The howler monkey (Alouatta species) is characterised by the production of howl vocalisations. These howls are suggested to function in territorial demarcation, social mating and resource defence. Preliminary observations of captive housed black howler monkeys (Alouatta caraya) found that the majority of groups do not perform howl vocalisations. To stimulate vocalisations, playback experiments were conducted on 11 groups of captive A. caraya, five pairs and six family groups, in UK zoos. Four acoustic stimuli were tested; howls of other A. caraya, howls of a non-sympatric howler species (A. palliata), the roars of a different primate species, the red ruffed lemur (Varecia rubra), and the low frequency sound of a chainsaw. The lemur calls and chainsaw sounds acted as controls. All four sounds were played on a random schedule, each morning, for six consecutive days for each study group. Responses of the adult male and female in each group were classified as orientation to the speaker location, physical approach towards the speaker and whether a vocal response was performed. A significant effect of sex was found, with males responding more to all stimuli than females. Behavioural responses to all acoustic stimuli were significantly higher when there was more than one howler monkey group housed in the zoo. This effect was most pronounced in female subjects. There were significantly more responses towards the playback of conspecific calls and responses increased with subsequent playback days. This study provides evidence that playback can elicit the performance of natural calls, providing an environment that may promote the welfare of this captive housed primate species.
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