Call type signals caller goal: a new take on ultimate and proximate influences in vocal production

Abstract

After 40 years of debate it remains unclear whether signallers produce vocalizations in order to provide receivers with information about call context or external stimuli. This has led some researchers to propose that call production is arousal- or affect-based. Although arousal influences certain acoustic parameters within a call type, we argue that it cannot explain why individuals across vertebrates produce different call types. Given emerging evidence that calls are goal-based, we argue that call type is a signal of a caller's goal to elicit a change in receiver behaviour. Using chimpanzees (Pan troglodytes) and vervet monkeys (Cercopithecus aethiops) as case studies, we demonstrate the two benefits of viewing call production as signalling both caller goal (which determines call type) and caller arousal (which affects within-call-type variation). Such a framework can explain first, why a single class of calls is apparently given in multiple contexts, and, second, why some species have larger call repertoires than others. Previous studies have noted links between sociality and repertoire size, but have not specified exactly why animals living in societies that are more complex might require a greater number of differentiated signals. The caller-goal framework potentially clarifies how social complexity might favour call diversification. As social complexity increases, callers may need to elicit a larger number of distinct behaviours from a wider range of distinct audiences.

Call order within vocal sequences of meerkats contains temporary contextual and individual information

“…First, the over-representation of repetitions of the same call type could allow for individual-specific repetition patterns before transitioning to the next call type. Secondly, consistent differences in arousal state between individuals (reviewed in [ 28 , 29 ]), which may reflect differences in personality [ 30 , 31 ], could result in some individuals moving up or down the risk level much quicker than others. Closely related to this, it is possible that although all individuals produce all six types of sentinel calls, the rate of using specific call types may differ between different individuals (e.g.…”

Acoustic variation of spider monkey (Ateles geoffroyi) contact calls is related to caller isolation and affects listeners’ responses

“…This phenomenon can be explained if we consider that when arousal level increases, subglottal pressure increases, which in turn leads to a decrease of F0 due to irregular vibrations in the vocal folds [30, 31]. However, the negative relationship between arousal level and whinny frequency (Hz) is opposite to what has been found for other mammals’ calls [7, 32, 33]. Hence, modeling spider monkeys’ vocal tract configurations (e.g., vocal folds tension: [34]) during the production of whinnies, and analyzing the relationship between a proxy of arousal level (e.g., breathing frequency) and whinnies’ acoustic variation, would help to identify whether an increase in caller’s arousal level is negatively related to whinny frequencies (Hz) [7, 35].…”

Why does the chimpanzee vocal repertoire remain poorly understood and what can be done about it?

“…However, as more studies are conducted with respect to context-specificity, the idea that chimpanzee vocalisations only convey emotion is difficult to sustain (Schamberg et al 2018). Chimpanzees do indeed have a number of context-specific calls in their vocal repertoire.…”

“…Given the contexts of emission mentioned above, this however seems unlikely. What seems more likely is that call type may delineate a particular underlying goal, as Isaac Schamberg, Roman Wittig, and I recently suggested (Schamberg et al 2018). Hoos as a call type, for example, may function to keep the current party of chimpanzees together.…”

“…In dense forest habitat, individuals 20 m apart often cannot see each other and thus visual and other cues that could provide additional context become unreliable. Thus, if hoos emitted during resting or travelling carry acoustic markers of the rest or travel context, receivers invested in remaining with the signaller, can act appropriately (Schamberg et al 2018).…”