We explored whether rats could discriminate a familiar song from a version that preserved the rhythm but not the melody, and from a version that preserved the melody but not the rhythm. The results suggest that most animals successfully discriminated among the complex auditory sequences by focusing on their underlying rhythmic structure. In the main analysis, we observed no differences between the original excerpt and its constant-pitch version (the isotonic rhythm excerpt). In contrast, we did observe differences between them and the version that disrupted the rhythmic organization of the song. Importantly, we observed some individual differences in the cues on which the animals focused to process the tune. Although the majority of the rats focused on the rhythmic structure of the tune to discriminate among the unfamiliar stimuli, eight out of 40 animals seem to have focused on the melodic organization of the tunes. This suggests that there might be individual differences in the strategies to process complex auditory signals and discriminate musical tunes. Nevertheless, the general tendency we observed is to focus on the rhythmic structure of the song rather than on its melodic organization.
Previous studies reported that rats found it difficult to discriminate between sequential patterns of notes (d’Amato & Salmon, 1984) and a tune from its reversed sequence (Poli & Previde, 1991). In these studies, the rodents seem to have focused on a single pitch of the sequences or on a difference in timbre rather than on their underlying structure to perform their discrimination. In our study, however, the order of the pitches and the timbre were identical between the familiar song and its rhythmically scrambled version. The only difference between these two excerpts was the duration pattern of the tones that composed them. Under these conditions, our rats seem to have focused on the rhythmic structure, rather than on the melodic organization of the song, to discriminate among the excerpts. It is an open question, however, whether the group of rats that did not discriminate the rhythmically scrambled from the familiar stimuli was paying attention to the whole frequency contour or just to a single pitch, as in the Experiment 3 by d’Amato and Salmon (1984). If they were focusing on certain tones, the rats that did not discriminate any stimuli could have been basing their responses only on the pitch C6, present in both unfamiliar stimuli.
Our study suggests that rats are sensitive to the rhythmic structure of a familiar tune. One possibility could be that, to discriminate between the stimuli, rats may have used the metrical organization of the three different durational values that the tones composing the excerpts could take. This may mean that they were sensitive to the temporal changes in the tune regardless of the pitch of the tones. In fact, they did not discriminate the familiar tune from the version in which all notes were transposed to a single tone (the constant-pitch version). This suggests that when the underlying structure of an auditory sequence provides enough information, rodents can compensate for changes in absolute frequency (e.g., Crespo-Bojorque & Toro, 2016). Another possibility could be that rats just focused on the first or the last few durational values of the unfamiliar excerpts to detect whether they matched or mismatched those of the familiar song. This alternative possibility implies that rats can discriminate the temporal organization of few durational values but without requiring the processing of higher metrical structures. Indeed, this is a limitation of this study, because we cannot ascertain whether rats focused on the whole rhythmic structure or just the temporal organization of the few first or last tones. A way to solve this issue would be by testing rats with unfamiliar excerpts that uniquely scramble the beginning, the middle, or the end of the tune.
Despite this limitation, and in contrast with the findings reported in Poli and Previde (1991), our main results indicate that rats’ discriminative behavior might go beyond the surface features of the musical excerpts (such as fundamental frequency or timbre) and is based on the structured rhythm of the tunes (or part of them). This suggests striking abilities in rodents regarding the processing of complex auditory signals, which would be interesting to explore using more naturalistic and biologically meaningful stimuli. Interestingly, we observed that the rodents responded more (produced more nose-poking responses) to the unfamiliar excerpts than to the familiar ones. A similar preference for novel stimuli has also been reported in rats in the domain of taste (Kalat, 1974; Welker and King, 1962) and in the spatial rearrangement of objects (Pisula & Siegel, 2005; Pisula, Stryjek, & Nalecz-Tolak, 2006), as well as in new-born chicks during difficult tasks (Bateson & Jaeckel, 1976; Santolin, Rosa-Salva, Vallortigara, & Regolin, 2016). The animals in our experiment were, therefore, responding to the novel durations of the tones that made up the tune.
The fact that the ability to detect the rhythmic organization of tones in a sequence – be it the whole sequence or part of it – is present in a distantly related mammal suggests that certain musicality features may have evolved independently across species to process the relevant sensory information of their environment. More importantly, it suggests that the cognitive architecture required for the processing of core aspects of music universals (i.e., temporal organization (metrical hierarchies) of few durations constrained to two to three subdivisions of the beat; Savage et al., 2015) might have deep biological roots shared across species. In fact, studies with avian species suggest strong rhythmic processing abilities probably linked to their natural vocalizations (Slabbekoorn & ten Cate, 1999; Hoeschele, Merchant, Kikuchi, Hattori, & ten Cate, 2015). Beyond the temporal organization of rhythms that we studied, other universal properties of music might be found in the animal kingdom (e.g., Fitch, 2006; Hauser & McDermott, 2003; Honing, ten Cate, Peretz, & Trehub, 2015), such as pitch identification or melodic organization, which opens the door to a better understanding of the phylogenetic origins of music.