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A schematic demonstrating the variety of brain sizes in the primate order. To date, marmosets and humans are the only two primate species demonstrated to vocally cooperate via an extended series of vocalizations with any other conspecific. (Photographs used under Creative Commons license; photographers as follows: marmoset by Simon Harris; chimpanzee by S.B.J.; rhesus macaque by Stacey Osburn; capuchin by Dave Spangenburg; squirrel monkey by Tracy Lynn.)
