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Introduction
Because of its functional and phylogenetic significance, the middle ear has occupied far more of the attention of zoologists than this tiny region of the body would, at first glance, appear to merit. Although all mammals have three middle ear ossicles, a defining characteristic of the class, middle ear morphology otherwise differs substantially both between and within mammalian orders.
Middle ear structures are particularly variable among the Rodentia and have long been used in rodent taxonomy. Features compared between groups include malleus morphology (Tullberg, 1899; Carleton and Musser, 1984), number of middle ear septa (Moore, 1959), stapedial arterial supply (Bugge, 1985) and the relationships between the bony components of the middle ear cavity (Lavocat and Parent, 1985). Although morphological phylogenies of living rodents have largely been supplanted by the molecular, the bony structures of the middle ear retain taxonomic value because of their preservation as fossils.
Rodents are central to current experimental studies of ear function, the mouse (Mus musculus), guinea pig (Cavia porcellus), chinchilla (Chinchilla lanigera) and gerbil (Meriones unguiculatus) representing model species of particular importance. The choice of these rodents is, of course, largely based on convenience: apart from ease of maintaining captive colonies, the relatively large middle ear cavities of the guinea pig, chinchilla and gerbil greatly facilitate surgery to expose the cochlea and other structures. To what extent their ears are representative of rodents as a whole, or mammals in general, often remains unaddressed.
Following a brief functional overview, this chapter will introduce the anatomy of the middle ear and then review details of its morphology in each of the major rodent clades. This is followed by a consideration of rodent ear evolution, including a discussion of the likely adaptive purposes of some of the features which distinguish the various groups. It is hoped that zoologists will gain some functional insight to help in the evolutionary interpretation of middle ear morphology, while the anatomical data provided may prove useful in the comparison of experimental results from different species, and in the consideration of what may safely be extrapolated to other mammals.
The middle ear structures of nine species of golden moles (family Chrysochloridae) were examined under the light microscope. Auditory structures of several of these species are described here for the first time in detail, the emphasis being on the ossicular apparatus. Confirming previous observations, some golden moles (e.g. Amblysomus species) have ossicles of a morphology typical of mammals, whereas others (Chrysospalax, Chrysochloris, Cryptochloris and Eremitalpa species) have enormously hypertrophied mallei. Golden moles differ in the nature and extent of the interbullar connection, the shape of the tympanic membrane and that of the manubrium. The stapes has an unusual orientation, projecting dorsomedially from the incus. It has been proposed that hypertrophied ossicles in golden moles are adapted towards the detection of seismic vibrations. The functional morphology of the middle ear apparatus is reconsidered in this light, and it is proposed that adaptations towards low-frequency airborne hearing might have predisposed golden moles towards the evolution of seismic sensitivity through inertial bone conduction. The morphology of the middle ear apparatus sheds little light on the disputed ordinal position of the Chrysochloridae.
Some genera of golden moles are known to possess enormously hypertrophied auditory ossicles. These structures have been implicated as potentially mediating a form of inertial bone conduction, used by the golden mole to detect seismic vibrations. A simple model of ossicular inertial bone conduction, based on an existing model of the human middle ear from the literature, was used in an attempt to examine vibrational sensitivity in these animals. Those golden moles with hypertrophied ossicles are predicted to possess a sensitive inertial bone conduction response at frequencies below a few hundred hertz, whereas species lacking these middle ear adaptations are predicted to have a far less sensitive response in this ecologically important frequency range. An alternative mode of inertial bone conduction in golden moles, potentially conferring sensitivity to vertically-polarized seismic vibrations such as Rayleigh waves, is proposed. Certain behaviours of golden moles described in the literature are interpreted as augmenting seismic sensitivity.
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