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The howler monkeys are notorious for the low pitched but high volume howls emitted in particular circumstances. The mechanism for achieving such high voice intensities is still little understood. The present study is based on the dissection of the hyolaryngeal apparatus in 6 Alouatta seniculus, supplemented by the observation of these organs in whole block serial sections (30–40 µm) obtained from 3 A. seniculus and 6 A. caraya.

Morphological adaptations to climbing (a scansorial mode of quadrupedal, arboreal locomotion practised on twigs and small branches) are identified by relating anatomical details of limb bones to a sample of 6,136 instantaneous observational recordings on the positional behavior and support uses of 20 different free-ranging, adult red howlers. Our findings are used to infer the original habitat in which proto-red howlers may have acquired such adaptations and to hypothesize that climbing and its related anatomy are a primitive condition for anthropoids.

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