, 2009). Yeon et al. (2011) showed that feral cats Felis catus produce vocalizations with higher energy distributions, F1 and peak frequencies in affiliative compared with agonistic situations. However, the ‘affiliative’ situation in this case was an approach by a familiar caretaker, and it is not clear how positive

or intense this experience was for feral cats. Pond et al. (2010) found spectral differences between vocalizations produced in two situations of similar arousal and different valence using Hidden Markov Models, but the shifts in individual vocal parameters are not detailed in this study. There is also evidence for a shift towards low frequencies during positive situations. Jovanovic & Gouzoules (2001) and Scheumann et al. (2007) showed that infant Rhesus monkeys and gray mouse lemurs PLX4032 produce different kinds of calls during positive contexts (‘coos’ and ‘purr’ respectively) compared with negative contexts. ‘Coos’ and ‘purr’ are both characterized by low frequencies. Fichtel, Hammerschmidt & Jürgens (2001) found that in squirrel monkeys Saimiri sciureus, call level of ‘negativity’ (aversion) is generally

correlated with longer duration, higher F0 contour, energy distribution, peak frequency, dominant frequency band contour, wider frequency range, and more noise. However, it is not clear how much of this variance is explained by arousal or valence. Tame and aggressive silver foxes Vulpes vulpes differ in their reactions to humans; tame foxes show a decrease and aggressive foxes buy EPZ-6438 an increase in peak frequency during approach (Gogoleva et al., 2010a), suggesting that low-peak frequencies reflects positive emotions. Soltis et al. (2011) found that African elephant rumbles produced in a positive situation have lower F0, H1–H2 and narrower F0 range than those produced in a negative situation. However, because the shifts in these parameters occurring between the neutral and positive contexts were similar (i.e. same direction),

yet less intense, than the shifts exhibited between the neutral and negative contexts, the authors suggested that their results Sclareol were more consistent with an effect of emotional arousal than valence. Similarly, the variations between contexts in vocal parameters found by Collins et al. (2011) in Weddell seals were more consistent with the expression of emotional arousal. Therefore, the only parameter shift that is supported by three studies, without any opposite shift, is duration, with positive situations characterized by shorter vocalizations (Table 4). There are some good examples in the literature of vocal expression of positive emotions: purr, laughter and rat ultrasonic 50-Hz vocalizations. Felid purrs are low pitched vocalizations (mean F0 = 26.