Characteristics of Hearing and Echolocation in Under-Studied Odontocete Species

Abstract

All odontoctes (toothed whales and dolphins) studied to date have been shown to echolocate. They use sound as their primary means for foraging, navigation, and communication with conspecifics and are thus considered acoustic specialists. However, the vast majority of what is known about odontocete acoustic systems comes from only a handful of the 76 recognized extant species. The research presented in this dissertation investigated basic characteristics of odontocete hearing and echolocation, including auditory temporal resolution, auditory pathways, directional hearing, and transmission beam characteristics, in individuals of five different odontocete species that are understudied. Modulation rate transfer functions were measured from formerly stranded individuals of four different species (Stenella longirostris, Feresa attenuata, Globicephala melas, Mesoplodon densirostris) using non-invasive auditory evoked potential methods. All individuals showed acute auditory temporal resolution that was comparable to other studied odontocete species. Using the same electrophysiological methods, auditory pathways and directional hearing were investigated in a Risso's dolphin (Grampus griseus) using both localized and far-field acoustic stimuli. The dolphin's hearing showed significant, frequency dependent asymmetry to localized sound presented on the right and left sides of its head. The dolphin also showed acute, but mostly symmetrical, directional auditory sensitivity to sounds presented in the far-field. Furthermore, characteristics of the echolocation transmission beam of this same individual Risso's dolphin were measured using a 16 element hydrophone array. The dolphin exhibited both single and dual lobed beam shapes that were more directional than similar measurements from a bottlenose dolphin, harbor porpoise, and false killer whale.