Purr

Key takeaways

Purr belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Purr to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Purr from memory before moving on to harder problems.

Reference excerpt

A purr or whirr is a tonal fluttering sound made by some species of felids, including both larger, wild cats and the domestic cat (Felis catus), as well as two species of genets and the Caquetá titi monkey (Plecturocebus caquetensis). It varies in loudness and tone among species and in the same animal. Although true purring is exclusive to felids and viverrids, other animals such as raccoons produce vocalizations that sound similar to true purring. Animals that produce purr-like sounds include mongooses, kangaroos, wallabies, wallaroos, badgers, rabbits, foxes, and guinea pigs. Animals purr for a variety of reasons, chiefly to express happiness, but also as defense mechanism for pain. Cats also purr to manage pain and soothe themselves. Purring is a soft buzzing sound, similar to a rolled 'r' in human speech, with a fundamental frequency of around 25 Hz. The sound occurs with noticeable vibrations on the surface of the body, varies in a rhythmic pattern during breathing and, for cats, occurs continuously during inhalation and exhalation, while other animals (such as foxes) that purr only purr on exhale. The intensity and length of the purr can also vary depending on the level of arousal of the animal.

Mechanism The mechanism by which cats purr is an object of speculation, with different hypotheses proposed. An early idea was that purring is a hemodynamic process where sound is produced as the blood runs through the thorax. Although the mechanism has not yet been fully explained, recent studies have inferred it could be the result of oscillatory mechanisms in the central nervous system. There is a unique "neural oscillator" in the cat's brain of uncertain significance. Studies have also shown that purring can be caused through electrically stimulating the infundibular region of the cat's brain, suggesting central control.

Vocal folds/laryngeal muscles One hypothesis, backed by electromyographic studies, is that cats produce the purring noise by using the vocal folds 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 of air as the cat breathes, a purring noise is produced with strong harmonics.

Degree of hyoid ossification No cat can both purr and roar. The subdivision of the Felidae into "purring cats" (Felinae) on one hand and "roaring cats" (Pantherinae) on the other goes back to Owen and was definitively introduced by Pocock, based on whether the hyoid bone of the larynx is incompletely ("roarers") or completely ("purrers") ossified; however, Weissengruber et al. argued that the ability of a cat species to purr is not affected by the anatomy of its hyoid. The "roaring cats" (lion, Panthera leo; tiger, P. tigris; jaguar, P. onca; leopard, P. pardus) have an incompletely ossified hyoid, which, according to this hypothesis, enables them to roar but not to purr; however, the snow leopard (Uncia uncia, or P. uncia), as the fifth felid species with an incompletely ossified hyoid, purrs. All remaining species of the family Felidae ("purring cats") have a completely ossified hyoid, which enables them to purr but not to roar. Based on a technical acoustic definition of roaring, the presence of this vocalization type depends on specific characteristics of the vocal folds and an elongated vocal tract, which is rendered possible by an incompletely ossified hyoid.

Frequency, amplitude, and respiratory variation Domestic cats purr at a frequency of 20 to 30 vibrations per second. Eklund, Peters & Duthie, comparing purring in a cheetah (Acinonyx jubatus) and a domestic cat (Felis catus) found that the cheetah purred with an average frequency of 20.87 Hz (egressive phases) and 18.32 Hz (ingressive phases), while the much smaller domestic cat purred with an average frequency of 21.98 Hz (egressive phases) and 23.24 Hz (ingressive phases). Schötz & Eklund studied purring in four domestic cats and found that the fundamental frequency varied between 20.94 and 27.21 Hz for egressive phases and between 23.0 and 26.09 Hz for ingressive phases. Schötz & Eklund also observed considerable variation between the four cats as regards relative amplitude, duration and frequency between egressive and ingressive phases, but that this variation occurred within the same general range. In a follow-up study of purring in four adult cheetahs, Eklund, Peters, Weise & Munro found that egressive phases were longer than ingressive phases in all four cheetahs. Likewise, ingressive phases had a lower frequency than egressive phases in all four cheetahs. Mean frequency were between 19.3 Hz and 20.5 Hz in ingressive phases, and between 21.9 Hz and 23.4 Hz in egressive phases. Moreover, the amplitude was louder in the egressive phases in four cheetahs. Eklund & Peters compared purring in adult, subadult and juvenile cheetahs and reported that while there was considerable variation across most of the parameters analyzed (amplitude, phase duration, cycles per phase and fundamental frequency) – mainly attributable to degree of relaxation/agitation in the animals resting or playing– previously reported observations that ingressive phases tend to be lower in frequency were largely confirmed. There were no major differences in these parameters as a function of age.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Purr

Start with the simplest possible case. Write down what Purr claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Purr before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Purr ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Purr

In research: Purr appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Purr in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Purr is common in secondary-school and first-year university syllabi. It links to neighbouring topics Animal sounds, Cat behavior, Ethology, so understanding it makes those chapters shorter.
In everyday life: Look for Purr outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Purr in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Purr means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Purr out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Purr in simple terms?

A purr or whirr is a tonal fluttering sound made by some species of felids, including both larger, wild cats and the domestic cat (Felis catus), as well as two species of genets and the Caquetá titi monkey (Plecturocebus caquetensis). It varies in loudness and tone among species and in the same ani…

Why does Purr matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Purr?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Purr.