Call perception

When humans hear a word or phrase they tend to infer meaning from the arbitrary conventional relationship that exists between words and external referents (semantics-related meaning), and from the context in which the utterance was made (pragmatics-related meaning).

Objectives of this thesis concerning proximate mechanisms of call perception were thus to investigate whether green monkeys attribute meaning to their alarm calls, and whether contextual cues in the form of previous experience influence this process.

Semantics

In Chapter 4 I show that green monkeys were more likely to respond to leopard-chirps than to snake-chirps by climbing more than 2m up into a tree, a typical leopard avoidance behaviour as established previously with predator-model presentation experiments. This finding suggests that green monkeys may be capable of categorical perception, although further study using a habituation-dishabituation paradigm (described in Fischer 2013) is required to assess how intermediate vocal structures are perceived. Categorical perception was once thought to be unique to humans (Liberman and Pisoni 1977), but it is now recognised as being widespread within the animal kingdom (insects: Wyttenbach et al. 1996; amphibians: Baugh et al. 2008;

birds: Nelson and Marler 1989; mammals: Kuhl and Miller 1975; Morse and Snowdon 1975). An interesting finding is that while Barbary macaques respond to conspecific's graded calls

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categorically (Fischer 1998), chacma baboons ignored intermediate call variants and may pay more attention to contextual than acoustic cues (Fischer et al. 2001b). How different sources of information are integrated is discussed under the heading of pragmatics just below.

That green monkeys respond appropriately to leopard-chirps could be interpreted as green monkeys having learnt to associate leopard-chirps with leopard presence, or equally it could be explained by affect-conditioning (Owren and Rendall 1997) if green monkeys experience a more fearful state when hearing leopard-chirps than snake-chirps and if tree-climbing behaviour is related to the degree of fear experienced. In some species, studies have shown that subjects look up in response to aerial alarm calls (Saguinus fuscicollis and S. mystax, Kirchhof and Hammerschmidt 2006; Callicebus nigrifrons, Cäsar et al. 2012). This is more supportive of a representational interpretation, and this is in keeping with the finding that aerial alarm calls often show the highest degree of predator-specificity (Zuberbühler et al. 1997; Fichtel and Kappeler 2002; Wheeler 2010). For calls with lower predator-specificity as is often the case with terrestrial/general alarm calls, learnt associations between calls and motivational states, and/or the incorporation of contextual information (Wheeler and Fischer 2012) may well be a more adaptive solution. Experimentally manipulating the salience of different predator types has the potential to offer more conclusive support for representation-based associative learning (Evans 1997); for example if subjects associate a leopard alarm call with a leopard rather than with a state of fear, habituating subjects to leopards should subsequently reduce responses to the leopard-associated call. A study like this was carried out on Diana monkeys (Zuberbühler et al.

1999a) and supported a representational conclusion; longer temporal separation of habituation and stimulus presentation would, however, be beneficial to more strongly negate an affective explanation.

Vervet barks also provide cues to caller identity at the individual and species level, and responses to playback experiments indicate that adult males distinguish between known and unknown conspecific calls, and conspecific and heterospecific calls (Chapter 4). These are differentiations that are highly relevant in a territorial species living in multi-male groups, in keeping with the prediction that individuals' social attention corresponds to the specifics of the species' social organisation (Maciej et al. 2013). Earlier studies suggest that vervets are also capable of more complex vocal recognition, recognising third party relationships (Cheney and Seyfarth 1980) and which neighbouring group a caller belongs to (Cheney and Seyfarth 1982a).

Pragmatics

Looking at the incorporation of contextual cues, Chapter 4 also shows that priming condition did not have a strong effect on green monkeys' immediate responses to chirp

General Discussion

97 playbacks, but did influence the amount of time that the individual stayed in a tree before descending to the ground again. Bees (Apis mellifera) and ants (Lasius niger) also make use of private contextual information (that is, previous experience) when making foraging decisions (Grüter and Ratnieks 2011; Grüter et al. 2011), but incorporate such cues in immediate rather than subsequent responses. That foraging decisions are likely less urgent than responses to alarm calls may have an influence on how contextual cues are incorporated within decision-making events, but private contextual cues also influence the immediate responses of Diana monkeys (Zuberbühler 2000c) and putty nosed monkeys (Cercopithecus nictitans, Arnold and Zuberbühler 2013) within predator-contexts. In these last two experiments priming conditions were given just minutes or seconds prior to the test stimulus, however, which may have meant the primed information was more easily retrieved than in my study when priming occurred up to an hour before. Alternatively, the findings of these studies could be viewed within the framework of affect-based cognitive bias, whereby priming induced a change in subjects' emotional state which subsequently influenced the processing of test stimuli; I discuss affect-based cognitive biases in more detail below, but an important point to make here is that, within this scenario, duration between prime and test stimuli would likely be highly relevant. These descriptions highlight that whilst the incorporation of contextual cues is likely widespread, little is known about species and individual differences in the type of contextual cues that are incorporated and the level of automaticity at which they are processed.

Summary

Learnt associations likely underlie green monkeys' and vervet monkeys' responses to conspecific chirps and barks, which may be based upon a representation of the stimulus or affect-conditioning. Context also influences later response behaviour. As with call production, there are still many unanswered questions concerning the cognitive processes involved in information processing, and how this interacts with more basal emotional states. A theoretical starting point for future investigations is that the meaning inferred by a signal receiver and their subsequent responses could be viewed as two separate processes, and that context could play a different role in both (Fischer 2013). I propose that, in addition to providing a framework for understanding call production in animals, appraisal theories could help to disentangle the processes involved in the perception of, and responses to, these calls.

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