Curling up with a cat and its soothing melodic purring is most cat lovers' idea of heaven. But how do they actually manage that continuous, comforting purr? 

Cats were initially divided into two groups, those that can’t purr but can roar (lions, tigers, leopards, snow leopards and clouded leopards), and those that can’t roar but can purr (domestic cats, along with all other smaller cats, such as cheetahs, lynx and pumas).

This was based on the observation that the “big” cats only have a partially ossified (bony) hyoid in the larynx, and the “small” cats have a fully ossified hyoid bone. The hyoid is effectively the structural support for the tongue and upper vocal tract. This, it was speculated, is what allowed some to roar but not others, but this is now questioned, and it's suggested that roaring is actually a result of other morphological features.  

The best guess so far of how kitties manage their pleasing purring is the intermittent contracting of their laryngeal and diaphragmatic muscles, which also allows them to purr on both the inhale and the exhale. This is backed up by evidence that shows that cats with paralyzed laryngeal muscles cannot purr. But how they start this is still unknown; we don't know whether they actively make the decision to start purring, or if it is an involuntary reaction.

Why they do it is not so straight forward either. Many might think that a purring cat is a happy cat, but this is not always the case. We might assume that purring is an expression of pleasure, especially when they do it at their kittens, or while you’re stroking them, but cats have been observed purring in all sorts of situations, including when under stress or injured.

When investigating how cats purr, scientists have discovered that cats tend to make the noise at a frequency between 25 and 150 Hertz. This low pitched vocalization is generally associated with “friendly” contexts, such as nursing and grooming, but it might have another, more interesting function. Vibrations in the body have been shown to promote healing.

This raises the fascinating possibility that purring is actually a way for cats to heal themselves. While ultrasound has been shown to accelerate skin healing, and high frequency vibration training increases muscle power, some suggest that a cat's purring can help to increase its bone density. This would fit the idea that athletes have denser bones on average as a result of vibrations when their feet hit the floor during exercise.  

With such sedentary life styles, it might actually make sense that cats would need to increase healing and bone density while effectively lounging around for most of the day. It would also explain why cats purr when sick or injured.

But in the same way that laughter in humans has many different uses, depending entirely on the context, it’s also probably true with cats and purring. For example, purring is also used by them to “exploit” humans in order to get attention and food. One study found that some even start purring at the same frequency to a human baby’s cry, in what’s been called a “soliciting purr,” and only stop when they get what they want.  

So super-healing feline or expert manipulator, the science behind your cat's yowling is still up for debate.