Requesting gestures in captive monkeys and apes: Conditioned responses or referential behaviours?

Captive monkeys and apes almost inevitably develop gestures to request food and objects from humans. One possibility is that these gestures are just conditioned responses without any understanding of the socio-cognitive causality underlying their efficacy. A second possibility is that they do involve some understanding of how they are (or fail to be) effective upon the behaviour of others. Observational evidence suggest that most apes and some monkeys coordinate their request gestures with joint attention behaviours — a criterion for early referential communication in human infants. However, experimental evidence about apes and monkeys‘ understanding of the causal role of joint attention in gestural communication is equivocal, with test pass and failure patterns that can be due to cognitive and/or motivational factors. Current evidence suggests that the gestures of apes and monkeys can neither be dismissed as simple conditioned responses nor be uncritically accepted as fully equivalent to human gestures.