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| Risso's dolphin during a physiological hearing test. |
As a tool to study hearing, we use auditory evoked potentials, or AEPs. This involves measuring the brain's response to sound. In cetaceans, just as with humans, we can do this with sensors on the surface of the skin. It is the same way that hearing is tested in human infants.
Below, we describe a few projects in which we're using AEPs to measure hearing in different species of whales and dolphins. We travel to do this so we can collaborate and investigate species that we don't normally get to see. They may be deep water species, or live a different part of the world. In all, by finding out more about these animal's hearing, we hope to learn how they find food, communicate, cope with a changing environment, and may be impacted by human-produced noise.
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Horta, Azores, March-April, 2013
Hearing is a common and key sense for many marine animals, and is fundamental to many critical vertebrate behaviors. Yet, there has been little investigation of hearing or sound responses in most marine invertebrates. It is particularly surprising that hearing in the cephalopods (octopus, squid, cuttlefish, nautilus) have not been well investigated since they have sophisticated behaviors, are a significant component of the ocean’s biomass, and play a critical role in many ecosystems both as predator and prey. My recent work demonstrated that sounds stimulate physiological ‘auditory’ responses in one squid species, the inshore Loligo pealeii, providing the first strong evidence that squid can hear. This is a previously unsupported sensory modality which may influence squid (and invertebrate) fisheries and ecology. Despite physiological responses, it is unclear how squid may use sound, if they behaviorally respond, and whether this sensory modality occurs in other species. Addressing these questions is necessary to understand the function and importance of hearing in this crucial marine invertebrate.
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| Loligo pealeii, a squid that detects sound. |
This work will address the broader relevance of cephalopod hearing by comparatively investigating the acoustic physiological response range and sensitivity of a squid species with very distinct habitat and ecological characteristics: the neritic L. forbesi.
L. forbesi maintain a key niche as prey for cetaceans and fishes, and predator to smaller fishes and invertebrates (Porteiro 1994). L. forbesi hearing tests will be conducted in the Azores where access will be coordinated with the assistance of local fishermen and collaborator Pedro Afonso (Univ. of the Azores, Portugal). Pedro is a postdoctoral fellow with the University of the Azores, Institute of Marine Research.
From WHOI, we are working with Kakani Young. Kakani is a posdoctoral researcher at WHOI who specializes in experimental fluid dynamics, biogenic mixing, and propulsive mechanisms of marine organisms.
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| The smaller, inshore squid during a physiological hearing test. |
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Taiwan, April-May, 2012
We have begun another field season. Aran is in Taiwan studying the hearing of Risso's dolphins (see the figure above). He's working at Farglory Ocean Park for the better part of two weeks. This is part of a larger project investigating the auditory diversity of cetaceans. By in large, almost all our hearing data has come from just a few "representative" species such as the bottlenose dolphin. But there are dolphins and whales of all shapes and sizes. Here, we're investigating how the Risso's dolphin (Grampus griseus) hears. Dolphins generally receive sound through their lower jaw, just as our pinnae gathers and funnels sound to our middle and inner ear. The Risso's has a unique shaped head, with a blunt rosturm and melon (forehead) with a groove down the middle. We are hypothesizing that there will be subtle differences in how the Risso's dolphin receives sound. If this is true, it means they might use sound, and be affected by human-produced noise, somewhat differently than other dolphins. That's why it's important to investigate hearing diversity and how different species hear.
For this Risso's dolphin project, Aran is working with Darlene Ketten (also from WHOI), Wei-Cheng "Jack" Yang (National Chiayi University), Ula Yu (National Taiwan University), and I-Fan Jen (Farglory).
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| Finless porpoise (left) and a bottlenose dolphin. Notice the clear anatomical differences in the head and lower jaw. Both are vital for sound production and hearing. |
Wuhan, China, 2010
This was one of our first projects to investigate the differences in how toothed whales and dolphins hear. So, we started with a highly divergent species, the Yangtze River finless porpoise (Neophocaena phocaenoides). To test the animal's hearing, we took it out of the water. We presented sounds using a special speaker (transducer) which allowed us to pinpoint sound entry at different locations on the animal's head and body. Using AEP hearing tests, we could then evaluate differences in hearing ability based on sound presentation location. We could then compare these "best" hearing locations to CT scans of the porpoises (i.e., compare the physiology to the anatomy). We also investigated how fast porpoises can hear and whether they hear porpoise clicks better than dolphin clicks (turns out they do!).
For this study, we worked at the Institute of Hydrobiology in Wuhan China
