Encoding of Arousal and Physical Characteristics in Audible and Ultrasonic Vocalizations of Mongolian Gerbil Pups Testing Common Rules for Mammals

SIMPLE SUMMARY: Mammals provide information about their emotional state and physical characteristics through vocal signals. Due to a similar vocal production apparatus across mammals, common rules for encoding this information are proposed. With increasing age/body weight, it is suggested that mammals utter elongated calls of lower frequency. With increasing arousal, it is proposed that mammal vocalizations become higher in frequency and longer in duration. However, in recent years an increasing number of mammalian species have showed discrepancies with these common rules. In this study, we investigated whether developing Mongolian gerbil pups fulfill the proposed common rules by recording vocalizations in a low and high arousal condition in four age groups. We discovered they produce three call types covering the audible (<20 kHz) and ultrasonic (>20 kHz) frequency ranges. Interestingly, the call types differed in the encoding of arousal state as well as physical characteristics and only partly fulfilled the common rules suggested for mammals. Thus, our results show that divergent encoding patterns do not only differ between species but also between call types within a species, indicating that rules to transmit information can be shaped by social, developmental, and environmental factors or different production mechanisms for vocalizations in certain frequency ranges. ABSTRACT: In mammals, common rules for the encoding of arousal and physical characteristics of the sender are suggested based on a similar vocal production apparatus. In this study, we want to investigate to what extent vocalizations of developing Mongolian gerbil pups fulfill these rules. We recorded vocalizations of 28 Mongolian gerbil pups in four developmental stages using a separation paradigm, suggested to induce different arousal levels. For low arousal, a pup was placed in an arena isolated from its siblings and parents; for high arousal, the pup was additionally stressed through the simulation of a predator. An unsupervised cluster analysis revealed three call types: ultrasonic (USV), audible vocalizations (ADV), and transitions between both (USV-ADV). The USV and USV-ADV rate showed an age-dependent decrease, contrasting an age-dependent increase for ADVs. Vocal correlates for the encoding of arousal were found for USVs and of physical characteristics for USVs and ADVs. However, the pattern of encoding these cues differed between call types and only partly confirmed the common rules suggested for mammals. Our results show that divergent encoding patterns do not only differ between species but also between call types within a species, indicating that coding rules can be shaped by socio-ecological factors or call type specific production mechanisms.