Despite widespread interest in inter-specific communication, few studies have examined the abilities of companion animals to communicate with humans in what has become their natural environment — the human home []. Here we report how domestic cats make subtle use of one of their most characteristic vocalisations — purring — to solicit food from their human hosts, apparently exploiting sensory biases that humans have for providing care. When humans were played purrs recorded while cats were actively seeking food at equal amplitude to purrs recorded in non-solicitation contexts, even individuals with no experience of owning cats judged the ‘solicitation’ purrs to be more urgent and less pleasant. Embedded within the naturally low-pitched purr, we found a high frequency voiced component, reminiscent of a cry or meow, that was crucial in determining urgency and pleasantness ratings. Moreover, when we re-synthesised solicitation purrs to remove only the voiced component, paired presentations revealed that these purrs were perceived as being significantly less urgent. We discuss how the structure of solicitation purrs may be exploiting an inherent mammalian sensitivity to acoustic cues relevant in the context of nurturing offspring.

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3 Bateson P.

Turner D.C. Questions about cats. 4 Moelk M. The development of friendly approach behaviour in the cat: a study of kitten-mother relations and the cognitive development of the kitten from birth to eight weeks. 1,500 = 248.26, P < 0.0005; and pleasantness: F 1,500 = 138.24, P < 0.0005) and when given the choice between pairs of non-solicitation and solicitation purrs from the same cats they identified the solicitation purr as the more urgent and less pleasant of the two (urgency: t 49 = 17.11, d.f. = 49, p < 0.0005; pleasantness: t 49 = 15.42, p < 0.0005). While participants consistently selected the solicitation purrs as more urgent irrespective of previous cat experience (owners: t 29 = 18.05, p < 0.0005; non-owners: t 19 = 8.22, p < 0.0005), individuals that had owned a cat did perform significantly better than non-owners, suggesting that the ability to identify these purrs can improve through learning (F 1,45 = 10.71, p = 0.002). In the domestic cat, many signals given when interacting with humans seem to originate from the period of dependency on the mother — which is also the time when social behaviour in this ancestrally asocial species is most prevalent []. Purring in domestic cats is one such signal, with kittens being first observed to purr whilst suckling from the mother []. Although humans typically interpret purring as indicating a happy, contented cat, some cats also purr at feeding time, actively soliciting food from their owners (see Supplemental Data available on-line with this issue). When we conducted playbacks of purrs from 10 cats recorded in both solicitation and non-solicitation contexts to 50 human participants at equal amplitude ( Supplemental Data ), they consistently judged the solicitation purrs to be more urgent and less pleasant than the non-solicitation purrs (urgency: F= 248.26, P < 0.0005; and pleasantness: F= 138.24, P < 0.0005) and when given the choice between pairs of non-solicitation and solicitation purrs from the same cats they identified the solicitation purr as the more urgent and less pleasant of the two (urgency: t= 17.11, d.f. = 49, p < 0.0005; pleasantness: t= 15.42, p < 0.0005). While participants consistently selected the solicitation purrs as more urgent irrespective of previous cat experience (owners: t= 18.05, p < 0.0005; non-owners: t= 8.22, p < 0.0005), individuals that had owned a cat did perform significantly better than non-owners, suggesting that the ability to identify these purrs can improve through learning (F= 10.71, p = 0.002).

1 Nicastro N.

Owren M.J. Classification of domestic cat (Felis catus) vocalisations by naive and experienced human listeners. 5 Frazer-Sissom D.

Rice D.

Peters G. How cats purr. 2,17 = 15.13, p < 0.005, Adjusted R2 = 0.598; VP height: β = 0.619, t = 4.09, p = 0.001; purr rate: β = 0.706, t = 4.67, p < 0.0005), and VP height and purr harmonicity as the key predictors of pleasantness (F 2,17 = 9.76, p = 0.002, adjusted R2 = 0.480; voiced peak height: β = −0.555, t = -3.34, p = 0.004; harmonicity: β = 0.423, t = 2.55, p = 0.021). To directly investigate the specific effect of the VP, we re-synthesised solicitation purrs to remove this spectral component while leaving other acoustic parameters unchanged ( 49 = 6.39, p < 0.0005). Interestingly, stimuli with the VP removed were not judged as more pleasant in these comparisons (t 48 = -0.65, p = 0.518), perhaps because their lower harmonicity (unchanged between the experimental conditions) played an important role here. Figure 1 Purr spectra illustrating intensity of voiced peak in a solicitation and non-solicitation purr from the same cat (Pepo). Show full caption For comparison with position of formant peaks and fundamental frequency see Supplemental Data ; audio files of these purr types (solicitation and non-solicitation purr) are also supplied as Supplementary material. We conducted analyses to identify the acoustic cues that both distinguished the purr types and predicted the mean urgency and pleasantness ratings that each of the purr stimuli in the independent rankings trial had received ( Supplemental Data ). While examining the acoustic structure of purrs we identified the presence of a frequency peak (range 220–520 Hz, mean 380 Hz) that was particularly pronounced in solicitation purrs and did not match the predicted formant structure of the call ( Figure 1 ). This peak was taken to indicate voicing (activation of the vocal folds via air movement), at a frequency more typical of a cry or meow [], occurring alongside the unusual low frequency muscular activation of the vocal folds that gives the purr its extremely low (∼27 Hz) fundamental frequency []. The height of this spectral peak was the acoustic feature that most consistently defined purr stimuli in the solicitation context, solicitation purrs having more intense voiced peaks (Wilcoxon-signed-ranks test z = −2.67, p = 0.008). Moreover, the height of the voiced peak (VP) was crucial in determining the urgency and pleasantness ratings that participants gave individual stimuli. A multiple regression on the mean ratings that each of the stimuli received identified VP height and purr rate as the key predictors of urgency ratings (F= 15.13, p < 0.005, Adjusted R= 0.598; VP height: β = 0.619, t = 4.09, p = 0.001; purr rate: β = 0.706, t = 4.67, p < 0.0005), and VP height and purr harmonicity as the key predictors of pleasantness (F= 9.76, p = 0.002, adjusted R= 0.480; voiced peak height: β = −0.555, t = -3.34, p = 0.004; harmonicity: β = 0.423, t = 2.55, p = 0.021). To directly investigate the specific effect of the VP, we re-synthesised solicitation purrs to remove this spectral component while leaving other acoustic parameters unchanged ( Supplemental Data ). In paired presentations, stimuli with the VP removed were consistently judged by participants as less urgent than matched stimuli with the voiced peak present (t= 6.39, p < 0.0005). Interestingly, stimuli with the VP removed were not judged as more pleasant in these comparisons (t= -0.65, p = 0.518), perhaps because their lower harmonicity (unchanged between the experimental conditions) played an important role here.