Dr. Matthew Mason: Further Information...
University Physiologist Tel: +44 (0)1223 333829, Fax: +44 (0)1223 333840, E-mail: mjm68@cam.ac.uk
Mason, M.J. (2013) Of mice, moles and guinea-pigs: functional morphology of the middle ear in living mammals. Hearing Research 301: 4-18.
The middle ear apparatus of mammals has been examined over the years by palaeontologists, zoologists and physiologists, but as is the case in many branches of science, there has not been a great deal of cross-over between these different groups of researchers and the literature has tended to remain distinct. In this paper, I attempt to bring the results of experimental and evolutionary studies together to form a more coherent view of how and why the middle ear differs in different groups of living mammals. Although I consider a wide range of mammals, I concentrate on the microtype mammals such as mice and rats, subterranean mammals such as moles and the Ctenohystrica rodents, a group including guinea-pigs and chinchillas.
Mice and rats are widely used in experimental studies, often as control groups. As discussed by Fleischer (1978) in his influential paper on mammalian middle ears, these microtype species are characterized by the presence of an orbicular apophysis of the malleus. I show here that these animals tend to have small middle ear structures for their body size, but the size-relationships between the structures within a given middle ear are relatively normal. One exception is the malleus which is heavier than expected due to the addition of the orbicular apophysis, the function of which remains enigmatic. The use of microtype species as "unspecialized" control groups is criticized, as are recent developmental studies which have mis-identified the orbicular apophysis as the lateral process of the malleus (doubtless by analogy with non-microtype human ears). It is the orbicular apophysis, not the lateral process, which is a second branchial arch derivative!
I then review the nature of the middle ear apparatus in moles and golden moles. These animals are unusual in several respects: many have connected middle ear cavities which might potentially act as pressure-difference receivers, something otherwise unique among mammals. Others have massively enlarged middle ear ossicles which may be specialized towards inertial bone conduction. The loss of middle ear muscles is common among subterranean mammals, and a new functional explanation for this is proposed.
Finally, I consider the ears of Ctenohystrica rodents, the group including guinea-pigs and chinchillas which are widely used in experimental studies of hearing. I show that the middle ears of these animals are unusual in a number of respects including the possession of a peculiarly-shaped malleus, fused malleus and incus, reduction or loss of the stapedius muscle and a synovial stapedio-vestibular articulation. These features can all be interpreted as being related to the low-frequency hearing of these animals, but together they comprise a unique form of middle ear apparatus referred to here as the "Ctenohystrica type". The unusual nature of the middle ear in these animals raises questions about their usefulness as models of human hearing.
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