Function and acoustic structure of drumming in Mongolian gerbils (Meriones unguiculatus)

Drumming is a non-vocal auditory display producing airborne as well as seismic vibrations by tapping body extremities on the ground/objects. It is mostly described as an alarm signal but it is also discussed to signal dominance or mating quality. In this study, we investigated the function of drumming in Mongolian gerbils by comparing the occurrence of drummings produced during predator, inter-sex and intra-sex encounters. We tested 48 captive Mongolian gerbils in three experiments. In the predator experiment, subjects were confronted alone or with their cage mate to aerial and terrestrial predator dummies. In the inter-sex encounter experiments, known and unknown male-female dyads and in the intra-sex encounter experiments, known and unknown same-sex dyads were confronted. For both experiments, a dominance index was calculated for each subject based on the number of won and lost conflicts. All experiments were audio- and videotaped. The occurrence of drummings as well as associated ultrasonic vocalizations was noted and a multi-parametric sound analysis was performed. During the predator encounter, subjects produced drummings more often when tested alone than with the cage mate. During the intra-sex and inter-sex encounters, subjects drummed almost exclusively when encountering an unknown individual. Furthermore, in all experiments combinations of drumming and ultrasonic vocalizations occurred. However, a cluster analysis could not separate the ultrasonic calls to the respective experiments. Since drumming behavior occurred in all experiments and the associated ultrasonic vocalizations did not differ between experiments, we suggest that drumming is not related to specific behavioral contexts. Instead, we argue, supported by the finding that drumming was not used to warn conspecifics and was almost solely uttered to unknown conspecifics, that drumming reflects an increased arousal state of the animal.