Animal Music: The Cat’s Meow – Part 2
It’s an old saying in the veterinarian trade that if you put a cat in the same room with a bunch of broken bones, the bones will heal; and we’ve all heard the saying that a cat has nine lives. Aphorisms like this are often relegated to urban myth and legend, but it looks as if science has found some truth in these old sayings. Anyone who has ever had a pet cat will be familiar with that unique feline quirk, the purr. Until quite recently it was thought that anything from rabbits, pigs and even elephants could purr. But studies have shown that it is only the cat, or Felidae species and possibly two kinds of genets from the Viverridae species, that actually purr – in the strictest sense.
A purr is defined by Dr Gustav Peters as a ‘continuous sound production [that] must alternate between pulmonic egressive and ingressive airstream’ (2002). He qualified it further by suggesting that it can go on for many minutes at a time. Essentially, it means that only cats and some genets can make the sound we all recognise as purring while they breath in and out, resulting in a continuous, rhythmic vibration.
The physics of the purr are a bit of a mystery to scientists, because cats don’t possess a unique anatomical feature that is responsible for the sound. But it seems that purring is facilitated by a fully ossified hyoid which exists in most cats of the Felis genus. The big cats of the Panthera genus include the tiger, lion, jaguar and leopard, and they cannot purr in the strictest sense. They can produce a similar sound, but only when exhaling. It is thought that this is because their hyoid is only partially ossified, a physical attribute that also enables them to roar. This distinction between roaring and non-roaring cats was suggested all the way back in 1834 by biologist and comparative anatomist Richard Owen, who would have used the differences in the hyoid to distinguish between the roaring and non-roaring cats. But to confuse things even further, there is an exception to this general rule. The snow leopard possesses a hyoid that is also not completely ossified and yet it can purr.
So even though scientists are not really sure how it’s done, they seem to have come to an agreement about what defines the purr. A domestic cat purrs at a frequency of 25 to 150 vibrations per second. With differences between cats and also between the ingressive and egressive output, we could generally say that they purr between 20-27 Hz.
It was once thought that a purr was indicative of a contented cat. And while this is certainly true, it is also true that cats purr when they are nervous, in pain, ill or asleep or even just to solicit a response – from both other cats and humans. Researchers now believe that the purr is linked with survival. It has been recorded in veterinarian science journals that cats have an uncanny knack of surviving falls from very high places, the highest reportedly being 45 storeys. Cats recover more quickly from bone injuries than dogs, suffer fewer complications from surgery and have less orthopaedic and muscular trauma. It is now thought that the cat’s ability to heal itself so efficiently is due to its purr.
Medical science has established that bone strength can be improved by up to 20 per cent when treated with frequencies of around 20-50 Hz. Higher frequencies, closer to 120 Hz, can promote healing in tendons and ligaments. The cat’s purr is also within the range of frequencies known to relieve chronic and acute pain. Vibrational stimulation is known to relieve pain, promote healing, bone and tissue strength and improve circulation and oxygenation. This relatively new discovery could mean that we can reverse the effects of conditions that deteriorate bones like osteoporosis and provide complementary health care for injuries and pain management.
So it seems that our urban mythology about cats and their seemingly magical ability to survive was spot on. Maybe the ancient Egyptians weren’t so unfounded in their worship of this creature. Empirical evidence has shown that biologically speaking, the purr has survived for a reason. The cat’s in-built healing system can not only benefit themselves, it can also benefit others.
BY TOBIAS FISCHER & LARA CORY
Eklund, Robert, Gustav Peters & Elizabeth D. Duthie. 2010. An acoustic analysis of purring in the cheetah (Acinonyx jubatus) and in the domestic cat (Felis catus).In: Proceedings of Fonetik 2010, Lund University, 2–4 June 2010, Lund, Sweden, pp. 17–22
Leslie A Lyons, Scientific American, April 2006, http://www.scientificamerican.com/article/whydocats-purr/
Elizabeth von Muggenthaler, ‘The Felid Purr; A healing mechanism?’, in the proceedings from the 12th International Conference on Low Frequency Noise and Vibration and its Control held in Bristol, UK, 18th to 20th September 2006. http://www.animalvoice.com/catpur.htm