Our results indicate that the vocal repertoires of adult common marmosets and golden-backed uakaris do not follow the pattern predicted by the law of brevity [2
], unlike the situation in Formosan macaques (Macaca cyclopis
]. For the latter, an inverse relationship existed between signal duration and frequency of use [1
]. However, even in Semple et al
.'s analysis, the macaques used the two briefest signals less frequently than the third briefest signal.
That the briefest signals are not the most frequently used is perhaps not surprising, given the different costs and benefits associated with signals of different durations. Very short calls may contain limited information, such as cues that identify the signaller. Although long calls may be costly to produce, they may nonetheless be honest signals of quality and hence will be favoured by sexual selection in some cases [8
]. Signal duration will also depend on factors such as home-range size: for example, animals that defend large territories tend to use long duration calls to advertise presence and location of signallers [9
In marmosets, the shortest signals are associated with vigilance (e.g. at territorial boundaries, when crossing between forest patches, when encountering novel observers), where rapid and clear signalling of potential danger is paramount. Vigilance behaviour is relatively uncommon, however, and so these signals are produced less frequently than other signals used in close-range contact communication among members of a social group. In the latter case, there is perhaps a greater need for encoding details about signaller identity and context, so there is selection for longer signals. In both species, the longest signals were produced for long-range communication, perhaps to maximize the likelihood of the signaller being detected.
Frequency of use will also depend on other factors that affect the costs of calling. For example, call amplitude—if brief, high-amplitude calls are more energetically expensive to produce, they may be selected against in comparison with longer duration signals of lower amplitude. Moreover, costs associated with increased conspicuousness to predators or prey [1
] are only applicable in certain species. In both our study species (and indeed in the Formosan macaques; [1
]), relatively brief signals were the most frequently used, perhaps suggesting that brevity is favoured to a certain extent, but that costs associated with very brief signals limit their use in vocal repertoires (i.e. there is stabilizing selection acting on signal duration).
Analogies between frequency of use in human language and animal communication may be misleading. Language involves extensive use of syntax whereby a large vocabulary can be arranged into sentences that convey a vast number of meanings to convey complex messages. Although Campbell's monkeys (Cercopithecus campbelli
) use ‘proto-syntax’ by producing combinatorial signal sequences associated with specific meanings [10
], the extent to which syntax is used is very limited in non-human animals. Language can convey complex messages of a virtually unlimited variety often about abstract situations via combinations of words, and the use of brief components in a vocabulary can be an efficient means of transmitting information in syntax-rich language.
Although human vocabularies can be very large, speech is broken up into a relatively small number of brief phonemes [11
] that facilitate the learning of a vocabulary, and here brevity may be important. Brevity is likely to be vital in memorizing the components of a large vocabulary, especially given the important role of vocal imitation in human language [12
]. Brevity brings benefits to both signallers and receivers in language [13
]. Conversely, vocalizations of non-human animals must be subject to a wider range of constraints, and single signals might need to convey accurate information about situation-specific contexts regarding, for example, risk, urgency, referential specificity and signaller identity [14
]. Such information is not necessarily best encoded in brief signals, and coding efficiency alone is insufficient to explain frequency of use in animal vocal communication. Semple et al
] encouraged a wider range of studies to test the generality of the law of brevity. Our study of two additional species does not support the hypothesis that the law of brevity is widely applicable in animal communication—we argue that relationships between signal duration and frequency of use are best considered on a species-specific basis, and must take account of the costs and benefits of different signal durations in a wide range of contexts.