Ultrasonic Behaviour of Apodemus sylvaticus and A. flavicollis

Differences in ultrasonic behaviour were found between Apodemus sylvaticus (L.) and A. flavicollis (Melch.). The calls ofA. sylvaticus were detected mainly at about 70 kHz, those of A. flavicollis at 40-50 kHz. The two species also differed in the behavioural contexts in which ultrasounds were commonly emitted. When single animals explored their home cage or a new, clean cage A. sylvaticus readily emitted calls whereas A. flavicollis seldom did. Established groups of A. sylvaticus emitted many calls when exploring, A. flavicollis groups emitted fewer detectable calls than A. sylvaticus groups but more than when exploring singly. Within the groups A. flavicollis produced calls mainly in association with social encounters (approaching or leaving each other, and contact behaviour), whereas in A. sylvaticus the calls did not appear to be especially associated with social encounters, but often seemed to be emitted by an animal moving about alone. Finally, in social situations, the two species differed in the status of the animals apparently producing most of the ultrasounds- In A. sylvaticus, ultrasonic calls appeared to be emitted by the more aggressive animal of a pair and more often by the more dominant animals of the groups than by other members; in A. flavicollis ultrasounds appeared to be emitted more by subordinate individuals.