<P> Brown et al. categorised vocal responses of cats according to the behavioural context: (1) during separation of kittens from mother cats, (2) during food deprivation, (3) during pain, (4) prior to or during threat or attack behavior, as in disputes over territory or food, (5) during a painful or acutely stressful experience, as in routine prophylactic injections and (6) during kitten deprivation . Less commonly recorded calls from mature cats included purring, conspecific greeting calls or murmurs, extended vocal dialogues between cats in separate cages, "frustration" calls during training or extinction of conditioned responses . </P> <P> Miller classified vocalisations into 5 categories according to the sound produced: the purr, chirr, call, meow and growl / snarl / hiss . </P> <P> The purr is a continuous, soft, vibrating sound made in the throat by most species of felines . Domestic kittens can purr as early as two days of age . This tonal rumbling can characterize different personalities in domestic cats . Purring is often believed to indicate a positive emotional state, but cats sometimes purr when they are ill, tense, or experiencing traumatic or painful moments . A more expansive definition is "purring signals a friendly social mood, and it can be given as a signal to, say, a vet from an injured cat indicating the need for friendship, or as a signal to an owner, saying thank you for friendship given ." </P> <P> The mechanism of how cats purr is elusive . This is partly because cats do not have a unique anatomical feature that is clearly responsible for this vocalization . One hypothesis, supported by electromyographic studies, is that cats produce the purring noise by using the vocal folds and / or the muscles of the larynx to alternately dilate and constrict the glottis rapidly, causing air vibrations during inhalation and exhalation . Combined with the steady inhalation and exhalation as the cat breathes, a purring noise is produced with strong harmonics . Purring is sometimes accompanied by other sounds, though this varies between individuals . Some may only purr, while other cats include low level outbursts sometimes described as "lurps" or "yowps". </P>

Why do cats meow at humans but not other cats