Effects of Rearing on the Behaviour of Zoo-Housed Chimpanzees (Pan troglodytes)

Early-life experiences may considerably affect the behavioural patterns of adult primates. Particularly, atypical rearing practices might lead to abnormal behaviours and social-sexual deficiencies in captive, adult non-human primates. We conducted behavioural observations of mother-reared (<i>n</i> = 5) and hand-reared (<i>n</i> = 6) adult chimpanzees in a social group at Parco Natura Viva, Italy. We used continuous focal animal sampling to collect behavioural data focusing on individual and social behaviours. We found that all study subjects performed individual and social species-specific behaviours. However, mother-reared chimpanzees performed locomotion and affiliative behaviours significantly more than hand-reared subjects. In addition to these species-typical behaviours, hand-reared chimpanzees showed significantly more abnormal behaviours than mother-reared subjects. Therefore, these findings suggest that hand-rearing could have wide-reaching effects on the behavioural repertoire in adult zoo-housed chimpanzees. Hence, even if sometimes human intervention in rearing may be necessary to ensure the survival of captive infant chimpanzees, our results suggest that zoo-housed chimpanzees might benefit from minimised human-animal interactions and exposure to conspecifics throughout their development. These suggestions should be implemented in regular husbandry practices.

Flexibility in wild infant chimpanzee vocal behavior

How did human language evolve from earlier forms of communication? One way to address this question is to compare prelinguistic human vocal behavior with nonhuman primate calls. An important finding has been that, prior to speech and from early on, human infant vocal behavior exhibits functional flexibility, or the capacity to produce sounds that are not tied to one specific function. This is reflected in human infants’ use of single categories of protophones (precursors of speech sounds) in various affective circumstances, such that a given call type can occur in and express positive, neutral, or negative affective states, depending on the occasion. Nonhuman primate vocal behavior, in contrast, is seen as comparably inflexible, with different call types tied to specific functions and sometimes to specific affective states (e.g. screams mostly occur in negative circumstances). As a first step toward addressing this claim, we examined the vocal behavior of six wild infant chimpanzees during their first year of life. We found that the most common vocal signal, grunts, occurred in a range of contexts that were deemed positive, neutral, and negative. Using automated feature extraction and supervised learning algorithms, we also found acoustic variants of grunts produced in the affective contexts, suggesting gradation within this vocal category. In contrast, the second most common call type of infant chimpanzees, the whimpers, was produced in only one affective context, in line with standard models of nonhuman primate vocal behavior. Insofar as our affective categorization reflects infants’ true affective state, our results suggest that the most common chimpanzee vocalization, the grunt is not affectively bound. Affective decoupling is a prerequisite for chimpanzee grunts (and other vocal categories) to be deemed ‘functionally flexible’. If later confirmed to be a functionally flexible vocal type, this would indicate that the evolution of this foundational vocal capability occurred before the split between the Homo and Pan lineages.

Early vocal production and functional flexibility in wild infant chimpanzees

ABSTRACTHow did human language evolve from earlier forms of communication? One way to address this question is to compare prelinguistic human vocal behavior with nonhuman primate calls. Here, an important finding has been that, prior to speech, human infant vocal behavior exhibits functional flexibility, the capacity of producing protophones that are not tied to one specific function. Nonhuman primate vocal behavior, by contrast, is comparably inflexible, with different call types tied to specific functions. Our research challenges the generality of this claim, with new findings of flexible vocal behavior in infant chimpanzees. We used artificial intelligence consisting of automated feature extraction and supervised learning algorithms to analyze grunt and whimper vocalizations from free-ranging infants during their first year of life. We found that grunt production was highly flexible occurring in positive, neutral and negative circumstances, as already shown in human infants. We also found acoustic variants of grunts produced in different affective contexts, suggesting gradation within this vocal category. By contrast, the second most common call type of infant chimpanzees, the whimpers, was produced in only one affective context in line with standard models of nonhuman primate vocal behavior. We concluded that the most common chimpanzee vocalization, the grunt, qualifies as functionally flexible, suggesting that evolution of vocal functional flexibility occurred before the split between the Homo and Pan lineages.