Abstract
Measuring the affective state of an individual across species with comparable non-invasive methods is a current challenge in animal communication research. This study aims to explore to which extent affect intensity is conveyed in the vocal behaviours of three nonhuman primate species (Campbell's monkeys, De Brazza's monkeys, red-capped mangabeys), which vary in body size, ecological niche and social system. Similarly in the three species, we experimentally induced a change in captive social groups' affect by locking all group members together in their outside enclosure. The two experimental conditions which varied in affect intensity consisted in imposing a pre-reunion 90 mn-separation by splitting up the respective group into two subgroups (High affect condition) or not (Low affect condition). We measured call rates as well as voice features at the time of reunion in both conditions. The three studied species reacted in a very similar way. Across species, call rates changed significantly between the behaviourally defined states. Furthermore, contact call duration and, to some extent, voice pitch increased. Our results suggest, for the first time in arboreal Old World monkeys, that affect intensity is conveyed reliably in vocal behaviour and specific acoustic characteristics of voice, irrespective of body size and ecological niche differences between species. Cross-taxa similarities in acoustic cues of affect intensity point to phylogenetic constraints and inheritance from a common ancestor, whereas variations in vocal behaviour and affect intensity-related acoustic cues between species may be an adaptation to specific social requirements and depend on social systems. Our findings as well as a comparison with published works on acoustic communication in other vertebrate groups support the hypothesis that affect intensity in human voice originates from precursors already found deep inside the vertebrate phylogeny.
Highlights
Language and music are multifaceted acoustic communication phenomena conveying both linguistic and paralinguistic properties, such as emotions [1,2]
Influence of affective state on contact call features The acoustic structure of the contact calls emitted just after the observation started differed between the Low and the High affect intensity conditions
Differences between mangabeys and guenon species were observed with regards to the latency needed to come back to Low affect intensity condition
Summary
Language and music are multifaceted acoustic communication phenomena conveying both linguistic and paralinguistic properties, such as emotions [1,2]. The fact that shared acoustic cues in affective prosody of nonverbal acoustic expressions, speech and music code for the respective quality and intensity of an emotion across human cultures [2,8,9,10,11,12,13,14,15,16] provides support for the hypothesis that specific components of affective prosody in humans may have derived from a prehuman basis (‘‘prehuman origin hypothesis of affective prosody’’ [8]). To our knowledge, studies on nonhuman primates covered solely four species, ranging from nocturnal and arboreal prosimians (mouse lemurs: [23]) to diurnal and arboreal New World monkeys (e.g. squirrel monkeys: [24]) and to terrestrial Old World monkeys (e.g. macaques: [25], baboons: [26])
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