Supplementary Data from Flexible use of contact calls in a species with high fission–fusion dynamics

The ‘social complexity hypothesis' posits that complex social systems (that 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’.

社会复杂性假说（social complexity hypothesis）提出，复杂的社会系统（伴随高度不确定性）需要具备高发声灵活性的复杂通讯系统。在具有裂变-融合动态的物种中，临时亚群的组成会不断变化，这增加了群体成员处理社会关系时的不确定性，因此其发声通讯需要具备尤其突出的灵活性。本研究针对野生蜘蛛猴，探究了联络鸣叫率是否会随发声个体与听众的特征、以及裂变与融合事件发生变化。成年雌性与未成年个体在听众规模较小时的鸣叫频次更高，而听众规模对成年雄性并无显著影响。当群体中有异性个体存在时，成年个体的鸣叫频次更高；而未成年个体仅在全雌性亚群中的鸣叫频次更多。雌性个体在与其成年儿子共处时，鸣叫频次也会升高。我们发现，在发生裂变与融合事件的时期，相较于群体组成更为稳定的时期，以及在裂变或融合事件后群体组成从单一性别变为两性混合的时期，蜘蛛猴的联络鸣叫率均更高。灵活运用联络鸣叫能够帮助个体在加入其他群体时完成身份识别，尤其是当对方为异性成员时。这一社会空间联结功能，能够降低亚群组成相关的不确定性。本文隶属于‘灵长类的认知、通讯与社会联结’专题栏目。