Syntax sets human language apart from other natural communication systems, although its evolutionary origins are obscure. Here we show that free-ranging putty-nosed monkeys combine two vocalizations into different call sequences that are linked to specific external events, such as the presence of a predator and the imminent movement of the group. Our findings indicate that non-human primates can combine calls into higher-order sequences that have a particular meaning.

Since de Waal & van Roosmalen (1979) first documented the occurrence of reconciliation between former opponents in captive chimpanzees, the study of the post-conflict behaviour of primates has provided valuable information about some of the details of primate social organisation. The vast majority of these studies have been carried out on captive subjects and it has been assumed that these findings are representative of wild primates. We set out to investigate whether this was true for the Sonso community of wild chimpanzees (Pan troglodytes schweinfurthii) of the Budongo Forest, Uganda, using controlled procedures comparable with those used in captive studies. We found that these chimpanzees were much less likely to reconcile than their captive counterparts. Only one dimension of relationship quality had an effect on the likelihood of reconciliation. Individuals which were highly compatible, in terms of time spent affiliating, reconciled conflicts more often than those with weak relationships. Captive chimpanzees have also been shown to 'console' one another (de Waal & van Roosmalen, 1979; de Waal & Aureli, 1996), where uninvolved bystanders initiate affiliative contacts with victims of aggression. This study did not confirm that consolatory behaviour was characteristic of wild chimpanzee post-conflict behaviour. Nor did these chimpanzees use explicit gestures during post-conflict interactions as they have been shown to do in two out of three captive studies. We conclude that the post-conflict behaviour of chimpanzees is more variable than has previously been thought and is likely to be dependent on the prevailing social environment.

No 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.

International audienceWe develop a formal semantic analysis of the alarm calls used by Campbell’s monkeys in the Tai forest (Ivory Coast) and on Tiwai island (Sierra Leone)—two sites that differ in the main predators that the monkeys are exposed to (eagles on Tiwai vs. eagles and leopards in Tai). Building on data discussed in Ouattara et al. (PLoS ONE 4(11):e7808, 2009a; PNAS 106(51): 22026–22031, 2009b and Arnold et al. (Population differences in wild Campbell’s monkeys alarm call use, 2013), we argue that on both sites alarm calls include the roots krak and hok, which can optionally be affixed with -oo, a kind of attenuating suffix; in addition, sentences can start with boom boom, which indicates that the context is not one of predation. In line with Arnold et al., we show that the meaning of the roots is not quite the same in Tai and on Tiwai: krak often functions as a leopard alarm call in Tai, but as a general alarm call on Tiwai. We develop models based on a compositional semantics in which concatenation is interpreted as conjunction, roots have lexical meanings, -oo is an attenuating suffix, and an all-purpose alarm parameter is raised with each individual call. The first model accounts for the difference between Tai and Tiwai by way of different lexical entries for krak. The second model gives the same underspecified entry to krak in both locations (= general alarm call), but it makes use of a competition mechanism akin to scalar implicatures. In Tai, strengthening yields a meaning equivalent to non-aerial dangerous predator and turns out to single out leopards. On Tiwai, strengthening yields a nearly contradictory meaning due to the absence of ground predators, and only the unstrengthened meaning is used