Sarah Gouzoules

Design features and developmental modification of pigtail macaque, Macaca nemestrina, agonistic screams

Scream vocalizations produced by pigtail macaques during agonistic encounters were studied using spectrographic and multivariate analyses. These calls are important in the recruitment of support from allies against opponents. Direct discriminant analysis was used to classify screams recorded from 45 monkeys living in a stable captive group at the Yerkes Regional Primate Research Center. Pigtail macaques employed acoustically distinct classes of screams depending upon features of the agonistic context: four types of screams were associated with the relative rank of the opponent and with the severity of the aggression. A comparison of rhesus macaque, Macaca mulatta, screams with those of pigtail macaques revealed that the acoustic features of screams used by the two species in identical agonistic contexts were very different. Matrilineal relatedness did not emerge as a factor related to specific call types in pigtail monkeys as it had for rhesus monkey screams. Significantly more classification errors were made for the calls recorded from monkeys under 3 years of age than for older monkeys in each of the four agonistic contexts. Calls correctly classified into the four agonistic contexts were assigned with a significantly higher probability for older monkeys, suggesting that call renditions of older monkeys were closer to the ‘prototype’ for a particular context than were those of younger monkeys. Proper contextual usage and the production of screams appear to undergo developmental modification.

Matrilineal signatures in the recruitment screams of pigtail macaques, Macaca nemestrina

In macaques and baboons, scream vocalizations play a major role in the recruitment of allies against agonistic opponents. Pigtail macaques make use of 4 acoustically distinct scream types, with each associated with a particular agonistic context (defined in terms of the opponents relative dominance rank and the intensity of the aggression). Information about caller identity must also be encoded in recruitment screams if spatially distant allies are to make decisions about intervention. In this study, the agonistic screams of pigtail macaques were analyzed for evidence of vocal signatures that may serve to identify matrilineal kin groups. Maternal genealogical relationships were known for all individuals in the 56 member study group. Direct discriminant analysis was used to classify calls of individuals on the basis of their acoustic structure to one of 3 groups defined by matrilineal relatedness (two homogeneous groups, or matrilines, and one heterogeneous control group). Two scream types associated with higher-ranking opponents were analyzed separately: contact aggression screams and noncontact screams. A highly significant proportion of calls was classified to the correct matrilineal group for both scream classes. The acoustic basis for matrilineal vocal signatures in these calls apparently exists. Efficient vocal communication may require monkeys to classify group members at different levels, depending upon the degree of specificity needed.

Skeptical responding in rhesus monkeys (Macaca mulatta)

Signalers that misinform sufficiently open may become devalued as sources of information; however, “skepticism” and any comparison involved in testing reliability entail a cost that involves delays and energy expenditure. Skepticism may be less costly though, if, as a rule, animals are not equally skeptical of the signals of all conspecifics. Animals with the ability to recognize individual conspecifics and to recall past encounters with them may have the capacity to restrict skepticism to subsets of animals that are most likely to benefit from deception. We played tape-recorded alarm calls of high- and low-ranking rhesus monkeys(Macaca mulatta) to their groups in a feeding context once daily over 8 consecutive days at the Yerkes Primate Center Field Station. Over the sequence of playbacks, response was greater to the calls of high-ranking monkeys, adult response patterns were different from those of juveniles, and for adults especially, decline in responsiveness was punctuated by partial resurgences of response. These differences may be the consequence of the adults’ more extensive histories of interaction with group members that, though generally reliable, vary with respect to the potential benefits of deceptive signaling.

Representational Signaling in Non-Human Primate Vocal Communication

Knowledge about non-human primate vocal communication has grown substantially in recent years and has resulted in a radically different view of the role vocalizations play in animals’ lives. Some 25 years ago, anthropologist Jane Lancaster summarized the then prevalent view by noting that, “.... field and laboratory workers have emphasized that vocalizations do not carry the major burden of meaning in most social interactions, but function instead either to call visual attention to the signaller or to emphasize or enhance the effect of visual or tactile signals” (Lancaster, 1968). Downplaying the role of vocal communication even further, Lancaster went on to speculate that “a blind monkey would be greatly handicapped in his social interactions whereas a deaf one would probably be able to function almost normally” (Lancaster, 1968, p.442). In general, the communication systems of monkeys and apes were thought to express the emotional or motivational states of the animals (reviewed in Marler, 1985, 1992; Cheney and Seyfarth, 1990a).

Agonistic screams and the classification of dominance relationships: are monkeys fuzzy logicians?

Scream vocalizations of group-living rhesus macaques provoked by higher-ranking aggressors were examined in two contexts: encounters in which the rank difference between opponents was either large or small. Such vocalizations are important in eliciting support from the callers allies in the group (usually matrilineal kin). Five acoustically distinct recruitment screams encode specific information about features of the agonistic context, for example, relative rank of the opponent and the severity of the attack. Against higher-ranking opponents, noisy screams are most likely to be given during encounters that involve contact aggression, and tonal or undulated screams are most likely for non-contact aggression. We predicted that scream bouts directed to small rank difference opponents would be more likely to comprise calls from scream classes other than the expected type, and that scream bouts from large rank difference encounters would be more in accord with the expected class. For large rank difference bouts, on average 74.2% of calls were of the expected class; for small rank difference bouts, a significantly smaller per cent (53.8%) of calls was accurately predicted. In the large rank difference encounters, 65.6% (55/84) of bouts were composed entirely of expected scream types, but only 47.6% (39/82) were uniformly correct for small rank difference encounters. Large rank difference interactions also yielded significantly fewer bouts where none of the screams produced by the victim of attack belonged to the predicted class. We suggest that a multi-valued or fuzzy logic system, that is one with more than two truth values, might be a more realistic way to conceptualize the categorization of certain referents of monkey vocalizat. Copyright 1998 The Association for the Study of Animal Behaviour.

Recruitment screams of pigtail monkeys (Macaca nemestrina) : ontogenetic perspectives

In several species of nonhuman primates screams given by victims of attack elicit interventions from allies in the social group. In the present study, the screams of 16 pigtail macaques (M. nemestrina) under three years of age were assessed with respect to production and contextual usage. Data obtained from these animals three years later were compared with those from the previous period for evidence of changes in usage and production. It was hypothesized that these monkeys (8 males and 8 females), when three years older, would show patterns of contextual usage and production for agonistic screams that were more similar to those of adult animals. For control and comparison, 10 monkeys (9 females and 1 male) who had been older than three years of age during the earlier study were also sampled during the subsequent period. Calls of all subjects were assigned to one of four agonistic contexts (defined by the severity of the aggression and the dominance rank of the opponent) using discriminant functions that had been generated from the earlier data. While the successful classification rate improved for the 16 subjects that were originally three years of age or younger, no significant change was found for screams of the 10 older control monkeys. The probabilities with which screams of the 16 younger subjects were assigned correctly to context (a measure of call production) also were significantly higher during the second period. No such change was evident in the calls of the 10 control subjects. Data from the earlier study period had suggested that, among juveniles, females appeared to be more proficient than males in both the production and usage of these calls. During the second period of study, the proportion of calls that was classified correctly increased significantly for both males and females. The results of tests comparing the proportion of correctly classified calls for individuals revealed no differences between males and females during the second period, confirming similar results for pooled data analyses. Furthermore, there were no significant differences between males and females in the assignment probabilities with which calls were correctly classified. These results suggest that, with age, the younger monkeys became more proficient in the contextual usage and production of screams.

Primate Communication: By Nature Honest, or by Experience Wise?

We review evolutionary views on honesty and deception and their application to studies of nonhuman primate communication. There is evidence that some primate signals are likely to be accurate on the basis of costliness. They appear most often in contexts that include overtly competitive interactions in which unrelated individuals have limited access to information about one another. However, both game theoretic models and most empirical work suggest that costly signals are not often likely to be the basis for honest communication in nonhuman primates. Inexpensive signaling can exist in contexts wherein communication occurs among related animals, something common among many nonhuman primate societies. Another condition in which inexpensive signaling is possible and that is also typical of nonhuman primates, is when sender and receiver both benefit from coordinated interactions. Additionally, when individuals interact repeatedly and can use past interactions to assess the honesty of signals and to modify future response to signals, low-cost signals can evolve. Nonhuman primates appear to deal with the problem of deception via skeptical responding, which can be largely accounted for by learning rules and the fact that they live in stable social groups and can recognize one another and recall past interactions.