gas exploration and offshore mining, have resulted in concerns about potential impacts on cetaceans.

Two species of particular interest are the resident, coastal Indian Ocean humpback dolphin Sousa plumbea

and the migratory humpback whale Megaptera novaeangliae. As the two species belong to different hearing

groups (high-frequency odontocetes vs low-frequency mysticetes), we review the available literature

on their differing hearing mechanisms (physiology), specialities and sensitivities here as there is a lack of

data available for these two species in the Indian Ocean region. The reviewed information included audiogram

data, species-specific frequencies and sensitivity ranges, ear morphology and adaptations for hearing

in their respective groups where available. For odontocetes, most information stems from animals

under human care, while for mysticetes bioacoustic measurements, like audiograms, are more difficult

to access, resulting in a lack of data on hearing for M. novaeangliae. Our review highlights an absence

of baselines upon which to measure future impacts from anthropogenic developments in the Indian

Ocean and we suggest future work to address this.

Our work is not only timely in view of the planned anthropogenic developments in the Indian Ocean,

but also has wider implications in the global context as cumulative impacts on cetaceans grow due to

increased international demand for resources and associated Ocean Economy developments.