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A domestic cat purring
A different domestic cat purring
Different cats can sound somewhat different when purring.
Purring and meowing
Domestic cat purring mixed with pronounced meowing
Guinea pig producing purring-like sound
This sound is made when a guinea pig is content, examples of this include when it is being petted or held, when groomed, investigating a new place, or given food. It is neither continuous nor does it correspond to respiration, and thus is not a true purr.
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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. In smaller and domestic cats it is known as a purr, while in larger felids, such as the cheetah, it is called a whirr.[1]

Although true purring is exclusive to felids and viverrids,[2] 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 and guinea pigs.[citation needed]

Animals purr for a variety of reasons, including to express happiness or fear, and as a defense mechanism. It has also been shown that cats purr to manage pain and soothe themselves.[3] Purring is a soft buzzing sound, similar to a rolled 'r' in human speech, with a fundamental frequency of around 25 Hz.[4] This sound occurs with noticeable vibrations on the surface of the body, varies in a rhythmic pattern during breathing and occurs continuously during inhalation and exhalation. The intensity and length of the purr can also vary depending on the level of arousal of the animal.[4]

Mechanism

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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.[5]

There is a unique "neural oscillator" in the cat's brain of uncertain significance.[6] 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.[7]  Studies have also shown that purring can be caused through electrically stimulating the infundibular region of the cat's brain, suggesting central control.[8]

Vocal folds/laryngeal muscles

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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.[9] Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced with strong harmonics.[10]

Degree of hyoid ossification

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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[11] and was definitively introduced by Pocock,[12] 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.[13]

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.[14]

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

[edit]
  • Domestic cats purr at a frequency of 20 to 30 vibrations per second.[3]
  • 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).[15]
  • 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.[16]
  • 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.[17]
  • 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.[18]

Purpose

[edit]

In domestic cats, many signals that occur when interacting with humans seem to originate from when the animal was dependent on the mother. Cats have been observed to purr for most of their lifespan, starting from when they were young and suckling from their mother.[19] Purring may be a signaling mechanism of reassurance between mother cats and nursing kittens. Post-nursing cats often purr as a sign of contentment when being petted, becoming relaxed or eating. Some purring may be a signal to another animal that the purring cat does not pose a threat.

Cats have been shown to have different types of purrs depending on situations. For example, purring appears to be a way for cats to signal their caretakers for food. This purring has a high-frequency component not present in other purrs. These are called solicitation purrs (when the cat is seeking a result) and non-solicitation purrs, and the two are distinguishable to humans. In a study, 50 people listened to recordings of purrs recorded in solicitation and non-solicitation situations at the same amplitude. Humans regularly judged the solicitation purrs as less pleasant and more urgent than the non-solicitation purrs.[19] This variety of purring seems to be found more frequently in cats in a one-to-one relationship with a caretaker. Similarities have been drawn between an infant's cry and the isolation cry of domestic cats. The high-frequency aspect of the purr may subtly exploit humans' sensitivity to these cries. Using sensory biases in communication between species provides cats with a productive means of improving the care that they receive.[19]

Cats often purr when distressed or in pain, such as during the three stages of labor. In the first stage, the uterus begins to contract, the cervix relaxes, the water breaks and the cat begins to purr. The female cat will purr and socialize during the first stage of labor. The purring is thought to be a self-relaxation technique.[20]

See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Purr

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from Grokipedia
Purring is a distinctive, low-frequency vocalization produced primarily by cats ( catus) and other felid species, as well as some non-felids like genets, characterized by a continuous rumbling or vibrating that occurs during both and exhalation, typically at frequencies ranging from 25 to 150 Hz. This serves multiple functions beyond the commonly perceived indicator of , including communication, self-soothing, and potentially therapeutic effects on the cat's own . The mechanism of purring has been the subject of scientific investigation, with long-held theories positing active contraction of laryngeal and diaphragmatic muscles to generate at rates of about 25–30 times per second. However, a 2023 study published in challenged this view, demonstrating that excised cat larynges can produce purring frequencies (25–30 Hz) passively through MyoElastic-AeroDynamic (MEAD) principles—similar to vocal fry—without neural input or muscular activity, relying instead on airflow across specialized fatty pads in the vocal folds. This passive production allows cats to purr continuously during respiration, distinguishing it from other vocalizations like meows, which are intermittent. Cats purr in diverse contexts, reflecting its role as a versatile communication tool and physiological regulator. Importantly, cats do not purr constantly but situationally, employing it as an adaptive mechanism for self-regulation in contexts such as relaxation, stress, or bonding. Kittens purr to signal and location to their mothers during , fostering bonding and coordination. In adults, it often accompanies positive social interactions, such as greeting owners or conspecifics, but also emerges during stressful situations like or veterinary examinations, suggesting a self-calming function that may release to manage pain and anxiety. Additionally, the purr's frequency range overlaps with therapeutic vibrations known to promote and tissue repair in other mammals, leading to hypotheses that it aids cats in maintaining skeletal health during rest or recovery, though direct evidence in felids remains exploratory. Purring is absent in high-arousal states like or , underscoring its association with low-energy, affiliative behaviors.

Overview

Definition and characteristics

Purring is defined as a continuous, low-frequency rumbling vocalization produced by felids, characterized by its rhythmic and continuous nature, which occurs throughout both and phases of the respiratory cycle. This distinguishes it as a unique form of feline communication, typically lasting from several seconds to minutes, and often audible as a soft, vibrating hum. Key observable characteristics of purring include its tonal quality and consistent rhythm, with fundamental frequencies generally ranging from 25 Hz to 150 Hz in felids, producing a soothing, repetitive that can be felt as through the animal's body. Purring originates from the larynx in the throat, and these vibrations transmit throughout the body, often leading many owners to mistakenly believe it comes from the back or belly. Unlike discontinuous vocalizations, purring maintains continuity across the full breathing cycle, creating a seamless auditory pattern that is self-sustaining without interruption from other mouth movements. The phenomenon was first systematically documented in domestic cats by naturalist in 1872, who described it as the "purr of " observed during states of , such as when a caresses its kittens or owner. This early observation highlighted its association with positive emotional states, though subsequent research has expanded understanding of its contextual uses. Purring differs markedly from related felid sounds like or meowing in its production and acoustic profile: serves as a low-frequency display with a harsher, broadband quality often paired with other aggressive vocalizations, while meowing is a higher-pitched, pulsed call typically produced with an open mouth for interspecies signaling. In contrast, purring's closed-mouth, continuous tonal rumble underscores its specialized role in feline expression.

Distribution across species

Purring is predominantly observed within the family Felidae, encompassing all 40 recognized wild cat species and the domestic cat (Felis catus), totaling approximately 41 extant species, though with consistent production mechanisms primarily in smaller species. Among felids, purring is universal in all small cats of the subfamily Felinae (including the cheetah, Acinonyx jubatus) and the puma (Puma concolor), accounting for the majority of the family's approximately 41 extant species; most larger felids in the genus Panthera, such as lions (Panthera leo), tigers (Panthera tigris), leopards (Panthera pardus), and jaguars (Panthera onca), lack the ability due to anatomical adaptations in the hyoid apparatus that favor roaring over purring; however, the snow leopard (Panthera uncia) is an exception that purrs. This pattern reflects evolutionary divergence within Felidae, where smaller body sizes correlate with purring as a versatile communication tool, while larger species prioritize powerful roars for territorial signaling. Beyond felids, true purring—defined as continuous tonal fluttering produced during both inhalation and exhalation—is confirmed in the family (civets, genets, and related carnivorans), based on acoustic analyses of species like the small-toothed civet (Arctogalidia trivirgata) and (Genetta genetta). Purr-like vocalizations, sharing similar low-frequency characteristics and mechanisms, occur in non-felid mammals such as raccoons (Procyon lotor) in the family , where mothers produce comforting purrs during grooming and nursing, and rock hyraxes (Procavia capensis) in the order Hyracoidea, which emit purring sounds during social bonding or contentment. These instances suggest , as the independent development of purring across distantly related lineages likely serves analogous functions in affiliation and stress reduction, despite differing anatomical bases from felids. The English term "purr" originates from imitative , with earliest records in around 1398, evoking the sound's rhythmic quality; it evolved without direct ties to forms like "purpen," instead drawing from broader Indo-European mimetic roots seen in related terms across .

Production Mechanism

Laryngeal and muscular processes

The production of the purr in felines primarily involves the , where the vocal folds undergo rapid oscillations to generate the characteristic low-frequency sound. The , the space between the vocal folds, experiences rhythmic dilation and constriction at frequencies typically ranging from 25 to 50 cycles per second, facilitated by the intrinsic laryngeal muscles such as the , which tenses the vocal ligaments to enable these vibrations. This oscillatory action is driven by rhythmic contractions, or muscular twitching, of the laryngeal muscles, initiated by neural impulses originating from the brain's vocal , which sustain self-oscillations without requiring continuous external airflow modulation. These contractions produce the purr's steady hum, integrating with the hyoid apparatus for structural support during prolonged emission. The foundational understanding of this mechanism emerged in the through studies proposing an active muscle model, where cyclical contractions of the laryngeal musculature directly generate the purr via neural and mechanical coordination in cats. This model emphasized the role of intrinsic muscles in creating self-sustained oscillations at purr frequencies. Recent histological investigations have refined this view, revealing specialized in the vocal folds that allow purring without the fatigue associated with constant muscular activity in the original model. Analysis of cat larynges identified embedded pads within the vocal folds, up to 4 mm in diameter, which facilitate passive vibrations at 25-30 Hz when passes through, reducing reliance on active muscle twitching for sustained production. These findings demonstrate how the laryngeal structure enables efficient, low-energy sound generation, updating earlier assumptions of muscle-driven fatigue.

Hyoid apparatus involvement

The hyoid apparatus in felids consists of a series of small bones that suspend the larynx and support the tongue, playing a critical role in vocal production by anchoring and stabilizing the vocal tract. In species capable of purring, such as domestic cats (Felis catus) and other members of the subfamily Felinae, the hyoid is fully ossified, forming a rigid bony chain typically comprising nine linked elements that transmit laryngeal vibrations effectively. This complete ossification contrasts with the hyoid structure in roaring species of the genus Panthera, such as tigers (Panthera tigris), where the apparatus is incompletely ossified, featuring ligamentous connections—often seven bony elements linked by cartilage or ligaments—that provide flexibility but limit sustained low-frequency oscillations. The rigid hyoid in purring felids enables the transmission of vibrations from the oscillating to surrounding tissues without , facilitating the characteristic 25-50 Hz rumble of purring. In domestic cats, this fully bony structure allows the to vibrate against the hyoid during both and , producing continuous sound without the flexibility that would absorb in roaring cats. Biomechanically, the hyoid's suspension of the amplifies these low-frequency vibrations through in the rigid framework and adjacent soft tissues, enhancing the purring signal's propagation while minimizing fracture risk under repetitive oscillation. Comparative anatomy across felids reveals that purring ability correlates with smaller body size and fully ossified hyoids in over 30 species of , whereas the partial ossification in the five roaring species supports broader laryngeal movements for powerful, low-pitched roars. This structural dichotomy reflects evolutionary trade-offs: roaring felids, adapted to large territories and long-distance communication, sacrifice purring's sustained resonance for vocal flexibility, while smaller purring felids prioritize close-range social signaling and self-soothing through efficient vibration mechanics. Muscular attachments, such as those from the sternohyoid and thyrohyoid muscles, further stabilize the hyoid in purring species to optimize vibration transfer.

Neural and respiratory control

The neural basis of purring in felids involves a in the that produces rhythmic impulses to coordinate the contraction of intrinsic laryngeal muscles, generating the characteristic oscillations without requiring continuous external sensory input. This generator operates as a high-frequency neural oscillator, sending stereotyped bursts of activity at rates aligned with purring, as evidenced by electromyographic recordings showing regular activation patterns in laryngeal muscles during sustained purring episodes. Respiratory integration ensures that purring persists across both and exhalation phases, with the diaphragm and adapting to maintain continuity despite the rapid laryngeal vibrations. Specifically, during inspiration, the diaphragmatic electromyographic activity becomes intermittently modulated or "chopped" in synchrony with laryngeal bursts, limiting fluctuations in the trachea and facilitating efficient without disrupting overall respiration. This coordination allows purring to overlay normal breathing patterns seamlessly, as the oscillatory mechanism does not rely on full respiratory cycles for timing. Purring is triggered by various sensory stimuli, such as tactile input or stress responses, processed through the , particularly regions like the , where electrical stimulation has been shown to elicit purring directly. The duration and intensity of purring are further regulated by feedback loops involving the , which innervates the via its recurrent laryngeal branch and provides sensory modulation to sustain or terminate the based on ongoing physiological states. Recent from 2023 highlights a passive biomechanical component in purring production, where laryngeal structures can self-sustain at purr frequencies without active neural drive or muscular contraction, thereby minimizing neural expenditure in a manner analogous to human vocal fry. This mechanism suggests that while neural initiation is essential, ongoing maintenance may rely more on intrinsic tissue properties than constant input, potentially explaining the low metabolic cost of prolonged purring. A 2025 study further elucidated the genetic underpinnings of purring, finding that polymorphisms in the gene, specifically shorter repeat lengths (≤18 repeats), are associated with higher purring scores in domestic cats, as assessed by owner questionnaires. This may influence neural pathways regulating vocalization frequency and intensity, providing evidence for a heritable basis in purring behavior.

Acoustic Properties

Frequency and amplitude profiles

The purring vocalization in domestic cats features a characteristic low- profile, with the overall range typically spanning 25 to 150 Hz, encompassing the and its harmonics. The lies between 25 and 50 Hz, while harmonics extend up to approximately 200 Hz, contributing to the sound's tonal quality. This arises from the periodic of the vocal folds, governed by the equation f=1Tf = \frac{1}{T}, where ff is the in Hz and TT is the glottal cycle time, approximately 0.02 to 0.04 seconds. Amplitude characteristics of purring reflect its subtle, non-intrusive nature, with sound pressure levels at the source generally ranging from 20 to 50 dB, comparable to a quiet whisper. Peaks in occur primarily during the phase of the respiratory cycle, when through the is maximized. In extended purring bouts, often displays gradual decay patterns, as the intensity wanes over successive cycles due to diminishing or modulation. Spectrographic analysis serves as the primary measurement method for dissecting these profiles, employing Fourier transforms to isolate tonal structures from any noisy, aperiodic components arising from irregular glottal closures. Such analyses confirm the dominance of periodic, line-spectrum elements in the purr's .

Variations with respiration and context

Purring in cats follows a biphasic acoustic pattern synchronized with the respiratory cycle, producing continuous vibrations during both (ingressive phase) and (egressive phase) without full interruption. A comparative acoustic analysis of four domestic cats revealed slight variations in between these phases, typically shifting by a few Hz due to differences in and laryngeal tension, with ingressive phases showing frequencies around 20-27 Hz and egressive phases often exhibiting higher amplitudes and longer durations. These respiratory-linked modulations maintain the purr's characteristic low-frequency profile while adapting to breathing dynamics. The acoustics of purring also vary with contextual and emotional states, reflecting adaptive signaling. In distress or anxiety, purrs tend to increase in amplitude compared to the quieter, softer purrs associated with contentment. Acoustic research highlights context-specific harmonics, where embedded high-frequency elements (like subtle cries) within the purr enhance emotional conveyance, such as urgency in solicitation scenarios.

Biological Functions

Communication and social roles

Purring serves as a primary signal of in felines, particularly in contexts involving solicitation for , , or . Kittens begin purring as early as two days old while from their , using the to communicate satisfaction and well-being, which in turn stimulates let-down and reinforces the . Adult cats similarly employ purring to express pleasure during petting or feeding interactions with humans or conspecifics, fostering positive social exchanges. In social settings, purring facilitates bonding among cats, especially in multi-cat households or colonies. Mutual purring occurs during affiliative behaviors such as allorubbing, where cats exchange and physical contact, helping to maintain group cohesion and reduce tension within the . This vocalization strengthens alliances by signaling non-aggression and comfort, contributing to the cooperative dynamics observed in feline groups. Purring also plays a role in distress communication, particularly through the "solicitation purr," a variant embedded with a high-pitched cry-like element resembling an infant's vocalization. This modified purr, produced when cats seek or from , elicits faster responses due to its urgent acoustic profile, exploiting sensitivity to such . In cross-species interactions, purring extends to calming companions like , promoting relaxed states and aiding in the process. Evolutionarily, purring likely provided a selective advantage in feline social structures by enabling effective signaling for care and affiliation across generations.

Physiological and therapeutic effects

Purring is a situational behavior in cats, occurring when relaxed, stressed, or bonding, rather than constantly, and serves as an adaptive tool for self-regulation when needed. It functions as a self-soothing mechanism particularly during times of stress, , or discomfort, where it is associated with of the , promoting relaxation and potentially aiding in pain and emotional distress management, thereby contributing to stress reduction and healing promotion that enhances feline resilience. This behavior is observed when cats purr while nursing wounds or in anxious situations, suggesting an internal calming effect that helps manage pain and emotional distress without external stimuli. The vibrations produced by purring, in the 25-150 Hz range, align with frequencies used in therapeutic vibration treatments that enhance and support . Studies from the Acoustical Society of America, including a 2001 presentation by Elizabeth von Muggenthaler titled "The felid purr: A healing mechanism?", along with veterinary research, support the hypothesis that these low-frequency oscillations could stimulate osteogenesis in felids, drawing parallels to cellular repair processes in clinical models for healing and pain reduction in cats, though remains exploratory. For , the same vibrational profile has been proposed to potentially accelerate tissue regeneration through mechanisms similar to and low-intensity applications that improve deposition and epithelialization. Purring vibrations have also been hypothesized to aid in muscle relief and the reduction of by enhancing circulation and mitigating , as the frequencies overlap with those employed in medical therapies for repair. In felids, this may potentially help alleviate strain and swelling during recovery periods, with the rhythmic oscillations proposed to promote lymphatic drainage and vascular flow to affected areas, though empirical support is exploratory. Preliminary suggestions from veterinary sources indicate that exposure to these purring vibrations may also contribute to stress relief in humans, such as lowering blood pressure and reducing anxiety, extrapolated from general vibration therapy effects. Notably, purring occurs passively during , providing these potential benefits with minimal energy expenditure, which supports ongoing tissue maintenance in cats that spend much of their time inactive.

References

  1. https://www.loc.gov/everyday-mysteries/zoology/item/why-and-how-do-cats-purr/
  2. https://www.livescience.com/animals/cats/are-cats-the-only-animals-that-purr
  3. https://www.aaha.org/resources/the-secret-feline-language-5-reasons-why-your-cat-purrs/
  4. https://www.scientificamerican.com/article/why-do-cats-purr/
  5. https://www.cell.com/current-biology/fulltext/S0960-9822(23)01230-7
  6. https://www.bbc.com/future/article/20180724-the-complicated-truth-about-a-cats-purr
  7. https://news.cvm.ncsu.edu/why-do-cats-purr-nc-state-university-college-of-veterinary-medicine-behavior-expert-explains/
  8. https://pmc.ncbi.nlm.nih.gov/articles/PMC7000907/
  9. https://pubs.aip.org/asa/jasa/article/110/5_Supplement/2666/550913/The-felid-purr-A-healing-mechanism
  10. https://www.gutenberg.org/files/1227/1227-h/1227-h.htm
  11. https://roberteklund.info/Purring.htm
  12. https://www.discoverwildlife.com/animal-facts/mammals/why-can-only-big-cats-roar
  13. https://www.nwf.org/Magazines/National-Wildlife/1995/Questions-and-Answers-About-Wildlife
  14. https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-2907.2002.00113.x
  15. https://www.skedaddlewildlife.com/location/durham-region/blog/raccoon-sounds-and-what-they-mean/
  16. https://www.oed.com/dictionary/purr_v1
  17. https://www.etymonline.com/word/purr
  18. https://www.cell.com/current-biology/pdf/S0960-9822%2823%2901230-7.pdf
  19. https://pubmed.ncbi.nlm.nih.gov/4644061/
  20. https://onlinelibrary.wiley.com/doi/full/10.1002/jmor.21627
  21. https://www.zobodat.at/pdf/Zeitschrift-Saeugetierkunde_59_0087-0104.pdf
  22. https://www.sciencealert.com/these-are-the-largest-cats-known-to-purr
  23. https://www.sciencedirect.com/science/article/pii/0034568772900643
  24. https://www.livescience.com/32286-how-do-cats-purr.html
  25. https://apps.dtic.mil/sti/tr/pdf/AD0407391.pdf
  26. https://telepaws.com/why-do-cats-purr-the-most-surprising-scientific-theories/
  27. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0324055
  28. https://www.petmd.com/cat/behavior/why-do-cats-purr
  29. https://www.researchgate.net/publication/230125299_How_cats_purr
  30. https://www.ck12.org/flexi/physical-science/intensity-and-loudness-of-sound/suppose-the-purr-of-a-cat-has-a-sound-intensity-that-is-380-times-greater-than-the-threshold-level-find-the-decibel-value-for-this-cat%27s-purr-round-to-the-nearest-decibel/
  31. https://link.springer.com/article/10.1007/BF00228249
  32. https://www.researchgate.net/publication/258704857_A_comparative_acoustic_analysis_of_purring_in_four_cats
  33. https://www.sciencedirect.com/science/article/pii/S0003347224011683
  34. https://pmc.ncbi.nlm.nih.gov/articles/PMC10822437/
  35. https://www.sciencedirect.com/science/article/pii/S0960982209011683
  36. https://rutherfordvet.com/cat-purring-effect-on-humans/
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