Development of infant baboons' responses to graded bark variants

We studied the development of infant baboons' (Papio cynocephalus ursinus) responses to conspecific 'barks' in a free-ranging population in the Okavan go Delta, Botswana. These barks grade from tonal, harmonically rich calls i nto calls with a more noisy harsh structure. Typically: tonal variants are given when the signaller is at risk of losing contact with the group or a p articular individual ('contact barks'), whereas harsh variants are given in response to predators ('alarm barks'). We conducted focal observations and playback experiments in which we presented variants of barks recorded from resident adult females. By six months of age, infants reliably discriminat ed between typical alarm and contact barks and they responded more strongly to intermediate alarm calls than to typical contact balks. Infants of six months and older also recognized their mothers by voice. The ability to dis criminate between different call variants developed with increasing age. At two and a half months of age, infants failed to respond at all, whereas as at four months they responded irrespective of the call type that was prese nted. At six months, infants showed adult-like responses by responding stro ngly to alarm barks but ignoring contact barks. We concluded that infants g radually learn to attach the appropriate meaning to alarm and contact barks .