comes from the frequency of these vocalizations. The general rule is that bigger animals have longer vocal cords, which allows them to produce sounds of lower frequencies, and vice versa. However, domestic cats weigh only around 3.5-4.5 kg on average, with relatively short vocal cords, and yet their purr vocalizations can reach frequencies as low as 20 Hz (for reference, a typical adult male voice will have a lower register of around 90 Hz). So, how exactly can these little animals achieve such vocal depths?

In this blog, we discuss a recent paper by Herbst et al. (2023) in which they shine a new light on the mechanisms that drive purring in cats.

The Hiss-tory of Feline Vocalizations

Vocal production across most mammals and birds is mediated by what’s known as the myoelastic-aerodynamic (MEAD) theory of phonation. MEAD theory describes how the elasticity of the vocal cords influences the frequency of the vibrations produced, similar to how changing the tension of a guitar string modifies its pitch. The elastic properties of the vocal cords are dictated by the surrounding laryngeal muscles, which can act to stiffen or slacken the cords. The vocalization is initiated by air flowing through the larynx, and once the cords begin vibrating, further neural input is no longer required to maintain the vibrations. This process, known as flow-induced self-sustained oscillation, has been shown to be responsible for the hissing and meowing vocalizations of domestic cats (Svec et al. 2023, Herbst et al. 2023).

However, it has long been thought that purring is mediated by an alternative mechanism called the active muscle contraction (AMC) hypothesis. Originating from research performed in the 1970s, the hypothesis postulates that purring vocalizations are produced via the active contraction and relaxation of the laryngeal muscles at a rate of around 30 times per second (Remmers and Gautier, 1972). Despite a sparsity of direct evidence for this mechanism, the theory has been largely accepted. That is, until recently.

Cat Got Your Tongue?

Herbst et al. used larynxes excised from eight domestic cats that had been humanely euthanized due to terminal illness to investigate the source of the purring sound. The researchers used a support structure to prop up the larynxes and applied a flow of heated humidified air to the tissues. Despite the complete lack of neural input or muscle contractions in the isolated larynxes, all eight tissues were able to elicit self-sustained purring sounds at or below 25-30 Hz, in stark contradiction to the AMC hypothesis.

This, however, brings us back to the question of how exactly these animals, with relatively small vocal cords, can produce such low-frequency sounds. Well, Herbst et al. think the answer lies in a little pad of tissue that runs along the edge of the vocal cords of domestic cats. These pads can reach a size of around 4 mm and are made up of elastic and collagen fibers. This mass of connective tissue seems to increase the density and elasticity of the vocal cords, allowing slower vibrations that produce lower-frequency sounds. The authors likened this structure in cats to so-called “phonic lips” found in toothed whales, which allow them to produce the super-loud click noises associated with echolocation (Madsen et al. 2023).

However, Herbst et al. haven’t totally ruled out the involvement of neural control in purring in vivo. Instead, they propose a mechanism in which both AMC and MEAD work synergistically to produce these low-frequency vocalizations. They suggest that the passive vibration of the vocal cords via MEAD could be synchronized to the AMC-mediated contractions of the laryngeal muscles, which could help stabilize the oscillations and provide energy-based benefits.

Taken as a whole, this study refutes the long-held notion that the purr of domestic cats is governed by the AMC hypothesis alone and illuminates a role for the previously uncharacterized vocal pad found in the larynx of these animals. This research brings us one step closer to better understanding our mysterious little furry companions!