The Function and Acoustic Characteristics of ‘Egg’ Calls in Common Marmosets (Callithrix jacchus)

Like other species of primates, common marmosets (Callithrix jacchus) produce characteristic vocalizations when they confront threatening situations. For example, when intensely mobbing a predator, marmosets produced long series of “tsik” calls followed by or intermingled with “egg” calls, a calling pattern that may reflect a shift in motivational state from mobbing to vigilance. The egg call does not appear to serve the same recruitment function as tsik calls, but this has not been tested. I tested the general hypothesis that egg calls are emitted in contexts that are mildly threatening, and that they do not serve a recruitment function. I also hypothesized that the egg call carries vocal signatures across individuals. I tested these hypotheses in four separate experiments and through acoustic analyses. In accordance with my predictions, descriptive statistics showed that at least one tsik call preceded egg calls in mobbing bouts. This is consistent with the interpretation that egg calls reflect a perception of decreased threat relative that associated with tsik calls. Trends in the data provide some evidence that marmosets took longer to switch from tsik calls to egg calls when confronted with a predator alone than when group-mates were present. Likewise, non-significant trends revealed that marmosets stay in closer proximity to the speaker during playbacks of tsik calls than egg calls. These two results argue in favor of the conclusion that egg calls are less about recruitment than they are about announcement. There was not a significant difference between production of egg calls when the marmosets were presented with novel versus familiar objects, a result that poses the possibility that egg calls reflect both positive and negative arousal. Finally, as predicted, discriminant function analysis showed that egg calls are distinguishable by individual. Knowing the identity of the individual emitting an egg call would be beneficial if the call is used for monitoring the whereabouts of group-mates. Overall, the data provide tentative evidence that: 1) egg calls serve a different function than tsik calls; 2) variability within and across individuals in rates of egg calling is a reflection of both context and degree of arousal; and 3) individual differences in egg calls contain acoustic information that may be used to inform group-mates about identity of the caller.