Signs of Winter 6: Cats are Perfect!

Pizo. Photo by D. Sillman

(Click on the following link to listen to an audio version of this blog … Cats

“What greater gift than the love of a cat?” (Charles Dickens)

Purring is one of the most pleasant things a cat can do. As I write this, my cat, Pizo, is draped across my left arm and chest and is purring loudly. It is  cold day, and Pizo has a deep need to be warm. The conditions of our closeness more than satisfies her thermal needs, and her purring is lowering my blood pressure and stimulating the synthesis of all sorts of pleasant hormones and endorphins throughout my body.

Research has shown that cat owners who regularly hold their purring cats have a 40% reduced risk of heart attack and have measurable reductions in their blood pressures. Holding a purring cat can also ease migraine headaches, reduce anxiety and generate feelings of belonging. My twelve pound cat is making my writing difficult and slow, but it is a very fair trade!

Why do cats purr?

Pezz. Photo by D. Sillman

The simplest answer is that purring in a cat is like smiling in people or tail wagging in a dog. Often purring reflects contentment and happiness. Cats seldom purr when they are alone which suggests that purring is a mechanism of communication and, often, what is being communicated is happiness. A happy, contented cat typically stretches out, closes their eyes and sinks into deep, sustained purrs.

But cats also purr when they are afraid. The explanation for this from several behavioral scientists is that purring may be a way for a cat to both calm herself and also communicate a calming message to others around her or to the source of her distress or fear.

Pizo. Photo by D. Sillman

Similarly, a cat might purr to relieve its own pain or discomfort. Purring is common in a mother cat as she is giving birth to her kittens. That purring continues after birth and attracts the blind, new-born kittens to the mom-cat to nurse. Kittens also begin to purr when they are only two days old. This purring is thought to be positive feedback for the mom-cat and also a mechanism of communications among the kittens themselves.

A cat will also purr when it is hungry. This is a very strong “feed me” message sent loud and clear to the cat’s human even when they are sitting at their computers trying to write.

How do cats purr?

An old hypothesis explaining how purring sounds are generated starts with nerve impulses coming into the larynx from the brain. These nerve impulses stimulate muscles in the larynx that then cause the cats vocal folds (aka “vocal cords”) to vibrate with each inhalation and exhalation thus creating the purring sound.

Mazie. Photo by D. Sillman

Some recent observations and experiments, though, on the physiology of purring reveals that there are no initial nerve impulses when purrs are generated, and there are no active muscular contractions in the larynx during purring. Instead the very low frequency sounds of a purr (the sound  is between 20 and 30Hz) seems to come less from the vibration of the cat’s relatively short vocal cords (which are seemingly too short to generate these low-frequency sounds in the first place) but, instead, from the vibrations of masses of fibrous tissue scattered across the vocal folds. When a cat relaxes the muscles supporting her vocal folds, these vocal “pads” are then free to vibrate as the air flow from inspiration and expiration blow across them.

Taz and friend. Photo by D. Sillman

All cat species can purr, but the large cats (lions, tigers, leopards and jaguars) very seldom do so. The females of these large cats (members of the subfamily Pantherinae) only purr when they are heat or when their cubs are nursing. What these large cats are able to do, though, that all other cat species cannot accomplish is roar! So, cougars, cheetahs, ocelots, servals, lynx, domesticated cats and the whole array of small, wild cats all can purr, but they cannot roar (a cougar’s blood-curdling call sounds more like a woman screaming (loudly!) than anything else in nature!

Photo by Benh-Lieu Song, Wikimedia Commons

So there is a relationship between purring and roaring, and one seems to prevent the other. The focus of this relationship is the vocal folds. In the non-Pantherinae cats the vocal folds are relatively short and arrayed in a characteristic triangular configuration. These short vocal folds cannot be vibrated energetically enough to generate a roar. In the Pantherinae, with the exception of the snow leopard, which is unable to roar, the vocal folds are quite long (and very well supported with strong connective tissues). They are also arrayed in a square configuration. These folds and their geometries and anatomies allow for the extremely energetic vibrations and the high volume sound generation of a roar. A lion’s roar can be heard up to five miles away! A roar is an incredibly powerful vocalization made possible by very specific laryngeal structures.

There was a recent article about cats in Scientific American (October 4, 2023) that discussed the remarkable anatomical and ecological similarities among all of the cat species on Earth. From domestic cats to tigers, all of these species basically look alike and do the same things in their ecosystems.  They are all “hard core predators” and carnivores. They are, to quote the Scientific American article, “not jacks-of-all-trades bur, instead, masters of one.” There are a number of species that have tried to take on the same predator roles as cats, but when they have had to compete against actual cats, those other species have always failed!

Mora. Photo by M. Hamilton

The skull of a house cat looks like the skull of a lion (although it is significantly smaller). The basic body shape, the teeth patterns (no molars behind the “slicing teeth” of the upper 4^th premolar and the lower first molar) and the overall shape of their heads (all rounded, “baby-heads!” lacking the elongated face and jaw so common in other adult mammals) are all consistent among all of the cat species.

Cats have not changed significantly over evolutionary time. Their design and ecological role are perfectly matched. Why change when you are perfect? Or, to loop this ending back to the title of this blog, why change when you are “purr-fect?”