How do cats purr?  Yesterday I asserted that this was a mystery. An alert reader, Mr. Sven DiMilo,  has called my attention to a scientific paper published in 1991:  Sissom, D. E. F., D. A. Rice, and G. Peters. 1991. How cats purr. Journal of Zoology 223:67-78. This was the very title I envisioned as a youth when I wanted to solve this long-standing problem, with visions of Stockholm dancing before my eyes.  Well, Sissom et al. seem to have made significant inroads into the problem, or even solved it.   They studied purring in housecats, cheetahs, and pumas. Their solution is given by this summary from the paper:

This evidence indicates that purring arises from the gating of respiratory flow by the larynx.
Increasing laryngeal resistance to respiratory flow by apposition of the vocal folds will cause a
relative pressure increase at the upstream side and apressure decrease downstream of the larynx.
This causes the phase differences observed across the larynx. Further, a point upstream of the
larynx during inspiration will be downstream during expiration. This explains the phase reversal
observed in the recordings.
During flow reversal when the respiratory flow stops, the amplitude of both the sound and
vibration diminishes, suggesting that laryngeal interaction with respiratory flow provides the
dominant source of both. A simple experiment demonstrates the effectiveness of this mechanism
für sound production: whisper the word ‘tee’ during both inspiration and expiration. Note that
significant sound is generated at ordinary flows when the tongue separates from the palate. When
the respiratory flow is exactly zero, the sound becomes inaudible or is limited to the sound of the
articulation itself, a distinctly different type of sound.
The source of the sound appears to be the sudden opening of the vocal folds (Remmers &
Gautier, 1972) that produces a sound very rich in harmonics. The vocal tract filters this sound and
conducts it to be radiated from the mouth and nose. Variable filtering in the vocal tract can
produce the variations in quality and loudness that is observed. We note that periodic incomplete
apposition avoids a sudden opening and will result in a signal that has a strong fundamental
frequency and weak harmonics. Under these circumstances purring could continue inaudibly.

The translation into normal-people speak:  a cat simply vibrates its larynx while inhaling and exhaling, and the movement of air over the vibrating larynx makes the sound.  There appears to be an endogenous neurological oscillator that drives this vibration, just as an endogenous oscillator drives our heartbeats.  Contrary to other hypotheses, the cat’s diaphragm is not involved.  These authors confirmed the knowledge of every ailurophile that cats purr on both the inhalation and exhalation phases of breathing.  They also found that different housecats purr at different frequencies, but each cat has a characteristic purr frequency that does not change with age.

Now, of course, the important evolutionary question: WHY do cats purr?  We know that cats purr when they are happy and contented, but also when they are under stress.  There are a multiplicity of theories, but it seems to me that at least one is correct: the mother and kittens purr as a form of bonding.  See this website for a discussion of other hypotheses.

There seems to be a bit of wiggle room in this conclusion, as the authors’ work was based on placing microphones over surfaces of the body (no cutting of cats!), correlation with respiration, etc.  They themselves admit that their solution is tentative, but for now it seems like a good one.