Can spider monkeys (Ateles geoffroyi) discriminate vocalizations of familiar individuals and strangers?

In a field experiment, tape-recorded vocalizations of spider monkeys (Ateles geoffroyi) were played back to investigate whether individuals were able to discriminate between group members and strangers. Monkeys responded remarkably similarly in the two cases, with no significant difference found between the numbers of calls given by an individual, or the types of call given. However, a group was more likely to give some vocal reaction when hearing a stranger's call than when hearing one from an individual of their own community. Further, the only instances in which agonistic territorial behaviours occurred were in reaction to strangers' playbacks. No significant effects on the response given were produced by the sex of the caller, the location and time of day of the broadcast, the size of the subgroup hearing the call or the activity in which they were involved. These results are discussed with respect to acoustic, social and ecological factors that may lead to the apparent lack of vocal discrimination of strangers within the community range.