The ‘vocal learning-beat perception and synchronization’ (VL-BPS) hypothesis states that only vocal learning species—those capable of producing new vocalizations or modifying existing ones based on auditory experience—may possess advanced rhythmic abilities [28 ,29 (link)]. This hypothesis is inherently cross-modal: it suggests a strong link between audition and timed movement. For example, Snowball, a sulfur-crested cockatoo (Cacatua galerita eleonora), was shown to perceive auditory rhythms at different tempi and to predictively synchronize his body movements to them [30 (link)]. Parrots are phylogenetically distant from humans and, among mammals, pinnipeds (seals, sea lions and walruses) are one of the vocal learning groups (besides humans, bats, elephants and cetaceans). Pinnipeds may well be the best mammalian model for testing the VL-BPS hypothesis—the ability to extract a beat from periodic acoustic stimuli and entrain to it in a predictive and adaptive manner—since some species show vocal mimicry and plasticity [31 (link),32 (link)] and others can keep a beat [33 (link)]. These characteristics, parallelling human abilities, make pinnipeds an ideal animal clade for comparative research on the origins of rhythmic communicative behaviour.
Harbour seals exhibit both vocal flexibility [32 (link),34 (link)] and rhythmic interactivity [20 (link)], and are particularly vocal in the first few weeks of life [35 (link)]. During the lactation period, harbour seal pups emit ‘mother attraction calls' (hereafter ‘calls’) to draw their mothers' attention [36 (link)]. Mothers are silent and use the individual vocal signatures in these calls to recognize their pups [35 (link),37 (link)]. Against the acoustically complex backdrop of large mother–pup rookeries, rhythmically tuned pup calls could constitute a socio-ecologically selected trait that allows individual pups to avoid conspecific call overlap by adjusting the timing of their own call onsets. Such timing plasticity could allow a pup to be more acoustically conspicuous and increase its chances of successful reunions with its mother. Unlike cooperative types of turn-taking (e.g. in humans and in common marmosets [38 (link)]) harbour seal pups’ interactions are a by-product of neighbouring pups vocalizing to attract their silent mothers and are thus probably competitive.
To date, only two papers studied vocal rhythms in harbour seals, crucially both focusing on single individuals [20 (link),27 (link)]. The first study was a playback experiment in which a pup vocally interacted with sounds broadcasted from a loudspeaker [20 (link)]. The pup adjusted the timing of its calls in an asynchronous manner by responding to the broadcasted conspecific calls with a non-uniformly distributed response phase whose mean approximated 90° [20 (link)]. The second study looked at the presence and development of vocal rhythms in three harbour seal pups [27 (link)]. Complementary analytical approaches showed how the pups' individual calling patterns gained more rhythmic structure over time [27 (link)]. However, a major limitation of both studies was the lack of sociality (i.e. individuals were tested alone) and, by extension, interactivity (i.e. the stimuli did not adapt to the response of the tested animals).
Harbour seals exhibit both vocal flexibility [32 (link),34 (link)] and rhythmic interactivity [20 (link)], and are particularly vocal in the first few weeks of life [35 (link)]. During the lactation period, harbour seal pups emit ‘mother attraction calls' (hereafter ‘calls’) to draw their mothers' attention [36 (link)]. Mothers are silent and use the individual vocal signatures in these calls to recognize their pups [35 (link),37 (link)]. Against the acoustically complex backdrop of large mother–pup rookeries, rhythmically tuned pup calls could constitute a socio-ecologically selected trait that allows individual pups to avoid conspecific call overlap by adjusting the timing of their own call onsets. Such timing plasticity could allow a pup to be more acoustically conspicuous and increase its chances of successful reunions with its mother. Unlike cooperative types of turn-taking (e.g. in humans and in common marmosets [38 (link)]) harbour seal pups’ interactions are a by-product of neighbouring pups vocalizing to attract their silent mothers and are thus probably competitive.
To date, only two papers studied vocal rhythms in harbour seals, crucially both focusing on single individuals [20 (link),27 (link)]. The first study was a playback experiment in which a pup vocally interacted with sounds broadcasted from a loudspeaker [20 (link)]. The pup adjusted the timing of its calls in an asynchronous manner by responding to the broadcasted conspecific calls with a non-uniformly distributed response phase whose mean approximated 90° [20 (link)]. The second study looked at the presence and development of vocal rhythms in three harbour seal pups [27 (link)]. Complementary analytical approaches showed how the pups' individual calling patterns gained more rhythmic structure over time [27 (link)]. However, a major limitation of both studies was the lack of sociality (i.e. individuals were tested alone) and, by extension, interactivity (i.e. the stimuli did not adapt to the response of the tested animals).
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