Ultrasonic Reaction of Bank Vole Males to the Presence of Females Varying in Hormonal Activity

Rodents are known to emit ultrasounds during social interactions. Despite the evidence that ultrasonic vocalizations are emitted during sexual encounters and may play a certain role in sexual selection, only a few studies have investigated this phenomenon, mainly in laboratory rodents. We analysed the ultrasonic calls of bank vole (Myodes glareolus) males from an outbred colony in the presence of females differing in hormonal activity. Sexually experienced males were tested during interactions with naive, ovariectomized, pregnant and post-partum oestrous females. We found that the males' ultrasound vocalizations depend on the phase of the bank vole females' reproductive cycle. During encounters with post-partum oestrous females the males emitted significantly more ultrasounds, and the total duration of ultrasound vocalization was longer. The presence of post-partum oestrous females also influenced the type of ultrasounds: the most typical constant-frequency sounds were significantly shorter, and two additional types of ultrasounds were presented during interactions with females ready to mate: U-shaped frequency-modulated sounds and frequency-modulated upsweep sounds. To our knowledge this is the first evidence that male voles of the subfamily Arvicolinae emit different types of ultrasounds in the presence of females depending on their reproductive stage. We suggest that these ultrasounds may be employed as an attractant during reproductive behaviour and that they are potentially an element of sexual selection.