Sources of variation in the long-distance vocalizations of spotted hyenas
It has long been recognized that vocal signals communicate information about the age, sex and affective state of callers. However, the mechanisms by which these types of information are communicated are less well understood. Here we investigated variation in the acoustic properties of the long-distance vocalizations, called 'whoops', emitted by free-living spotted hyenas, Crocuta crocuta. Specifically we investigated whether the fundamental frequency, length and rate of whoops provide information about the caller's age, sex and/or level of arousal. We determined the latter by contrasting whoops emitted spontaneously with whoops emitted during periods of social excitement, when callers typically also exhibited visual signals associated with heightened arousal. We found that the minimum fundamental frequency of a whoop provides reliable information about the caller's general age and, for adult callers, information about sex as well. The vast majority of adult male whoop bouts were emitted spontaneously, but juveniles and adult females produced many of their bouts during periods of social excitement. Although context did not significantly affect the whoop bouts of adult females, juvenile bouts emitted during social excitement had higher maximum fundamental frequencies, greater proportions of asymmetric whoop subtypes, and reduced inter-whoop intervals. By reducing the inter-whoop intervals of bouts, juvenile hyenas significantly increased the likelihood that conspecifics would respond to whoops by approaching the caller or its social companion. Peak fundamental frequency and the relative abundance of whoop subtypes did not appreciably affect response. We discuss the potential functions of whooping by juvenile and adult hyenas in light of these findings.