Can monkeys spontaneously synchronize to audio?

Setup for the three experiments (from discussed publication).

It was recently shown that rhythmic entrainment, long considered a human-specific mechanism, can be demonstrated in a select group of bird species, and, somewhat surprisingly, not in more closely related species such as nonhuman primates. This observation supports the vocal learning hypothesis that suggests that rhythmic entrainment is a by-product of the vocal learning mechanisms that are shared by several bird and mammal species, including humans, but that are only weakly developed, or missing entirely, in nonhuman primates. However, since no evidence of rhythmic entrainment was found in many vocal learners (including dolphins, seals, and songbirds), vocal learning may be necessary, but not sufficient for beat induction – the cognitive mechanism that supports the perception of a regular pulse from a varying rhythm (Honing et al., 2012).

Today a new study appeared in Nature Scientific Reports claiming to show rhythmic entrainment (or spontaneous synchronization as the authors refer to it) in the Japanese macaque (Macaca Fuscata). Intriguing! However, reading the paper it becomes clear quickly that the results might not be what they seemed at first sight.

[link to video for non-Flash supporting devices]

First, as was shown in several earlier studies, macaques can synchronize to an auditory metronome, but they tend to do this in reaction, and not in anticipation of the sound. They do not show the typical negative synchronization error: tapping or pressing a button slightly earlier than the actual sound, a sign that an anticipatory process (i.e. expectation) plays a role.

Second, it is unclear whether the experiments are evidence for rhythmic entrainment: it could well be imitative behavior. This hypothesis is actually confirmed by the third experiment in which the monkeys were asked to synchronize with a virtual monkey (see panel C above) of which the auditory and visual information was presented independently as well as combined. The monkeys performed better for the visual condition as opposed to the auditory condition. In contrast, in humans it is the opposite: rhythmic entrainment is much stronger in the auditory modality.

Lastly, the researchers only analyzed asynchronies between the button presses of the two monkeys sitting opposite to each other (see Panel B above). Therefore the results could well be simply support for an imitative, cq. reactive behavior instead of evidence for a periodic anticipatory reaction that is common to human rhythmic entrainment.

Nagasaka, Y., Chao, Z., Hasegawa, N., Notoya, T., & Fujii, N. (2013). Spontaneous synchronization of arm motion between Japanese macaques Scientific Reports, 3 DOI: 10.1038/srep01151

Honing, H., Merchant, H., Háden, G., Prado, L., & Bartolo, R. (2012). Rhesus Monkeys (Macaca mulatta) Detect Rhythmic Groups in Music, but Not the Beat PLoS ONE, 7 (12) DOI: 10.1371/journal.pone.0051369