Ontogeny of Ultrasonic Vocalizations in the Woodmouse (Apodemus Sylvaticus L.). I: Temporal Organization

Abstract

Abstract We have studied the ontogeny of ultrasonic vocalizations in the woodmouse, Apodemus sylvaticus, a high vocalizing palearctic murid species. Vocalization and behaviour were recorded as the pups were daily isolated for three minutes at ambiant temperature in two different experimental conditions: a plus-maze and a cup. The temporal organization of the vocalizations was analysed using log survivor functions of the intervals between the calls. These functions provided a reliable bout criterion indicating that ultrasound production by the youngest pups was clustered in bouts. Ontogenetic changes in the ultrasound production appeared different when measured in calls or in bouts per time unit. The call rate decreases from PND 11 onwards, whereas the bout rate remains elevated up to PND 15. Ultrasound production and locomotor activity arc associated from PND 11 onwards. The high ultrasound production by two-week old pups in the plus-maze is related to a very high locomotor activity in this experimental condition. We have identified four different steps in the ontogeny of ultrasonic vocalisations in the woodmouse: 1) Just after birth, the pups emit very few ultrasounds. 2) Around PND 3, there is a tremendous vocal output: the pups vocalize in a tonic mode, almost continuously. 3) The tonic mode goes gradually into a phasic mode in which ultrasounds are clustered in bouts; the total production decreases as the pups grow up. From PND 11 onward, most of the calls are produced during locomotion. 4) Around PND 15 there are mainly single calls, like in the adults. The very intense and frequent calls emitted during the second step are known to be distress calls. Since their production is accompanied by a high muscular activity, that might also delay cooling of the pups. The calls emitted by two-week old pups might serve as contact calls increasing the cohesion between the pups at the time of the first excursions out of the nest. In this way, these calls might diminish the predation risk and would facilitate a possible protection or retrieval by the mother.