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.