Observations on mechanisms and phenomena underlying underwater and surface vocalisations of grey seals

Air and water constitute acoustic propagation media with very different physical properties. Consequently, mechanisms for air- and water-borne sound generation observed in terrestrial and aquatic animals are also significantly different. Notwithstanding, some amphibious species are capable of making efficient use of their vocal tracts in both environments. The present study investi- gates such a phenomenon on the example of grey seals Halichoerus grypus. Grey seals vocalise both underwater and above the water surface emitting variety of sounds. Both kinds of vocalisations were recorded using microphones and hydrophones, and analysed for their acoustic parameters which could be related to the underlying generation mechanisms. Also, video with synchronised underwater audio recordings were captured, allowing to link the acoustic phe- nomena with specific behaviour of the animals. The vocalisations are categorised into three groups based on differences in acoustic characteristics. Temporal and spectral parameters of the sounds belonging to each group are determined and discussed in terms of potential underlying generation mechanisms. Based on the adopted criteria surface vocalisations do not constitute a separate group but are categorised together with other tonal sounds pro- duced underwater. Other two vocalisation groups include pulsed sounds with different bandwidths, duration times, repetition rates and associated behavioural signatures.