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Syntax is widely considered the feature that most decisively sets human language apart from other natural communication systems. Animal vocalisations are generally considered to be holistic with few examples of utterances meaning something other than the sum of their parts. Previously, we have shown that male putty-nosed monkeys produce call series consisting of two call types in response to different events. They can also be combined into short sequences that convey a different message from those conveyed by either call type alone. Here, we investigate whether "pyow-hack" sequences are compositional in that the individual calls contribute to their overall meaning. However, the monkeys behaved as if they perceived the sequence as an idiomatic expression rather than decoding the sequence. Nonetheless, while this communication system lacks the generative power of syntax it enables callers to increase the number of messages that can be conveyed by a small and innate call repertoire. (Contains 4 figures.)

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 pant-grunting 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.

Observations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide valuable comparative data for understanding the significance of conspecific killing. Two kinds of hypothesis have been proposed. Lethal violence is sometimes concluded to be the result of adaptive strategies, such that killers ultimately gain fitness benefits by increasing their access to resources such as food or mates (1-5). Alternatively, it could be a non-adaptive result of human impacts, such as habitat change or food provisioning (6-9). To discriminate between these hypotheses we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades. Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos. We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio). Variation in killing rates was unrelated to measures of human impacts. Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported., Substantial variation exists in rates of killing across chimpanzee study sites (2-5,10-12). The human impact and adaptive strategies hypotheses both seek to explain this variation, but have contrasting predictions, which [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2010.09.005 Byline: Thibaud Gruber (a), Zanna Clay (a), Klaus Zuberbuhler (a)(b) Abstract: Chimpanzees, Pan troglodytes, are the most sophisticated tool-users among all nonhuman primates. From an evolutionary perspective, it is therefore puzzling that the tool use behaviour of their closest living primate relative, the bonobo, Pan paniscus, has been described as particularly poor. However, only a small number of bonobo groups have been studied in the wild and only over comparably short periods. Here, we show that captive bonobos and chimpanzees are equally diverse tool-users in most contexts. Our observations illustrate that tool use in bonobos can be highly complex and no different from what has been described for chimpanzees. The only major difference in the chimpanzee and bonobo data was that bonobos of all age-sex classes used tools in a play context, a possible manifestation of their neotenous nature. We also found that female bonobos displayed a larger range of tool use behaviours than males, a pattern previously described for chimpanzees but not for other great apes. Our results are consistent with the hypothesis that the female-biased tool use evolved prior to the split between bonobos and chimpanzees. Author Affiliation: (a) Centre for Social Learning and Cognitive Evolution and Scottish Primate Research Group, School of Psychology, University of St Andrews, U.K. (b) Center for Advanced Study, Wissenschaftskolleg zu Berlin, Germany Article History: Received 15 February 2010; Revised 29 March 2010; Accepted 1 September 2010 Article Note: (miscellaneous) MS. number: 10-00114R

Despite ongoing and, in some regions, escalating morbidity and mortality rates associated with malaria-causing parasites, the evolutionary epidemiology of Plasmodium spp. is not well characterized. Classical studies of the blood [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2010.07.012 Byline: Anne Marijke Schel (a)(b), Agnes Candiotti (a)(b)(c), Klaus Zuberbuhler (a)(b) Abstract: Guereza colobus monkeys, Colobus guereza, produce acoustically conspicuous vocalizations, the roars, in response to their main predators, leopards, Panthera pardus, and crowned eagles, Stephanoaetus coronatus. Roaring alarm utterances generally consist of the same basic call types but differ in overall structural composition. Leopards trigger roaring alarms containing many roaring sequences of only a few calls each, while eagles trigger few sequences with many calls each. To investigate whether conspecifics extract meaning from these structural differences, we played back leopard and eagle alarm call sequences and compared the monkeys' responses in terms of their locomotor, gaze and vocal behaviour with their responses to the corresponding predator vocalizations. Locomotor responses did not differ between playback conditions; movement was always towards the simulated caller with no clear patterns in the vertical plane. Gaze direction, however, was highly predator specific. When hearing leopard-related stimuli, monkeys were significantly more likely to scan the area beneath them than when hearing eagle-related stimuli, which caused more scanning above. Vocal response rates to conspecific alarms were generally low but comparable with rates to the corresponding predators. If monkeys called, however, they produced the matching call sequences. Overall, our results showed that Guerezas discriminated between predator alarm call sequences produced by unfamiliar conspecifics and responded to them in predator-specific ways. Since the sequences were composed of the same basic call types, we concluded that the monkeys attended to the compositional aspects of these utterances. Author Affiliation: (a) School of Psychology, University of St Andrews, U.K. (b) Budongo Conservation Field Station, Masindi, Uganda (c) Departement de Biologie, Ecole normale superieure de Lyon, France Article History: Received 11 January 2010; Revised 4 March 2010; Accepted 9 July 2010 Article Note: (miscellaneous) MS. number: 10-00027R

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2010.06.005 Byline: Marion N.C. Laporte (a)(b), Klaus Zuberbuhler (a)(b)(c) Abstract: Chimpanzees, Pan troglodytes, are unusual among primates in that they express their social position with a unique vocal signal, the pant-grunt. The call is only produced when encountering a higher-ranking group member and has thus been interpreted as a 'greeting' signal. We monitored the calling behaviour of nine adult females in a group of free-ranging chimpanzees, the Sonso community of Budongo Forest, Uganda, when encountering higher-ranking adult males. We found that call production was by no means rigid, but that calls were given only if certain social conditions were met. Although all adult males received pant-grunts from females, the alpha male received a significantly larger proportion of calls. The number of pant-grunts given to males was not correlated with their hierarchical position or with the level of anticipated aggression. Instead, females were significantly more likely to vocalize to other males if the alpha male was absent, suggesting that their calling behaviour was moderated by social inhibition. The presence of the alpha female had a similar yet weaker inhibitory effect. Social inhibition was further increased with increasing numbers of bystanders, especially males. Our results thus demonstrate that chimpanzees use their 'greeting' signals flexibly by taking into account the social fabric of their community. Author Affiliation: (a) School of Psychology, University of St Andrews, U.K. (b) Budongo Conservation Field Station, Masindi, Uganda (c) Wissenschaftskolleg zu Berlin, Germany Article History: Received 18 November 2009; Revised 14 January 2010; Accepted 25 May 2010 Article Note: (miscellaneous) MS. number: 09-00733R

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2010.04.012 Byline: Robert M. Seyfarth (a), Dorothy L. Cheney (b), Thore Bergman (c)(d), Julia Fischer (e), Klaus Zuberbuhler (f), Kurt Hammerschmidt (e) Abstract: The concept of information plays a central role in studies of animal communication. Animals' responses to the calls of different individuals, to food calls, alarm calls, and to signals that predict behaviour, all suggest that recipients acquire information from signals and that this information affects their response. Some scientists, however, want to replace the concept of information with one based on the 'manipulation' of recipients by signallers through the induction of nervous-system responses. Here we review both theory and data that argue against hypotheses based exclusively on manipulation or on a fixed, obligatory link between a signal's physical features and the responses it elicits. Results from dozens of studies indicate that calls with 'arousing' or 'aversive' features may also contain information that affects receivers' responses; that acoustically similar calls can elicit different responses; acoustically different calls can elicit similar responses; and 'eavesdropping' animals respond to the relationship instantiated by signal sequences. Animal signals encode a surprisingly rich amount of information. The content of this information can be studied scientifically. Author Affiliation: (a) Department of Psychology, University of Pennsylvania, Philadelphia, PA, U.S.A. (b) Department of Biology, University of Pennsylvania, Philadelphia, PA, U.S.A. (c) Department of Psychology, University of Michigan, Ann Arbor, MI, U.S.A. (d) Department of Ecology & Evolutionary Biology, University of Michigan, Ann Arbor, MI, U.S.A. (e) Cognitive Ethology Laboratory, German Primate Center, University of Gottingen, Gottingen, Germany (f) Wissenschaftskolleg, Institute for Advanced Study, Berlin, Germany Article History: Received 17 December 2009; Revised 4 March 2010; Accepted 26 March 2010 Article Note: (miscellaneous) MS. number: AE-09-00802

Primate vocal behavior is often considered irrelevant in modeling human language evolution, mainly because of the caller's limited vocal control and apparent lack of intentional signaling. Here, we present the results of a long-term study on Campbell's monkeys, which has revealed an unrivaled degree of vocal complexity. Adult males produced six different loud call types, which they combined into various sequences in highly context-specific ways. We found stereotyped sequences that were strongly associated with cohesion and travel, falling trees, neighboring groups, nonpredatory animals, unspecific predatory threat, and specific predator classes. Within the responses to predators, we found that crowned eagles triggered four and leopards three different sequences, depending on how the caller learned about their presence. Callers followed a number of principles when concatenating sequences, such as nonrandom transition probabilities of call types, addition of specific calls into an existing sequence to form a different one, or recombination of two sequences to form a third one. We conclude that these primates have overcome some of the constraints of limited vocal control by combinatorial organization. As the different sequences were so tightly linked to specific external events, the Campbell's monkey call system may be the most complex example of 'proto-syntax' in animal communication known to date. alarm call | nonhuman primate | referential communication | semantic | syntax doi/10.1073/pnas.0908118106

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2009.09.010 Byline: Ilka Herbinger (a)(c), Sarah Papworth (b), Christophe Boesch (a)(c), Klaus Zuberbuhler (b)(c) Abstract: Wild chimpanzees can be dangerously violent against individuals that do not belong to their community, indicating a strong selection pressure on decision-making abilities in this context. The presence of a neighbouring male indicates a serious threat, although encountering an unfamiliar male is potentially even more dangerous because it indicates the arrival of a new group with whom the subjects have no previous history of interaction. We conducted playback experiments with members of three chimpanzee, Pan troglodytes verus, communities in the TaA[macron] National Park, CA[acute accent]te d'Ivoire, in which we simulated the unexpected presence of another chimpanzee. We tested subjects' responses to pant hoots of familiar group members, neighbouring individuals and unfamiliar strangers. We found that neighbours and strangers elicited a wider range of gestural signals than familiar group members. Vocal responses were common in all conditions. Familiar group members mainly triggered pant hoot replies, whereas neighbour and stranger trials caused screams. Across conditions, males responded more strongly than females. Male party size had an effect on subjects' vocal responses to strangers but not to neighbours, although neighbours were approached more closely. Our results show that chimpanzees are able to identify different classes of individuals by their pant hoot vocalizations, as judged by their differential responses to the calls of familiar group members, neighbours or strangers. The overall response patterns suggest that chimpanzees are aware of the different social consequences associated with encountering a neighbouring group or a group of strangers. Author Affiliation: (a) Max Planck Institute for Evolutionary Anthropology, Germany (b) School of Psychology, University of St Andrews, U.K. (c) Wild Chimpanzee Foundation/Centre Suisse de Recherches Scientifiques en CA[acute accent]te d'Ivoire, Germany Article History: Received 6 April 2009; Revised 28 April 2009; Accepted 4 September 2009 Article Note: (miscellaneous) MS. number: 09-00220R

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2009.03.014 Byline: Karim Ouattara (a)(b)(c)(d), Klaus Zuberbuhler (b)(e), Eliezer K. N'goran (c), Jean-Emile Gombert (d), Alban Lemasson (a) Abstract: Field studies on male forest guenon alarm-calling behaviour have revealed a number of intricacies about how these primates use vocalizations to protect themselves from predation. In these species, the vocal behaviour of adult females is often different from that of the males, but little systematic work has been done. Here, we describe the alarm call system of female Campbell's monkeys, Cercopithecus campbelli, in their natural forest habitat in western Ivory Coast. We found that in response to disturbing events, females produced three basic alarm call types, 'wak-oos', 'hoks' and acoustically variable 'trill' calls, consisting of repeated and rapidly ascending (RRA) pulses, which varied systematically in the temporal and frequency domains. Using observational and experimental data we were able to demonstrate that the RRA calls consisted of four acoustic variants, which could be associated with specific contexts, allowing listeners to draw inferences about the type of disturbance experienced by the caller. We also compared the alarm call behaviour of free-ranging individuals with published results from captivity. As predicted, captive individuals failed to produce predator-specific alarm calls, but they also produced an RRA variant in response to humans that was absent in the wild. We discuss the relevance of these findings in terms of their broader potential impact on evolutionary theories of primate communication. Author Affiliation: (a) Ethologie animale et humaine, U.M.R. 6552-C.N.R.S., Universite de Rennes 1, France (b) Centre Suisse de Recherches Scientifiques, TaA[macron] Monkey Project, CA[acute accent]te d'Ivoire (c) Laboratoire de zoologie et de biologie animale, Universite de Cocody, CA[acute accent]te d'Ivoire (d) Centre de Recherche en Psychologie, Cognition et Communication, Universite de Rennes 2, France (e) School of Psychology, University of St Andrews, U.K. Article History: Received 3 November 2008; Revised 23 December 2008; Accepted 12 March 2009 Article Note: (miscellaneous) MS. number: 08-00712R

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2009.02.016 Byline: Zanna Clay, Klaus Zuberbuhler Abstract: When encountering food, chimpanzees, Pan troglodytes, and some other primates produce specific calls, whose acoustic structure covaries with the caller's food preference. For chimpanzees, there is evidence that listeners use these contingencies to guide their own foraging behaviour. Here, we investigated the vocal behaviour of another great ape, the bonobo, Pan paniscus, in response to food. We were able to distinguish acoustically five different call types given during interactions with food. These calls were not given singly, but usually as part of long and complex call sequences. We established the food preference hierarchies for 10 different individuals, housed at two different facilities. We found that the composition of call sequences produced by these individuals was not random, but was related to the type of food encountered by the caller. Significant variation in call composition was explained by taking into account the caller's individual food preferences, suggesting that bonobo food-calling sequences convey meaningful information to other group members. Author Affiliation: School of Psychology, University of St Andrews, U.K. Article History: Received 20 October 2008; Revised 24 November 2008; Accepted 8 February 2009 Article Note: (miscellaneous) MS. number: 08-00675

Vervet monkey alarm calling has long been the paradigmatic example of how primates use vocalizations in response to predators. In vervets, there is a close and direct relationship between the production of distinct alarm vocalizations and the presence of distinct predator types. Recent fieldwork has however revealed the use of several additional alarm calling systems in primates. Here, the authors describe playback studies on the alarm call system of two colobine species, the King colobus (Colobus polykomos) of Tai Forest, Ivory Coast, and the Guereza colobus (C. guereza) of Budongo Forest, Uganda. Both species produce two basic alarm call types, snorts and acoustically variable roaring phrases, when confronted with leopards or crowned eagles. Neither call type is given exclusively to one predator, but the authors found strong regularities in call sequencing. Leopards typically elicited sequences consisting of a snort followed by few phrases, while eagles typically elicited sequences with no snorts and many phrases. The authors discuss bow these call sequences have the potential to encode information at different levels, such as predator type, response-urgency, or the caller's imminent behavior. Keywords: Colobus polykomos, Colobus guereza, alarm calls, predation, referential signaling

Wild chimpanzees produce acoustically distinct scream vocalizations depending on their social role during agonistic interactions with other group members. Here, we show that victims during such agonistic interactions alter the acoustic structure of their screams depending on the severity of aggression experienced, providing nearby listeners with important cues about the nature of the attack. However, we also found that victims of severe attacks produced screams that significantly exaggerated the true level of aggression experienced, but they did so only if there was at least one listener in the audience who matched or surpassed the aggressor in rank. Our results are consistent with the more general hypothesis that chimpanzees possess sophisticated understanding of third-party relationships, so-called triadic awareness, and that this knowledge influences their vocal production. audience effect | language evolution | Pan troglodytes | primate vocalizations | social intelligence

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2006.01.030 Byline: Katie E. Slocombe, Klaus Zuberbuhler Abstract: Chimpanzees produce specific vocalizations, 'rough grunts', when encountering food, and it has been suggested that these calls vary acoustically depending on the food type discovered by the caller. Nearby listeners often behave as if the calls are meaningful to them, indicating that the calls may function as referential labels for particular foods. We investigated whether rough grunt variants are the result of callers responding to specific food types or relative food preferences. We recorded calls from captive individuals in response to nine different food items, which could be ranked as high, medium or low preference. Individuals consistently produced three acoustically distinct grunt variants to the three food preference classes. There was no evidence that chimpanzees produced individual labels for food types of low and medium preference. However, calls to high-preference food types differed significantly in their acoustic structure. These acoustic patterns remained stable over trials, suggesting that rough grunts have the potential to serve as semantic labels for individual high-preference food types. We were unable to replicate these findings with a set of recordings from the wild, although most other aspects of calling behaviour remained identical. We discuss these discrepancies between the wild and captivity and suggest that the emergence of referential labels for food items may be a by-product of the special circumstances found in captive settings. Author Affiliation: School of Psychology, University of St Andrews and Budongo Forest Project, Uganda Article History: Received 29 July 2005; Revised 26 September 2005; Accepted 17 January 2006 Article Note: (miscellaneous) MS. number: 8638

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2005.12.009 Byline: Karline R.L. Janmaat, Richard W. Byrne, Klaus Zuberbuhler Abstract: We investigated the ranging behaviour of two groups of wild mangabey monkeys (sooty mangabeys, Cercocebus atys atys, and grey-cheeked mangabeys, Lophocebus albigena johnstoni) relative to a number of preselected target trees within their home range. We observed the groups' visiting patterns and speed when they approached within a critical distance of a target tree as a function of the tree's fruiting state. For both groups, the likelihood of coming into sight or actually entering these trees was significantly higher if fruit was available. Target trees with fruit were also approached significantly faster than were trees without fruits. These behavioural differences were unlikely to be the result of auditory, visual or olfactory cues available over long distances, suggesting that monkeys relied on spatial memory to relocate fruit trees and distinguish between trees that had and had not carried fruit in the immediate past. Results further indicated that the monkeys clearly distinguished between different types of fruit-bearing target trees. We suggest that the monkeys used memory of previous feeding experience to assess each tree's differences and were able to anticipate changes in fruit quality. We found no evidence that individuals belonging to a particular age or sex class led the group towards trees with fruit more often than did others. Author Affiliation: (a) School of Psychology, University of St Andrews, U.K. (a ) TaA[macron] Monkey Project, Centre Suisse de Recherches Scientifiques, Abidjan, France Article History: Received 26 January 2005; Revised 18 March 2005; Accepted 26 December 2005 Article Note: (miscellaneous) MS. number 8443R

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2005.11.017 Byline: Kate Arnold, Klaus Zuberbuhler Abstract: We investigated the calls of male putty-nosed monkeys produced in response to playbacks of leopard, Panthera pardus, growls and crowned eagle, Stephanoaetus coronatus, shrieks. Two call types, hacks and pyows were produced and both occurred within alarm-calling sequences regardless of the predator category simulated by the playbacks. Unlike previous studies of alarm calling in guenon monkeys, we therefore could not conclude that the alarm calls of putty-nosed males are functionally referential. There were, however, striking regularities in the patterning of the calls given in response to these stimuli and different call combinations were strongly associated with each of the two predator types. While we concluded that individual calls did not qualify as vehicles of semantic content, these males produced structurally unique call sequences that provided sufficient information for receivers to select appropriate antipredator responses. Author Affiliation: School of Psychology, University of St Andrews, U.K. Article History: Received 28 February 2005; Revised 20 May 2005; Accepted 16 November 2005 Article Note: (miscellaneous) MS. number: 8480R

Campbell's monkeys (Cercopithecus campbelli) frequently exchange vocalizations, the combined-harmonic calls, with individuals responding to one another's calls. Previous work has shown that these calls can be grouped into several structural variants. Adult females differ in their variant repertoires, which may change during their adult life, particularly after changes in the group composition. Playback of females' currently produced variants triggered vocal responses from other group members, whereas the same females' former, no longer used variants and those of stranger females never did. In contrast, former variants caused long-term cessation of vocal behavior, whereas stranger variants had no effect. Data showed that monkeys were able to distinguish between the different types of variants, indicating that these calls form part of a long-term social memory.

Some nonhuman primates have demonstrated the capacity to communicate about external objects or events, suggesting primate vocalizations can function as referential signals. However, there is little convincing evidence for functionally referential communication in any great ape species. Here, the authors demonstrate that wild chimpanzees (Pan troglodytes schweinfurthii) of Budongo forest, Uganda, give acoustically distinct screams during agonistic interactions depending on the role they play in a conflict. The authors analyzed the acoustic structure of screams of 14 individuals, in the role of both aggressor and victim. The authors found consistent differences in the acoustic structure of the screams, across individuals, depending on the social role the individual played during the conflict. The authors propose that these 2 distinct scream variants, produced by victims and aggressors during agonistic interactions, may be promising candidates for functioning as referential signals.

The communication systems of monkeys, Cercopithecus diana and Cercopithecus campbelli, were studied for signs of syntactic patterns. It was found that alarm calls understood by both species sometimes have a lexical element preceding the alarm call that renders it not as urgent, as perceived by the listening monkeys. This could be a sign of lexical syntax in nonhuman primates.

The predation and communication context of Diana monkeys (Cercopithecus diana diana) long-distance calls was studied. Monkey long-distance calls function as defense against predators, by conveying predator detection and by warning co-species about the type of danger present. It also functions as within-group symbolic signals that denote different types of predators. The experiment on Diana monkeys allowed not only the responses to alarm calls but also the factors that influence call production.

The findings of the behavioral study of the various types of triadic social games between four bonobos (Pan paniscus) and humans are given. The two main factors for shared intentionality, the ability to understand and respond to one another and share the focus with others are highlighted in the social game.

Moral Minds: How Nature Designed Our Universal Sense of Right and Wrong. Marc D. Hauser. HarperCollins, New York, 2006. 512 pp. $27.95 (ISBN 9780060780708 cloth). The main idea behind Moral […]

Author(s): Kate Arnold [1]; Klaus Zuberbuhler (corresponding author) [1] Syntax sets human language apart from other natural communication systems, although its evolutionary origins are obscure [1]. Here we show that [...]

Research into how wild Diana monkeys, Cercopithecus diana, of the Tai forest, Ivory Coast, label predation events is presented. These monkeys consistently respond to predator category irrespective of immediate threat or direction of attack.

Wild Diana monkeys, Ceropithecus diana, in the Tai forest, Ivory Coast, show differing responses to chimpanzees' alarm calls. Some monkeys seem to recognize that these alarm calls indicate the potential presence of a leopard, while others behave as if they are unable to distinguish between the different causes underlying chimpanzee vocalizations. It appears significant that the monkeys' chances of seeing an encounter between chimpanzees and a leopard are very low. The findings of this research support the view that monkeys responding cryptically to chimpanzee alarm calls do so because they cannot understand the meaning of these calls.