The development of a greeting signal in wild chimpanzees

Abstract

Adult chimpanzees produce a unique vocal signal, the pant-grunt, when encountering higher-ranking group members. The behaviour is typically directed to a specific receiver and has thus been interpreted as a 'greeting' signal. The alpha male obtains a large share of these calls, followed by the other adult males of the group. In this study, we describe the development of pantgrunting behaviour from the first grunt-like calls of newborn babies to the fully developed pant-grunts in adults. Although babies produce grunts from very early on, they are not directed to others until about 2 months of age. Subsequently, socially directed grunting steadily increases in frequency to peak around 7 months of age, but then decreases again to reach a nadir in older infants and juveniles, while the specificity in use increases. During adolescence, grunt production increases again with grunts given most frequently to socially relevant individuals. As young chimpanzees are closely affiliated to their mothers for the first decade of their lives, we also compared the grunting patterns of mothers and their offspring, which revealed some influences in pant-grunt production. In conclusion, the acquisition of pant-grunting behaviour in chimpanzees is a long-lasting process with distinct developmental phases in which social influences by the mother and other group members are likely to play a role.

Functional flexibility of infant vocalization and the emergence of language

“…Usually primatologists assess function by reasoning from contexts of use. Nonhuman primates often use a particular call in substantially varying contexts (37,38), but such variation still may be consistent with "a generalized function that transcends the different contexts" (58, p. 185). A key goal is to determine the generalized function or functions that each call may serve, both in terms of illocution and perlocution.…”

“…For example, across development from infancy, chimpanzee grunts have been shown to occur in increasingly variable "contexts" that may suggest variable functions (36,37). Additional sounds in chimpanzees and other primates later in life also have been shown to be used in a variety of contexts (38).…”

Wild Chimpanzees Show a Decrease in Pant Grunting over Their First 6 Years of Life

“…Our fourth inclusion criterion, in which faces must be visible, ensured inclusion of instances in which young infants' voices were so quiet that, while the greeting was obvious from the shape of the infant's mouth, it was not clearly audible from the researcher's vantage point. Laporte and Zuberbuhler [2011] note too, that, due to the effort required to produce an audible pant grunt, infants may sometimes be seen to form the greeting with their mouth even in the absence of an audible sound. Relatively few infant pant grunts were recorded in this way (i.e., seen but not heard); their inclusion demonstrates our focus on the development of attempt/motivation and not specifically skill in execution.…”

“…It would be useful to distinguish among simple mimicry, some kind of arousal-based contagion, and a "true" pant grunting based on recognition of a newly arrived other's higher status. Available data make the first two difficult to distinguish [though mother-infant concordance rates are useful, see also Laporte and Zuberbuhler, 2011], but for the third it is possible that infant positional information could yield insight.…”

“…Previous work [Laporte and Zuberbuhler, 2011] investigated pant grunt use from early infancy through adulthood, presenting initial evidence for a possible Ushaped developmental curve. Utilizing a mixed longitudinal/cross-sectional design, it was reported that the call was used at higher rates in early infancy, declined in late infancy and juvenility, and rose again in adolescence.…”