Functional morphology of the ear in fossorial rodents, Microtus arvalis and Arvicola terrestris

Abstract

Functionally relevant features and parameters of the outer, middle, and inner ear were studied morphologically and morphometrically in two species of voles, smaller Microtus arvalis and larger Arvicola terrestris. The findings in these fossorial (i.e., burrowing) rodents with components of surface activity were compared with respective findings reported for taxonomically related muroid rodents representing the same size classes but different eco-morphotypes: obligate subterranean rodents (Ellobius talpinus and Spalax ehrenbergi superspecies) and generalized rodents (Mus domesticus and Rattus norvegicus). The ear in voles was characterized by traits reported for subterranean rodents. The eardrum was round, without a distinct pars flaccida, and had an area of 5.4 mm2 in M. arvalis and 9 mm2 in A. terrestris. The middle ear exhibited reduced goniale, enlarged incus nearly parallel to the manubrium of the malleus. The malleus-incus lever ratio amounted to 2.1 (M. arvalis) and 2.0 (A. terrestris). The malleus-incus complex weighed about 0.8 mg in both vole species. The stapedial footplate had an area of 0.3 mm2 in M. arvalis and 0.4 mm2 in A. terrestris. The cochlea had 2.3 coils in both vole species; the basilar membrane was 8.5 mm and 10.5 mm long in M. arvalis and A. terrestris, respectively. There were on average 1,030 (M. arvalis) and 1,220 (A. terrestris) inner hair cells, and 3,760 (M. arvalis) and 4,250 (A. terrestris) outer hair cells in the organ of Corti. In quantitative terms, all these (as well as some further) traits and parameters were intermediate (related to body size) between those reported for generalized rodents on the one hand and subterranean ones on the other. The sound transmission system of the ear seems to be best tuned to frequencies of about 8-16 kHz with a high-frequency cut-off at about 50-60 kHz. The ear of A. terrestris seems to be tuned to somewhat lower frequencies than that in M. arvalis. In this aspect as well as regarding hearing sensitivity (as judged from the mechanical transmission parameters), voles can be considered intermediate not only in their lifestyle but also in their hearing abilities between the subterranean rodents (mole-vole and blind mole-rat) and the surface dwellers (house mouse and Norway rat).

Gross anatomy and arterial vascularization of the tympanic cavity and osseous labyrinth in mid‐gestational bovine fetuses

“…A number of studies carried out on the vascularization of middle ear did not report the presence of stapedial artery; however, its presence was determined in this study and some other studies on certain mammal species like a mongolian gerbil (Rosowski et al, 1999), rats (Ortug, 2001;Yamamoto et al, 2003), fossorial rodents (Lange et al, 2004), and Talpid moles (Mason, 2006). Lange et al (2004) and Ortug (2001) observed that stapedial artery passing through intercrural foramen was surrounded by an osseous canal. Ortug (2001) also reported that the artery continued in such a canal in the middle ear cavity after exiting from the foramen.…”

“…Cochlea was determined to make 3.25-3.5 turns in bovine fetuses examined in the study, which was surprisingly more compatible with the results reported for guinea pigs, cats, and dogs than adult cattle. In addition, Lange et al (2004) reported that cochlear spiral number appeared not directly related to hearing ability, but when examined more closely, subterranean mammals were found to have more spiral formation than their epigeic relatives (rat, mouse etc.). Subterranean mammals were normally expected to have better hearing ability (Webster, 1966); however, spiral number was found higher in aboveground mammals both in this study and the previous ones (Wysocki, 2001), which supported the idea that hearing ability was not related to spiral number.…”

“…However, this artery was found to persist in bovine fetuses during the mid-gestation period in the study. It was reported to be permanent in rats (Bugge, 1970;Ortug, 2001;Yamamoto et al, 2003) and fossorial rodents like a common vole and water vole (Lange et al, 2004). Degeneration of stapedial arteries was rather typical for some rodents and humans, and vertebral artery and external carotid artery undertook their functions (Bugge, 1978;Govaerts et al, 1993).…”

“…Some of the studies regarding ear morphology of animals were carried out to reveal functional roles of ear especially in fossorial rodents (Lange et al, 2004;Lange and Burda, 2005;Mason, 2006), and some other studies (Pracy et al, 1998;Seibel et al, 2006a;Borin et al, 2008) made morphological comparisons to humans. To understand the anatomic problems and vascular diseases, certain techniques like radiography, ultrasonography, computed tomography, and magnetic resonance were applied to animals as well, and carnivores (e.g., cats, dogs) were frequently used for this purpose (Allgoewer et al, 2000;Russo et al, 2002;Eom et al, 2008).…”

Methods for Estimating the Volume of the Tympanic Bulla in the Big Hairy Armadillo Chaetophractus villosus (Mammalia, Xenarthra, Dasypodidae)

“…Some authors have associated the shape of the bulla of different mammalian species to idealised geometric figures in order to calculate the bullar volume from external measurements; such figures include, for example, the cylinder (Lange et al, 2004), the rectangular prism (Huang et al, 2002;Lange & Burda, 2005), or the elliptical cone (Schleich & Vassallo; Begall & Burda; Francescoli et al). However, a comparison between the different methods employed has never been performed, nor is available a detailed study of the relationship between this type of estimations and the real tympanic volume.…”