This is a guest post by Margaret Wilson.
Turn-taking is fundamental to human conversation, so the question of whether it occurs in other social animals is extremely interesting. A new paper on turn-taking in marmoset monkeys (Takahashi et al., "Coupled Oscillator Dynamics of Vocal Turn-Taking in Monkeys", Current Biology, 2013) is to be applauded for tackling this issue.
Unfortunately, though, it is not clear that their data demonstrate turn-taking in any sophisticated sense: specifically (and this is the sense embraced by the authors), entrainment of timing mechanisms between two individuals to regulate the passing of the turn. They begin by asking, "Is this a simple call-and-response (‘‘antiphonal’’) behavior seen in numerous species, or is it a sustained temporal coordination of vocal exchanges as in human conversation?" They conclude that they have shown the latter, but, on my reading, all their data is compatible with simple call-and-response. What seems to be going on is that the authors have failed to appreciate just how weird human turn-taking is.
When humans take turns, there is a cyclic structure to the extremely short gaps between speakers' utterances (Sacks, Schegloff, & Jefferson, 1974; Wilson & Wilson, 2005; Wilson & Zimmerman, 1986). A between-turn gap of, say, 200 milliseconds is more likely to be broken by the second speaker at certain regular intervals (say, odd multiples of 50 ms) than during the "troughs" between those intervals. That is, short silences are not of arbitrary length, but reflect a cyclic passing back and forth of who has the "right" to speak next (Wilson & Zimmerman, 1986). The troughs represent moments when the right to speak has shifted back to the original speaker, hence the second speaker inhibits speech during those fractions of a second. And this is happening at the order of tens of milliseconds. This "structured silence" can only be explained by extremely tight coupling — entrainment — of some oscillatory mechanism in the brains of the two speakers. (For further research on this framework, see O'Dell, Neiminen & Lennes, 2012; Stivers et al., 2009).
In contrast, Takahashi et al. analyzed marmoset exchanges of up to 60 seconds, and found a cyclic pattern of about 12 seconds. This is completely different, because it includes the actual calls of Marmoset 1, and then Marmoset 2, and then Marmoset 1 again, etc. In other words, there is an actual signal being exchanged. Cyclicity in this case simply means that the timing has some consistency to it.
Furthermore, twelve seconds (six seconds per marmoset) is more than enough time to perceive an unanticipated stimulus and generate a response to it. Neither marmoset need be "tracking" the other marmoset's timing (which is what we mean by entrainment). Instead they could be simply reacting. This IS the call-and-response theory. The authors claim that their data are identical to the human case, except for the difference in time-scale. But that difference in time-scale is all.
With this core issue in mind, let's look in more detail at the findings. Takahashi et al. make their case for an entrainment model of marmoset turn-taking in three stages:
1) They disproved strict versions of two alternative hypotheses. The "reset" hypothesis holds that each monkey maintains a fixed interval between its own calls, and this interval is simply reset to zero by the intrusion of a call from another animal. The "inhibition" hypothesis holds that each monkey inflexibly preplans a series of calls (a minimum of three), and that a call by another animal suppresses one call in this series without altering the timing of the subsequent calls. The authors show that neither of these hypotheses fit the data. This seems fine as far as it goes, but these are very narrow hypotheses, and eliminating them doesn't force us to a human-like turn-taking account.
2) They showed counter-phased cyclicity of marmoset exchanges, as discussed above.
3) They claim to show that the marmosets are responsive to each others' timing, in the sense that each is faster or slower if the other is faster or slower. Unfortunately, it is not clear that they've shown this. Consider three calls uttered alternately by marmoset 1 and marmoset 2 (their Figure 4A), labeled M1a, M2, and M1b (p. 6). The thing to do would be to look for a correlation between the inter-call interval M1a-M2 and the inter-call interval M2-M1b.* That is, when Marmoset 2 jumps in quickly, does Marmoset 1 then jump back in quickly?
Instead, the authors looked for a correlation between M1a-M2 (marmoset 2's other-self interval), and M1a-M1b (marmoset 1's self-self interval). (The latter is normed to marmoset 1's self-self interval when alone. This is because they are basing their calculations on the phase response curve, which is used to model how an oscillator responds to a perturbing signal.) But notice that M1a-M1b contains within it the duration of M1a-M2, and will be partially determined by it, if marmoset 1 is in any sense at all responding to marmoset 2's call. The only way for there not to be a correlation between these two measures is if marmoset 1 proceeds with its own call timing (M1a-M1b), impervious to marmoset 2's call. This is a straw man, not reflective of any of the hypotheses under consideration. In short, while the authors have chosen a legitimate statistical technique, it is not clear that they have chosen the correct one for their question.
To see that the question of mutual speeding-up or slowing-down is not captured by this analysis, note that a positive correlation would be found if marmoset 1 simply waits a fixed 6 seconds after M2, regardless of the timing of M2. Marmoset 1 would not be speeding up or slowing down according to whether marmoset 2 speeds up or slows down (except in a degenerate sense in which, when marmoset 2 speeds up its response to marmoset 1, then marmoset 1's overall cycle is thereby necessarily shortened — that is, when "speed" is defined differently for marmoset 1 and marmoset 2).
To return to the big picture, none of the data presented here make a convincing case for anything other than the call-and-response hypothesis. Meaningful turn-taking might still be demonstrated with other statistical techniques, either in marmosets or in other non-human animals. And the efforts of these authors are certainly not wasted, in that they have begun to map the territory of how non-human call exchange occurs. But the conclusion that marmoset call exchange mirrors human turn-taking is simply not warranted.
[Above is a guest post by Margaret Wilson]