Development in the Usage and Comprehension of Greeting Calls in a Free-Ranging Group of Japanese Macaques (Macaca fuscata)

Greetings among female chimpanzees in Mahale, Tanzania

Greeting behaviors have been reported in several primate species, although their forms and context may vary across species. Chimpanzees (Pan troglodytes) greet each other in various ways when they meet; however, many recent studies of greetings in chimpanzees have mostly focused on pant grunt vocalizations, which are often viewed as equivalent to submissive signals. As most greetings in chimpanzees are directed toward adult males, either from other males or females, relatively few studies have focused on female-female greetings. Thus, the primary aim of this study was to describe the greetings (not limited to pant grunts) between chimpanzee females in the Mahale Mountains National Park based on long-term observational data. I observed 405 female-female greeting events (10.9 instances per 100 observation hours [obsn. h]) between 1994 and 2018, of which 242 were pant grunts (6.5 instances per 100 obsn. h); 42.3% of greetings were nonaudible, such as tactile or gestural greetings. Most pant grunts were directed toward older females; females under 20 years of age were generally responsible for this trend, as they were the most frequent greeters among females and tended to perform pant grunts toward older females. Nonetheless, among females 20 years of age or older, pant grunts from an older to a younger female were not rare (37%). Compared to previous studies in Mahale, pant grunts between females were an order of magnitude less than those directed toward males. There may also be a large difference in the frequencies of female-female pant grunts across study sites, which may be attributed to differences in female gregariousness.

The function and evolution of child-directed communication

Humans communicate with small children in unusual and highly conspicuous ways (child-directed communication (CDC)), which enhance social bonding and facilitate language acquisition. CDC-like inputs are also reported for some vocally learning animals, suggesting similar functions in facilitating communicative competence. However, adult great apes, our closest living relatives, rarely signal to their infants, implicating communication surrounding the infant as the main input for infant great apes and early humans. Given cross-cultural variation in the amount and structure of CDC, we suggest that child-surrounding communication (CSC) provides essential compensatory input when CDC is less prevalent-a paramount topic for future studies.

Temporal adjustment of short calls according to a partner during vocal turn-taking in Japanese macaques

Turn-taking is a common feature in human speech, and is also seen in the communication of other primate species. However, evidence of turn-taking in vocal exchanges within a short time frame is still scarce in nonhuman primates. This study investigated whether dynamic adjustment during turn-taking in short calls exists in Japanese macaques Macaca fuscata. We observed exchanges of short calls such as grunts, girneys, and short, low coos during social interactions in a free-ranging group of Japanese macaques. We found that the median gap between the turns of two callers was 250 ms. Call intervals varied among individuals, suggesting that call intervals were not fixed among individuals. Solo call intervals were shorter than call intervals interrupted by responses from partners (i.e., exchanges) and longer than those between the partner's reply and the reply to that call, indicating that the monkeys did not just repeat calls at certain intervals irrespective of the social situation. The differences in call intervals during exchanged and solo call sequences were explained by the response interval of the partner, suggesting an adjustment of call timing according to the tempo of the partner's call utterance. These findings suggest that monkeys display dynamic temporal adjustment in a short time window, which is comparable with turn-taking in human speech.

Vocal greeting during mother-infant reunions in a nocturnal primate, the gray mouse lemur (Microcebus murinus)

In human societies, ritualized greeting behavior includes gestural and vocal displays to signal the social acceptance of an encountering person. These displays are universal across cultures suggesting a pre-human origin. Vocal greeting displays are only reported for monkeys and apes with complex social systems, but none of these studies confirmed that greeting signals fulfill all criteria characterizing human greeting behavior. In this study, we analyzed for the first time whether vocal exchanges between mother and infants in a non-human primate fulfill the criteria of human greeting behavior and whether vocal greeting behavior is present in a basal primate with a less complex social system, the gray mouse lemur. By comparing spontaneous leave-takings and reunions, we found that vocal exchanges during mother-infant reunions fulfilled all six criteria characterizing human greeting behavior. Thus, predictable reciprocal vocal exchanges occurred at the start of the reunion (but not during leave-taking), when mother and infant had visual contact to each other. Thus, we argued that mother-infant vocal exchanges governing reunions are essential to establish social bonds and to ritualize the greeting function. Our findings suggest that ritualized vocal greeting has its origins deeply rooted in mammalian phylogeny and derives from vocal exchanges during parent-infant reunions.

Acoustic characteristics used by Japanese macaques for individual discrimination

The vocalizations of primates contain information about speaker individuality. Many primates, including humans, are able to distinguish conspecifics based solely on vocalizations. The purpose of this study was to investigate the acoustic characteristics used by Japanese macaques in individual vocal discrimination. Furthermore, we tested human subjects using monkey vocalizations to evaluate species specificity with respect to such discriminations. Two monkeys and five humans were trained to discriminate the coo calls of two unfamiliar monkeys. We created a stimulus continuum between the vocalizations of the two monkeys as a set of probe stimuli (whole morph). We also created two sets of continua in which only one acoustic parameter, fundamental frequency (f₀) or vocal tract characteristic (VTC), was changed from the coo call of one monkey to that of another while the other acoustic feature remained the same (f₀ morph and VTC morph, respectively). According to the results, the reaction times both of monkeys and humans were correlated with the morph proportion under the whole morph and f₀ morph conditions. The reaction time to the VTC morph was correlated with the morph proportion in both monkeys, whereas the reaction time in humans, on average, was not correlated with morph proportion. Japanese monkeys relied more consistently on VTC than did humans for discriminating monkey vocalizations. Our results support the idea that the auditory system of primates is specialized for processing conspecific vocalizations and suggest that VTC is a significant acoustic feature used by Japanese macaques to discriminate conspecific vocalizations.

Vocalizations during post-conflict affiliations from victims toward aggressors based on uncertainty in Japanese macaques

We investigated the use of vocalizations called "grunts," "girneys," and "coos" accompanied by post-conflict affiliative interaction between former opponents (reconciliation) in Japanese macaques (Macaca fuscata). Although reconciliation functions to repair bonds, such interactions sometimes entail risks of receiving further aggression. Vocalizations can be used at a distance from the former opponent; thus, we predict that vocalizations are used particularly by victims of a conflict, and are frequently used in situations of uncertainty when it is difficult for them to estimate whether the former opponent will resume aggression. In addition, we predict that vocalizations are effective in preventing further aggression. To test these hypotheses, we conducted observations of post-conflict and matched-control situations in female Japanese macaques living in a free-ranging group. We found that former opponents tended to be attracted to each other within the first minute following a conflict, thus demonstrating reconciliation behavior. Vocalizations were more frequently used by the victims in post-conflict interactions than under control situations; however, this tendency was not found in aggressors. When affiliation with the former opponent occurred, victims were more likely to use vocalizations towards less familiar opponents. These findings suggest that Japanese macaques used vocalizations more often when interacting with less predictable former opponents. Victims were more likely to receive aggression from former aggressors when engaged in affiliations with them than under no such affiliations. No significant differences were found in the probability of the victims receiving aggression, regardless of whether they used vocalizations; thus, whether the victim benefits from using vocalizations in these contexts remains unclear. Japanese macaques form despotic societies and therefore, further aggression was inevitable, to some degree, after a conflict. The use of vocalizations by a victim was found to depend on the nature of their relationship with the aggressor; however, the effectiveness of this behavior requires further investigation.

Function of grunts, girneys and coo calls of Japanese macaques (Macaca fuscata) in relation to call usage, age and dominance relationships

We investigated how the context of the production of vocalizations used in social interactions among Japanese macaques (Macaca fuscata) affects their outcome. We focused on a variety of soft vocalizations, including three acoustically distinct call types: grunts, girneys, and coo calls. We predicted that call outcomes would be influenced by call combinations and exchanges, and by the relationship between the caller and the recipient. We observed social interactions among female Japanese macaques, and found that individuals were less likely to initiate agonistic behaviour when they emitted calls. Call exchanges and call combinations increased the occurrence of affiliative interactions. The probability of affiliative interaction following a given type of call differed according to the relationship between the caller and the recipient. These findings suggest that recipients interpret these calls within a social context; they also demonstrate the existence of complex communicative abilities that integrate vocalizations and context in these monkeys.