Subjects

In Experiment 1, the participants were 16 domestic cats (8 males and 8 females; age range: 1–11 years, mean age: 3.69 years, SD = 3.01) living with 11 families (three male and eight female owners), each of whom lived with 2 or fewer other cats. By breed, there were 12 mongrels, two Scottish Folds, an American Shorthair, and a Himalayan. Fifteen of the cats had begun to live with their owner within one year of birth, and one cat when it was 5 years old. Fifteen of the cats were neutered (one female was not).

In Experiment 2, 34 domestic cats (16 males and 18 females) each of which was living with 4 or more other cats, participated. Twenty-four cats were owned by four families and the remaining 10 were part of a ‘cat café’, a business establishment where visitors can freely interact with cats. The cats had six female owners (two owners were members of the same household). Of the 34 cats, there were 24 mongrels, three LaPerms, a Devon Rex, a Somali, a Scottish Fold, an American Curl, a LaPerm Shorthair, a Tonkinese, and a Munchkin. Their ages ranged from 0.5 to 10 years (mean age: 5.51 years, SD = 2.95), and the ages when they began to live with their owners ranged from birth to 36 months after birth. All cats were neutered.

In Experiment 3, participants were 29 domestic cats (16 males and 13 females) living with 4 or more other cats. They were kept by three families and one cat café, which had four female owners; of the 29 cats, 9 were from the cat café. Breeds were 21 mongrels, three LaPerms, a Scottish Fold, an American Curl, a LaPerm Shorthair, a Tonkinese, and a Munchkin. Their ages ranged from 1 to 11 years (mean age: 6.48 years, SD = 3.29). The ages when they began to live with their owners ranged from birth to 36 months after birth. All cats were neutered. Of these 29 cats, 26 cats participated in Experiment 2. Interval between Experiment 2 and 3 was at least 2 weeks.

In Experiment 4, participants were 33 domestic cats (14 males and 19 females) living with from 0 to 5 other cats. Of them, 30 cats were kept in 21 families (2 male and 19 female owners) and 3 cats were kept in university laboratories. Of the 33 cats, 24 were mongrels, two LaPerms, two American Shorthair, a Scottish Fold, a Himalayan, a Russian Blue, a Norwegian Forest Cat, and a Bengal. Their ages ranged from 1 to 17 years (mean age: 6.48 years, SD = 4.14), and the ages when they began to live with their owners ranged from one month to 36 months after birth. All cats were neutered, excepting one female. Of these 33 cats, 3 had participated in Experiment 1 and 5 had participated in Experiments 2 and 3. Experiment 4 was conducted about 3 years after Experiment 3. In all experiments, all cats were indoor only except one, and cats were not subjected to food deprivation during the study period. Detailed information is presented in the electronic Supplementary Material (Tables S2–S5).

Apparatus and Stimuli

Before the experiments began, for each cat, five sound stimuli consisting of human voice were recorded. One stimulus consisted of a human calling the cat’s name. The other four stimuli consisted of a human vocalising four different general nouns (Experiments 1, 3, and 4) or four names of other cats living with the test cats (Experiment 2). For Experiments 1, 2, and 3, the stimuli were recorded by the owners of the tested cats. For Experiment 4, the stimuli were recorded by two women unfamiliar to the tested cats. Each owner was instructed to vocalise the cat’s names as he/she normally would; if the owner usually called the cat by a nickname instead of its real name, the nickname was used. In Experiments 1, 3, and 4, four different general Japanese nouns were selected from the list of Matsumoto41; all nouns had the same level of familiarity and were emotionally neutral. The numbers of moras and accents in the nouns were the same as in the cat’s name. Speakers were instructed to vocalise the nouns with the same intonation and manner as they vocalised the cats’ names. In Experiment 2, four of the other cohabiting cats’ names were recorded similarly to the test cats’ names. The orders of presentation of general nouns and cohabiting cats’ names were pseudo-randomized.

We recorded the vocalisations with a handheld digital audio recorder (ZOOM H2 Handy Recorder) in WAV format; the sampling rate was 44100 Hz with 16-bit quantisation. The sound stimuli were adjusted to the same volume level using sound editing software (Adobe Soundbooth CS4 or Adobe Audition CS6). During the experiment, the handheld recorder was used to present the stimuli through a speaker (Sony SRS-Z100), which was hidden from the test cat. The distance between the test cat and the speaker was about 3 m, and the volume of the voices was approximately 65 dB at 3 m from the speaker. A video camera (Sanyo DNX-CA9 or Panasonic HX-WA20) placed in front of the test cats recorded their reactions during the playback of the stimuli.

For Experiment 1, 3, and 4, the discriminant analysis was performed to confirm that there was no implicit difference in acoustic characteristics between noun and name stimuli. Vocal stimuli for cats which showed dishabituation (habituated cats with increasing response magnitude from noun 4 to own name: N = 9, 13, and 13 in Experiment 1, 3, and 4, respectively) were selected for analysis. Six acoustic parameters were extracted from each vocal stimulus by using Praat 6.0.43 software: total duration (sec), mean pitch (Hz), f1 (Hz), f2 (Hz), f3 (Hz), and mean intensity (dB). Then the discriminant analysis was applied with IBM SPSS Statistics 21. Above acoustic parameters were set as independent variables, and type of stimulus (noun or name) was set as a group. As a result of the analysis, high values of Wilks lambda were obtained (Experiment 1, Wilks lambda = 0.930, χ2 = 2.884, df = 6, P = 0.823; Experiment 3, Wilks lambda = 0.821, χ2 = 11.866, df = 6, P = 0.065; Experiment 4, Wilks lambda = 0.979, χ2 = 1.294, df = 6, P = 0.972), indicating that it was difficult to discriminate between noun and name stimuli by using implicit acoustical characteristics as a cue.

Procedure

Experiments 1, 2, and 3 were conducted from December 2012 to November 2013; Experiment 4 was conducted from September 2016 to April 2017. All experiments were held in each owner’s home or in the cat café, wherever the particular cats lived. The experimenter waited until cats were calm before beginning the experiment. During the experiment, the owners were out of their cat’s sight. We used a habituation-dishabituation procedure in which prepared stimuli were played serially with a 15-s inter-stimulus interval (ISI); the order of presentation was word 1, word 2, word 3, word 4, and test cat’s name. The number of habituation stimuli and the ISI were improved versions of those used in a previous study17. Cats’ responses to the stimuli were expected to decrease during the presentation of words 1 through 4 due to habituation; then, if the cats could discriminate their own names from the other words, responses were expected to increase again when their own names were presented, due to dishabituation. The experiment lasted around 1.5 minutes. During presentation, the test cat was not actively isolated from cohabiting cats, to keep the test cat’s behaviour natural. There was no need for any interruption in the experimental sessions due to cohabiting cats’ behaviour.

All procedures related to animal care and experimentation in our research adhered to the ‘Guidelines for the treatment of animals in behavioural research and teaching’ as published by the Association for the Study of Animal Behaviour in Animal Behaviour 71, 245–253 (2006) and to the ethical guidelines of the University of Tokyo. The study was approved by the Animal Experiments Committee of the Graduate School of Arts and Sciences of the University of Tokyo and by the Animal Experiments Committee of Musashino University.

Behavioural analysis

Video-recordings of cats’ responses were trimmed to show from 5 s before stimulus onset to 10 s after stimulus offset, using Adobe Premiere CS6. Vocalisation of the words and cats’ names in the clips was masked by pure tones to facilitate blind evaluation of the clips. In total, 80, 170, 145, and 165 clips were created for Experiments 1, 2, 3, and 4, respectively.

We conducted two kinds of analyses to investigate the cats’ response styles and magnitudes, as in our previous study17. The first analysis describes response style. One of the experimenters (KS) observed the clips of each cat in random order and classified the cat’s responses to the stimuli into five categories: ear moving, head moving, vocalising, tail moving, and displacement; each category is described in Table 1. These categories cover orienting responses (ear moving and head moving)40 and communicative responses (vocalising and tail moving)39. Each category was scored separately as 0 (absent) or 1 (present) for each clip, to determine the proportion of cats showing each response in each presentation trial. Then, the summed score was calculated as the total score for each clip, to enable examination of the correlation between the numbers of categories occurring simultaneously and response magnitude rated by blind raters (described in the next section). To check for reliability, the other experimenter (AS) observed a random selection of one-fourth of the clips and scored the cats’ behaviours. The indices of concordance were 0.75 for ear moving, 0.81 for head moving, 0.99 for vocalising, 0.97 for tail moving, and 0.99 for displacement (κ = 0.76, P < 0.001 for overall observation).

Table 1 Descriptions of categories for behavioural scores. Full size table

The second analysis was conducted to examine response magnitude. Raters who were blind to the stimuli and their presentation order scored each cat’s responses in the clips, which were presented in random order within each test cat. In Experiment 1, there were ten blind raters (6 men and 4 women; mean age = 21.7 years), whereas in Experiment 2 and 3 there were six blind raters (all women; mean age = 27.5 years), and in Experiment 4, nine blind raters (one man and 8 women; mean age = 22.9 years). The raters were instructed to compare each cat’s behaviours before and after the presentation of each stimulus and rate the magnitude of the cat’s responses to the stimuli from 0 (no response) to 3 (marked response). Kendall’s coefficient of concordance showed significant, moderate concordance among the raters (W = 0.73, df = 79, P < 0.001; W = 0.73, df = 169, P < 0.001; W = 0.65, df = 144, P < 0.001, W = 0.55, df = 164, P < 0.001 for Experiments 1, 2 3, and 4, respectively).

Mean response magnitude was calculated for each video clip and used for subsequent analysis. GLMM was applied using the lme4 package version 1.1–13 on R software version 3.4.1. Stimulus category (Experiment 1; noun 4 v. own name, Experiment 2; other cat’s name 4 v. own name, Experiment 3; noun 4 v. own name, Experiment 4; noun 4 v. own name) was set as a fixed effect. Environment (ordinary households v. cat café) and interaction of stimulus category * environment were also set as fixed effects for Experiments 2 and 3. Subjects were set as a random effect. Gaussian distribution with identity link function was specified for lmer function. Then, post-hoc analysis was conducted using the step function in the lmerTest package version 2.0–33; the step function reduced non-significant fixed effects and determined a final model. The random effect (subjects) was manually kept regardless of significance, to control pseudo-replication.