International Journal of Comparative Psychology

McCowan and Reiss first reported the “thunk” sound of the Atlantic bottlenose dolphin ( Tursiops truncatus ) during separations and discipline behavior of mother-calf dyads. This sound has been previously described as a wide-band, low frequency contact call, however the harmonic structure of this sound is more variable than previously described. Based on preliminary observations of the graded structure of thunks within our data set, we investigated the directionality of thunks with energies at higher frequencies. We recorded a bottlenose dolphin mother with her calf during the first 30-days of life, and analyzed thunk production during separation and discipline contexts. Two classifications of the thunk sound were compared to determine calf response and whether location cues were embedded in the higher harmonics of one of the thunk types. The mother oriented towards the calf significantly more during production of both thunk types during separation and discipline contexts. This sound may have potential directional information within the harmonic structure; however, we could not draw that conclusion based on our findings. Therefore, we present data here that indicates a graded structure to the harmonics of thunk sounds.