Banded mongoose close calls, consisting of a single syllable, were not only individually distinct, but also differed in their acoustic structure depending on the current behavior of the signaler. This acoustic variation depended on the behavioral context encoded within a harmonic extension of the basic noisy segment of the close call. To our knowledge this is the first example of temporal segmentation as a means of encoding multiple types of information within a call consisting of a single syllable in an animal vocalisation. Variation in spectral aspects (for example, fundamental frequency) of the more noisy call element verify previous findings of individual cues in close calls of banded mongoose . In that study, Müller and Manser  showed, using playback experiments that pups are able to discriminate between close calls of their escorting adult and the close calls of other adults. Their results suggest that individual vocal cues of these close calls are meaningful to receivers. Additionally, here we found group specific vocal cues. Such cues of group identity may arise because the physical characteristics that determine vocal characteristics of an individual (for example, vocal fold length (for F0) and/or vocal tract length (for formants)) are, on average, more similar among group members than non-group members. Another possibility in species with vocal flexibility and where individuals change groups is that individuals converge to match the vocal group cue of the new group after switching [48, 49]. At present it is unknown which of these two processes is applicable for the banded mongoose. In contrast, temporal features (for example, duration) of the tonal harmonic segment of the call seem to encode the behavioral vocal cues. Future research using playback experiments will need to be conducted to investigate if behavioral context vocal cues are used by receivers.
While many animal signaling systems, including human speech, use concatenation of acoustically-separate syllables to enrich and extend the signaling space (for example, birdsong [28, 29], rock hyraxes (Procavia capensis)  or cetacean species [31, 32]), human speech also encodes information into individual syllables. By combining stop consonants with different vowels at a phonological level, syllables are created that have different meanings. Thus, a stop consonant like/b/versus/p/can be combined with a vowel like/a/or/o/to create a richer signaling unit than either class (that is, stop consonants or vowel) alone could provide. Such combinations (versus 'syntactic' concatenation of syllables and words) are a core feature of the phonological component of human spoken language . The temporally segmented fashion in which banded mongooses encode multiple cues into a single syllable close call is analogous to this system. Moreover, our study provides an example of a discrete individual 'element' in a graded call containing information regarding individuality. The noisy, yet stable, segment of the close call, explained almost as much individual variation as the whole call. This implies that, despite the graded nature of the close call, individual identity is encoded in a discrete way.
The functional aspect of the discrete identity cue in combination with a graded behavioral cue seems analogous to human communicative contexts, when sender and receiver cannot see each other. For example, in the drum or whistle languages of tribes in the remote and isolated conditions of mountainous or densely forested areas, discrete signals are used to announce identity and other information to avoid ambiguity [50, 51]. Similarly, in radio conversations in aviation between pilots and control towers, identity and additional information are shared in a highly standardized order (that is, You Me Where What With; chapter 5, in ). Signals in these 'conversations' are intentionally chosen for their clarity to the receivers [53, 54]. In particular in species that are constantly moving as a cohesive unit, in their search for food or shelter, and where the identification of an individual cannot be based on its spatial position, acoustic individual identity may be a crucial aspect for the successful operation of the system. This is true for banded mongooses where coordination of foraging and movement facilitates the successful functioning of the overall social system. Temporal segregation of vocal cues may enable banded mongooses to reliably encode dual information sets regarding an individual's identity and its current behavioral context. Our study on banded mongoose close calls demonstrates temporal segregation within a single syllable call type. However, reviewing spectrograms of other species' calls, available in the literature, reveal that our findings may not be unique to banded mongooses. For example, the well-known 'whine-chuck' advertisement call of the túngara frog (Physalaemus pustulosus) provides another example of segregation of information within a single syllable, where whines encode the species identity and the chucks refer to male quality [55, 56]. Such a system is highly advantageous in providing detailed reliable information in an otherwise ambiguous graded system. Human speech [6, 54, 57, 58], and elements of some other species' vocal repertoires such as Barbary macaque (Macaca sylvanus) [59, 60], chimpanzee (Pan troglodytes) [61, 62] and Japanese macaque (Macaca fuscata)  are, from the production side, classified as a graded system, yet perceived by the receivers as discrete [6, 59–61, 63]. Graded signals have the potential to convey subtle and complex information, but potentially suffer from heightened ambiguity [54, 64]. This ambiguity can partly be resolved by meaningful, within-category, classification of a graded signal into perceptually discrete signals [64, 65]. It has been hypothesized that this perception of a graded continuum as a series of discrete units was a crucial stage in the evolution of human language [63, 64]. This analogous ability in banded mongoose demonstrates that animal communication systems also have the potential to convey a rich set of information in an acoustically sophisticated way.
Recent studies have shown that some free ranging primates use meaningful call- and element-combinations to vastly increase the range of information that can be decoded by listeners [3, 4, 66–71]. This may be particularly important for forest species living in dense vegetation, where no visual cues can be used to verify the information content or context of the signal [3, 4]. In the same way, we suggest that species that use vocal cues ultimately benefit from an increased informational repertoire and, therefore, similar species demonstrating combinatorial calling behavior could be expected to make use of multiple vocal cues and benefit from temporal segregation of information. Vocal cues predominantly encode individual related cues of the sender (for example, identity or male quality) and we, therefore, predict temporal segregation to evolve when signalers could benefit from unambiguous multiple vocal cues. Call combinations have been hypothesized to occur in response to discrete external events (for example, alarm calls) or behavioral contexts, but not directly related to characteristics of the signaler [3, 71]. Species with graded vocal systems would especially benefit from the use of unambiguous vocal cues, since these would; i) avoid the lack of clarity that generally occurs in graded vocalizations, and ii) potentially enhance the reliability of categorization by receivers of graded signals into discrete units.