Trusting in animal folklore can put you in a pickle, as I learned from an encounter years ago with a purring kitten. She was living hard, on mean city streets, and I was going to be her ticket out, and her audible purr — indicated contentment and friendliness, at least according to everything folklore has to say about it. Reading the message loud and clear, I reached for my new feline acquaintance, who, after taking full measure of the upright animal twenty times her size coming right at her, welcomed me with the equanimity of a multidirectional, ultra-speed buzz saw.
Of my current store of scars that mark hard-earned knowledge, those contributions from my street-dwelling kitten are now, after decades of sloughing skin cells, barely visible. And yet they carry fresh meaning: Only recently have we humans come to appreciate, if not fully grasp, the great depth of biological intelligence that underpins the behavior of nonhuman animals, such that my scars now serve as tangible reminders of what I shall call feline wisdom. I have in mind not her violent protest (which, given our size differential and the fact that she didn’t know me from Adam, was entirely logical, even if it may not have been in her best interest), but her purring.
Folklore on the subject is partly accurate. Cats may indeed purr when they are content, comfortable, and — dare I risk charges of anthropomorphism? — happy. But they may also purr in times of stress or pain. This is, in the parlance of evolutionary science, an adaptive behavior. Its value derives from the nature of sound, which may be thought of as a mechanical flow of energy in the form of waves, or vibrations. Subjecting these vibrations to measurement, scientists have found the deeper sounds cycle less frequently per unit of time than higher-pitched sounds. Purrs are deep sounds, ranging from 20 to 150 hertz (cycles per second). Housecats typically purr at 20 to 50 Hz. The smaller figure of this decidedly “low frequency” range happens to coincide with the lower limit to what is audible to the human ear (the upper limit is 20,000 Hz.). More important from the feline perspective, however: low frequency sounds promote health and healing.
One hesitates to cite research that has caused animals suffering, for fear of endorsing it. Surely such work cannot be defended morally. Fortunately, not all research on sound and its healing properties has included purposeful torture, however, and humane studies show exactly what inhumane studies show, namely that sound frequencies at 20 Hz to 50 Hz — precisely the frequencies of a housecat purring — increase bone density growth, induce the healing of injured tendons and muscles, and relieve pain.
Thinking back on my street kitten, it now seems she was venting her stress by purring. Likely she was also anticipating injury, which her purring would have done much to alleviate. That the healing benefits of her purring began well in advance of any actual injury was nothing less than a marvelous display of built-in preventive medicine. Of course the kitten turned the tables on me, which short-circuited everything, but at least she made the point that purring may signal discomfort as well as comfort. Any thoughtful person can find lessons here on how to best approach a purring cat. And yes, it’s a two-way street: the next time you suffer an injury you may want to call not a doctor, but a feline friend who readily purrs in pleasure of your company.
A purr is not entirely unlike a “grrr.” Tigers, although not known to actually purr, produce low frequency sounds — some that we humans cannot hear. Although most of their vocalizations are within the 40 to 60—Hz range, tigers occasionally create rumbling sounds of 18Hz, doubtless important for tiger-to-tiger communication. Any sound frequency below 20Hz constitutes “infrasound” and travels especially well. It not only covers long distances (up to seven miles) but also penetrates dense forests and even goes through mountains. Although many factors may come into play in the behavior of sound, generally the tiger’s rumbling, infrasonic grrr travels best under dry conditions and steady temperatures. The latter condition in the tiger’s natural habitat occurs at night, just when the animal is most active.
The tiger’s infrasonic communications resembles that of the more thoroughly researched elephant, two-thirds of whose vocalizations are infrasound. Although both male and female elephants trumpet, grunt, scream, and purr, females do most of the low frequency rumbling, in order to keep the typical matriarchate together. (Sexually aroused adult males rumble at a low frequency, probably to signal other males.)
The magnificence of their large ears notwithstanding, elephants use their feet to sense infrasonic vibrations often lifting one foot, or leaning forward on their toes, to discern the traveling direction of a distant rumble. When an elephant gently touches a dead calf or adult elephant carcass she or he is almost certainly searching for the sound of a pulse or heartbeat, an act that demonstrates not only a deep social bond but also a transcendent level of recognition.
Giraffes, alligators, hippos, and rhinos also produce infrasound: and many other species likely do as well. It seems a vast world of animal communication exists right under our noses, and our hearing range.
Low frequency sounds — say, growls above 20 Hz — have an emotional component to them. They get the serious attention of all vertebrates, partly because deep, low frequency sounds tend to come from large animals, and large animals, all other things being equal, command more respect than smaller animals. Furthermore, when low frequency sounds are harsh, they signal wide-awake attentiveness. Nonhuman animals regularly employ both these principles. When the situation calls for such, they use a deep voice to sound big, and a deep, harsh voice to signal heightened alertness. The whole performance is the auditory equivalent of piloerection, the phenomenon of hair standing on end, which makes the bearer appear larger and hyper alert.
Not infrequently, human behavior also conforms to the principles. Parents reprimanding a child, for example, tend to speak in harsh, deep tones; and rarely do people speak in other than harsh, deep tones in the heat of an argument. But here is where biological intelligence — and what often seems inability to demonstrate it — comes in. Nonhuman animals deploy deep, harsh tones to prevent fights from occurring. The strategy is simple: By sounding large and hyper alert, one forces a presumably rational potential opponent to back off.
Thus roaring lions, or growling bears, or bellowing alligators so vocalize not to precipitate violence but to prevent violence. Far from bellicose threats, their harsh sounds actually constitute a form of peace activism. Contrast this with human bellowing, which is nothing if not a direct threat of violence, one, moreover that is often carried out.
My long-ago buzz-saw kitten was not looking to fight me, of course, but reacting out of fear, which meant I should have done something to emphasize not large size and alertness, but their opposites. Nonhuman animals usually sense when that kind of behavior is called for; on occasion, even human animals do. When most of us talk to young children, for example, we use gentle, high frequency vocalizations. We talk to babies using baby talk. The effect is to sound small and relaxed, that is, non-threatening and reassuring. Precisely what that kitten needed. Here, then, is another lesson: Baby-talk to an animal who doesn’t know you. Yes, many people (mostly men, we can assume) will ever resist doing so, certain that it would be futile or even dangerous, and downright silly besides. Kittens, of course, know better.
William Mannetti is president and co-founder of Animal Rights Front, an all-volunteer, Connecticut activist group.