Flexible use of contact calls in a species with high fission-fusion dynamics

Abstract

The 'social complexity hypothesis' posits that complex social systems (which entail high uncertainty) require complex communicative systems (with high vocal flexibility). In species with fission-fusion dynamics, where the fluid composition of temporary subgroups increases the uncertainty with which group members must manage their social relationships, vocal communication must be particularly flexible. This study assessed whether contact call rates vary with caller and audience characteristics in free-living spider monkeys, as well as with fission and fusion events. Adult females and immature individuals called more when in small audience settings, while audience size did not influence adult males. Adults called more when in the presence of the opposite sex, whereas immatures vocalized more in subgroups composed only by females. Females also called more when with their mature sons. We found higher call rates in periods during which fission and fusion events took place than in periods with more stable compositions and when the composition after a fission or fusion event changed from one sex to two sexes. A flexible use of contact calls allows individuals to identify themselves when they join others, particularly if they are members of the opposite sex. This socio-spatial cohesion function reduces the uncertainty about subgroup composition. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

Keywords: New World monkeys; audience effect; contact calls; social uncertainty; vocal flexibility.

Figure 1.

At the top of the figure, we find the effects plots showing the expected influence of audience size on the individual call rates of (a) mature females and (b) males. The lines represent predicted means derived from the generalized linear mixed model. The grey area represents the 95% confidence interval and the rug plot at the bottom of the graphs shows the location of the audience size values. At the bottom of the figure, we find the effects plots showing the expected influence of social audience individual call rates of (c) mature females and (d) males calling. The points are the fitted values of each category of social audience and their standard errors which are based on the covariance matrix of the estimated regression coefficients.

Figure 2.

Effects plot showing the expected influence of mature son presence or absence on the call rate by mature female spider monkeys. The points are the fitted values of each category (the presence or absence) and their standard errors which are based on the covariance matrix of the estimated regression coefficients.

Figure 3.

Plots showing the effects of social events and changes in audience composition on mean call rate. Error bars show the standard error. Letters indicate the results of the post hoc test, where groups with the same letters indicate non-significant differences.