DA describes gestural communication as 'extremely eloquent' [p. 271] and such behaviour is well established in other mammals beside primates, for example cats and dogs. To attain this eloquence, gestural communication requires good visual acuity. In fact, gestures often rely on colour perception to enable individuals observing the gesture to respond accurately. For example, DA describes how De Brazza's guenons draw attention to their white-bearded dewlaps by shaking their heads when signalling aggression [p. 271], and dominant male vervet monkeys subdue subordinate males by flaunting their blue scrotums and red penises [p. 270]. The ability to use facial expressions has increased the complexity of social interactions between primates; therefore the brains of anthropoid primates also require a substantial amount of processing power to interpret gestures correctly.
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