December 17, 2008. A research project supported by the BBVA Foundation and led by Michel André, director of the Laboratory of Applied Bioacoustics at the UPC (Universitat Politècnica de Catalunya) has developed the world's first portable system for measuring cetacean hearing sensitivity.
This audiogram measurement system facilitates in situ diagnosis of cetacean hearing loss, allowing assessments to be run on the survival chances of stranded animals without having to transport them to a laboratory. Researchers in Spain, the United Kingdom, France, the Netherlands and the United States have taken part in this BBVA Foundation project.
Cetaceans rank among the world's most imperiled species, due, among other reasons, to the noise produced by artificial sound sources. A number of problems have recently come to light which bear a direct relation to sound sources of human origin; among them, the growing number of cetacean deaths in collisions with boats, or the mass beaching of whales after military maneuvers. Oil and gas extraction too have added their share of noise pollution to the marine environment.
To date, the only way to measure cetaceans' hearing sensitivity was to remove them to a laboratory. However this complex process entailed serious risks for their survival, given their large size and the delicate state of health of stranded individuals.
Among the innovative features of the new portable system developed by Michel André's team with the aid of the BBVA Foundation are its electrical autonomy, measurement speed – just a few minutes suffice to detect any auditory lesions – and its ability to generate stimuli from 10 Hz up to 200 kHz, encompassing the entire human hearing range (20 Hz- 20 kHz) and, naturally, that of cetaceans.
This novel system can ascertain how the animals' brains react to sound signals, as well as measuring cetaceans' hearing sensitivity to certain frequencies by analyzing the evoked potentials registered through the top of the skull. When an animal hears a sound, its brain registers this vibration through an electrical impulse that can be detected with simple suction cup electrodes. These electrical pulses are called auditory evoked potentials or auditory brainstem responses (ABR), and incorporate a short latency time and duration. In the case of a rehabilitating stranded cetacean, this hearing analysis is vital in order to determine whether it can correctly use its biosonar system and thus evaluate its survival chances on release.
ELUCIDATING THE SPERM WHALE SONAR AND ITS DETECTION POWERS
Cetacean hearing systems are characterized by a series of unique morphological adaptations allowing them to pick up frequencies that their auditory channels translate into accurate acoustic images. At the same time, by measuring the ear's sensitivity to certain frequencies we can gauge the physical and pathological state of an individual's auditory system and thereby its acoustic ability to negotiate its habitat.
The diversity of sound signals - there are around eighty cetacean species, every one with a rich acoustic repertoire - complicates the task of extracting the key components that determine the survival of an individual or a population, and is a constraint on our ability to estimate the effects of contaminating sound sources.
The BBVA Foundation project coordinated by Michel André has developed analysis models for the sound signal sequences that sperm whales emit while foraging in the ocean deeps, which conclude that this marine mammal is capable of detecting a 25 cm squid at a distance of more than two kilometers by means of the repeat emission of sonar clicks. But the presence of intrusive, man-made sound sources can impair this capacity, potentially disrupting the species in its feeding activities – occupying 80% of its time since its ingestion needs are a ton per day – and destabilizing the entire food chain.
Each of the species making up the cetacean order has its own acoustic repertoire directly related to the habitat where it has evolved over the course of millions of years. In order to detect their prey, coastal species must be able to accurately trace the outline and details of short-range features, while the absence of such features in the high seas means the cetaceans that inhabit them (pelagic species) have more need of medium- and long-range data on the presence of fish schools. However, all odontocetes or toothed cetaceans share the same method of sound production, which includes the passage of air though their nasal conducts and its expulsion by specialized tissues known as phonic lips located in the upper part of the head. While the mammal is immersed, this air is recycled and allows them to vocalize for the purpose of social communication or echolocation, depending on the need at hand.
The absence of vocal chords is accompanied by another trait unique among the mammals: that of not using an external auditory conduct for the purposes of hearing. Instead, they pick up sound waves through their jaws which transmit the information directly to the middle or inner ear where it is processed then relayed to the brain.
The work of the BBVA Foundation project team will help develop bioindicators of the damage caused by human-produced noise in the marine environment, as a first step to combating this pollution source and establishing a balance between the conduct of human activities and the conservation of marine mammals.
Source: Fundación BBVA