Abstract:
BACKGROUND: The ability to recombine smaller units to produce infinite structures of higher-order phrases is unique
to human language, yet evidence of animals to combine multiple acoustic units into meaningful combinations
increases constantly. Despite increasing evidence for meaningful call combinations across contexts, little attention
has been paid to the potential role of temporal variation of call type composition in longer vocal sequences in
conveying information about subtle changes in the environment or individual differences. Here, we investigated
the composition and information content of sentinel call sequences in meerkats (Suricata suricatta). While being on
sentinel guard, a coordinated vigilance behaviour, meerkats produce long sequences composed of six distinct
sentinel call types and alarm calls. We analysed recordings of sentinels to test if the order of the call types is graded
and whether they contain additional group-, individual-, age- or sex-specific vocal signatures.
RESULTS: Our results confirmed that the six distinct types of sentinel calls in addition to alarm calls were produced
in a highly graded way, likely referring to changes in the perceived predation risk. Transitions between call types
one step up or down the a priory assumed gradation were over-represented, while transitions over two or three
steps were significantly under-represented. Analysing sequence similarity within and between groups and
individuals demonstrated that sequences composed of the most commonly emitted sentinel call types showed
high within-individual consistency whereby adults and females had higher consistency scores than subadults and
males respectively.
CONCLUSIONS: We present a novel type of combinatoriality where the order of the call types contains temporary
contextual information, and also relates to the identity of the caller. By combining different call types in a graded
way over long periods, meerkats constantly convey meaningful information about subtle changes in the external
environment, while at the same time the temporal pattern of the distinct call types contains stable information
about caller identity. Our study demonstrates how complex animal call sequences can be described by simple rules,
in this case gradation across acoustically distinct, but functionally related call types, combined with individualspecific
call patterns.
