, 2004). Therefore, motivation levels should be taken into account in the interpretation of context-related changes in vocal parameters. In the case of non-linear phenomena, the results are not consistent. According to Table 4, harmonic-to-noise ratio decreases, spectral selleck products noise increases (more noise), but entropy decreases (more pure tone vocalizations) with arousal. The increase in spectral noise (Table 4)

is contradicted by Gouzoules & Gouzoules (1989), which showed that pigtail macaques Macaca nemestrina produced less noisy and more tonal screams during contact aggression (high presumed arousal) than during non-contact aggression. Similarly, Blumstein & Chi (2011) showed that yellow-bellied marmots Marmota flaviventris selleck inhibitor with more faecal glucocorticoid metabolites, indicating higher stress levels, produced less noisy calls (measured as entropy). Therefore, it seems that non-linear phenomena might increase or decrease with arousal depending on species or particular contexts and are not good indicators of arousal. Vocal correlates of valence have been considerably less studied than arousal (Table 4). There are only a few studies in which authors compared vocalizations produced in negative and positive situations. There are two main reasons for this lack of research. One is the difficulty to find calls

that are produced in positive situations (but see exceptions of positive vocalizations later). Because vocal correlates of negative states selleck kinase inhibitor signal urgency (e.g. alarm calls) and need (e.g. infant begging calls), these vocalizations are far more common than positive vocalizations, and probably emerged earlier during evolution. The evolution of positive vocalizations could have been facilitated later by the increased

importance of communication within social groups (Brudzynski, 2007). Expression of arousal can be studied by comparing vocalizations produced in negative situations that are characterized by varying degrees of arousal. By contrast, research on expression of valence must compare vocalizations produced in positive and negative situations that are characterized by a similar degree of arousal. This leads to the second reason for a lack of research on vocal correlates of valence; it is difficult to find situations of opposite valence, but similar arousal. Expressions of negative emotions (e.g. physiological, visual, vocal) are easier to study, because they are often more intense than expressions of positive emotions (Boissy et al., 2007). Therefore, it is difficult to find situations triggering positive emotions as intense as negative emotions. Because of this lack of research, knowledge on vocal correlates of valence listed in Table 4 is sparse. Some studies show a shift towards higher frequencies during positive situations.