Good vibrations by the beach boys: magnitude of substrate vibrations is a reliable indicator of male grey seal size

Communication via substrate vibrations can convey information on conspecific presence, individual quality, group cohesion and/or allow for predator avoidance. Although studies have identified that various species use this modality, few studies on mammalian taxa have investigated whether the information contained in substrate vibrations is a reliable indicator of resource-holding potential (RHP). The grey seal, Halichoerus grypus, breeding colony at Donna Nook, U.K., is part of a limited geographical region where the Body Slap (BS) behaviour is performed during male-male conflicts. This behaviour is thought to have a mechanical component. We examined whether the magnitude of the BS substrate vibrations contained reliable information on male mass and size as measures of RHP, and whether reliability varied across environmental conditions. To test this, we deployed seismometers during the breeding season that recorded continuous seismic data over a frequency bandwidth of 0.03-500 Hz. Locations and times of BS events performed by individual males were recorded, matched with the seismic data, and a distance-corrected magnitude was calculated for each event. Our results demonstrate the BS generates a stereotyped seismic signature measurable up to 126.3 m away. We found a positive correlation between the maximum and mean magnitudes of the substrate-borne vibrations and a male's length. Dampness of the sand substrate had no effect on magnitude. Results of this study confirm that the maximum magnitude substrate vibrations generated by the BS behaviour is an indicator of male size and that the substrate-borne vibrations are reliable across varying environmental conditions.