Why Do Cats Purr? - By Abigail Pinchbeck

One of the many joys of being a cat-owner is listening to the relaxing rumble of your cat’s purr. You may assume that this noise of contentedness simply means your cat is comfy and calm, though some recent research suggests a purr may be more complex than we initially thought. This familiar noise is in fact shrouded in mystery, with scientists currently unable to definitively reveal the reasons behind why and how cats make this calming noise.

What’s that purring sound?

It was previously thought that purring was caused by a surging of blood through the inferior vena cava, one of the main veins of the heart. This so-called ‘turbulent blood theory’ has since been disproven.

Current research suggests that the noise originates from the constriction of the cat’s larynx. This vibrates the vocal cords, producing noise that continues as the cat inhales and exhales, aided by the diaphragm muscles. You can feel these vibrations in both a cat’s throat and stomach, where the larynx and diaphragm are located respectively. We hear these vibrations as a purr.

Purring has shown to be triggered by part of a cat’s brain known as the neural oscillator, which is commonly associated with rhythmic motor outputs.

Despite popular belief, the phenomenon of purring is not unique to cats. Small mammals such as squirrels, guinea pigs and racoons have also been known to purr.

A purr is worth a thousand words.

Although our feline friends have been kept as domesticated pets for almost ten thousand years, scientists are yet to confirm the definitive reasons behind their characteristic purring.

Many cat owners simply assume that cats purr when they are happy and comfortable. However, this is only somewhat true. Scientists predict that there are in fact a whole host of reasons behind this endearing vibration, rather than being merely a display of contentedness. Cats rarely purr whilst alone – suggesting that communication is a key factor in the reasons behind this distinctive rumble.

Kittens let out their first purr at just a few days old, helping their mothers to find them for feeding time. Mothers also often purr during labour – it’s suggested that this could release endorphins to provide pain relief. Cats have also been shown to purr in other stressful situations, such as to calm an opponent in a fight or perhaps as a display of submission.

Some scientists even predict that a cat’s purr can have mind-controlling properties, taking advantage of our nurturing instincts by mimicking a baby’s cry. Dr Lauren Finka, a cat behavioural expert from Nottingham Trent University, noted that “the purrs indicating a cat was hungry actually shared aural signatures with the noises that hungry babies make when they’re crying,” in regards to a study conducted in 2009. This shows that cats can attempt to solicit food from their owners in this way, disguising the message of ‘feed me’ in an adorable purr that humans just can’t resist.

A study published in the New Zealand Veterinary Journal suggested that the frequency of a cat’s purr (20Hz to 150Hz) could have healing properties, with the vibrations produced corresponding to those often used in treatment for bone fractures. Cats have been shown to purr in response to injury or distress, possibly in attempt to self-soothe or heal. A purr is frequently observed as a cat’s final act before death.

Lions and Tigers: Purring or Roaring?

Early 19th century taxonomists split the cat family Felidae in two, with distinct groups of ‘purrers’, including housecats, bobcats and ocelots, and ‘roarers’, such as lions, tigers and leopards. The mechanism a lion uses to produce a terrifying roar is surprisingly similar to how your cat produces a gentle purring.

The ability of a cat to purr or roar depends on the structure of twiglike bones in the larynx known as the hyoid. Small cats have hyoid bones which are completely ossified and rigid, allowing it to resonate against the vibrating larynx and produce a purr. In larger cats however, this bone is more stretchy and flexible, allowing them to produce deep roars. This means that big cats such as lions are incapable of purring, whilst your pet cat probably can’t emit a deafening roar.

Right now, the mystery of the purr is far from solved, with scientists still unable to agree on the exact reasons behind these vibrations. Through purring, your cat could be trying to tell you it’s comfy, hungry, hurt, stressed or even in labour!