One of the wackier outcroppings now appearing on the coastlines of the internet has been the bevy of purring videos, or super loops of cats purring, sometimes up to 10 hours long.
Occasionally someone will add crackling fire or rain sounds into the background for extra atmosphere, presumably to help chill y’all out. And here perhaps lies the key to the purpose of purring, in its entirety. Cats purr far more than they meow or hiss, and they can do it for a long time. Purring is continuous sound production and operates both on exhalation and inhalation, so it usually go on for minutes at a time (with no looping!).
Yes, cats do it when they’re apparently content, but also under stress, want food, or if they’re ill. They purr when mating and also when giving birth. And straight out the womb, kittens purr when suckling and their mothers (or “queens”) purr while nursing. You have to be close to hear most purrs, as the act involves intimacy and it isn’t a sound alone. Because the average frequency of purring occurs around 26 Hz (range 20-150 Hz), the vibrations are palpable — we feel them, but that’s something YouTube purrrr-veyors miss out on.
Not all cats purr, however. The puma, cheetah, lynx, and our cute domesticated house cats all do. Cats that “roar” instead are the lion, tiger, jaguar, and leopard. Oddly, the snow leopard is able to purr, unlike his cousin. Behold the mighty cheetah!
And you might expect that since the cheetah is a much larger animal than your average house cat, its purring frequency would be much lower. But in fact, they actually have a very similar range!
Below are the purring frequency ranges of the cheetah and the domestic cat mapped onto an extended piano keyboard, in the green horizontal line. The grey octave to the left is too low to exist on the piano, as it exits in the infrasonic frequency range (but you knew that, eh!). The green vertical line, coloring in a piano key, represents the mean average frequency at which each cat purrs — so as you can see, on average, cheetahs purr a low D, while your little Muffy produces an F.
Images courtesy of purring.org.
Cats produce their trademark rumbling sounds through intermittent signaling of the laryngeal, intercostal, and diaphragmatic muscles, initiated by a neural oscillator in the brain. And purring is reflexive — not consciously provoked. But here’s the kicker: scientists have shown that sound frequencies in the purring range can improve bone density and promote healing. Domestic cats, servals, ocelots, and pumas all produce fundamental, dominant, or strong frequencies at exactly 25 Hz and 50 Hz, the two low frequencies that best promote bone growth/fracture healing.
Furthermore, these four species have a strong harmonic overtone within 2 Hz of 100 Hz, a frequency used to treat oedema, wound repair, and respiratory dysfunction, as well as relieve pain.
Now purring really starts to make sense, as a built-in low energy sonic healing mechanism that aids recovery after trauma or illness, but also keeps muscles and bones strong when sedentary. Well known for regularly lying around or sleeping 16-20 hours a day, cats have miraculously adapted to conserve energy via long periods of rest. Brings some serious science to the nine lives theory!
Recent research has added a new twist to the story: there’s a cry within the low purr.
Embedded within the naturally low-pitched purr is a high-frequency component, one comparable to the fundamental frequency of a healthy human baby’s cry (300–600 Hz). This frequency is measurably emphasized when cats actively solicit for food. And guess what? Humans are highly sensitive to it (just like we are to a baby’s cry)!
Chalk one up for evolution and adaptation!
And maybe this research also explains how our best human purrer (“purrette?”) stayed so limber throughout her fabulous career. Behold the mighty Eartha Kitt!