The cat's purr is thought to communicate pleasure. However, cats will also purr when stressed or injured. The purr consists of a variable voiced region and a low frequency conserved region (the fundamental frequency (F_-0) and its harmonics). The purr is thought to originate from oscillations of the vocal cords, stimulated by a centrally located neural oscillator. In people, stress causes the vocal cords to shorten and increases F_-0 of phonation; this change allows detection of emotion. This study hypothesised that stress (environmental and surgical) and centrally acting opioids would alter F_-0 of purring in the domestic cat.

Cats were recruited from domestic households (control, n = 50), a rescue centre (environmental stress, n = 28), and on presentation for neutering at two veterinary clinics (environmental stress, n = 26). Purrs were recorded using a handheld device. Cats at the neutering clinics were further recorded at opioid premedication, postoperative recovery and four hours post surgery. Cats were excluded from the study if they had preexisting medical conditions. Analyses of purrs were performed using Audacity® audio editing software, plotting time against amplitude (dB), and amplitude against Log₁₀ frequency (Hz). F_-0 was identified as the peak with the lowest frequency; peaks for the 2nd, 3rd and 4th harmonics (multiples of the fundamental frequency) were identified and used to verify the identity of F_-0. Statistical analyses were performed using Minitab®.

84/104 (81%) cats purred when stroked; cats were more likely to purr in their home environment (90%) or the rescue centre (89%) than at the veterinary clinic (54%). The majority of cats were domestic shorthairs (70%). Siamese, Burmese and Bengal cats were most likely to purr (95.0%); British shorthairs least likely (62.5%). The average F_-0 26.7 Hz ±2.3 SD was similar to that previously reported. However, our results indicated a range for each frequency peak, and the third harmonic, not previously reported, was identified at 79.4 Hz ±6.92 SD. F_-0 was not significantly affected by opioid administration, environmental or surgical stress.

Willingness to purr reduced as stress increased. F_-0 of purring was not affected by environmental or surgical stress, and is unlikely to communicate stress. However, the voiced region of the purr may alter with stress and needs further investigation. In adult cats, the function of the conserved region may be to facilitate the production of the voiced region, thereby allowing a more complex form of communication.