The cetacean's auditory system is characterised by a series of unique morphological adaptations: one of the most interesting is the capacity to select the frequencies that allow detailed discrimination in acoustic images through the auditory channels that act as frequency filters. In a healthy organism, the selection of frequencies by the ear (and therefore the selection of acoustic signals that are produced) is evolutionary and directly related with the specific use of its habitat and therefore is characteristic in every cetacean species. On the other hand, the sensitivity of the ear towards a few frequencies permits measuring the physical and pathological state of the auditory system of an individual, and to estimate its acoustic capacity for living in its habitat.
The intra- and interspecific diversity of acoustic signals - there are around 80 cetacean species, every one with a complex acoustic repertoire - complicates the analysis in terms of the extraction of the basic components that contain the necessary information for survival of an individual or a population, and therefore considerably limits our capability of estimating the effects of a contaminating sound source.
Each of the species that form the order of cetaceans introduces a unique acoustic repertoire, directly related to the habitat where it has evolved over the course of millions of years. It is clear that in order to detect its prey, a coastal species needs to extract the precise details of its surroundings at short range, while the absence of these surroundings requires that pelagic species (that live in open sea) obtain information about the presence of schools of fish at medium of large distances. Without doubt, all odontocetes (toothed whales) share a similar mechanism for sound production, that includes the propulsion of air through the nasal air conduits, and its passage through the vocal lips situated in the superior part of the head. This air is recycled during most of the dive and it allows them to vocalise, with the goal of echo-localisation or communication, depending on the social context in which they find themselves.
The absence of vocal chords is accompanied with another particularity, unique among mammals, namely that of not using an external auditive conduit for hearing. They receive the acoustic vibrations through their jaws that direct the information directly to the middle and inner ear, where it is processed before arriving to the brain.
We invite you to explore with us and to listen to the peculiar acoustic characteristics of each species over the course of the months in the "Cetacean of the Month" section.