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., Lin, C.C. & Narins, P.M. (2003) Sex differences in the middle ear of the bullfrog (Rana catesbeiana). Brain, Behavior & Evolution 61: 91-101.
The bullfrog has long been known to have sexually dimorphic eardrums, much larger in the male. In this paper, we sought to establish whether other components of the middle ear apparatus are similarly dimorphic. In particular, we were interested in the stapes footplate, because the ratio of tympanic membrane to stapes footplate areas (the "anatomical area ratio") is conventionally believed to represent an important component of the impedance-matching function of the middle ear. Although we anticipated that the male footplate might be larger than that of the female, so as to maintain a similar area ratio, we actually found a small but statistically significant difference in the other direction: males have slightly smaller footplate areas than females of similar body size, which only serves to exaggerate the difference in anatomical area ratios. No sexual dimorphism was found in the area of the otic operculum, which occupies the caudal half of the oval window in these frogs.
Combining the area ratio results with vibrometric velocity ratios from a previous study (Mason & Narins, 2002a), we calculate that the impedance transform ratio is much higher in male frogs than in females. Given that there are no obvious differences between the inner ears of male and female bullfrogs which might result in a significant difference in impedance, we conclude that models of impedance transformation based on simple, anatomical measurements may represent inadequate descriptions of middle ear function, in the case of these animals.
To access this article via the Brain, Behavior & Evolution website, please click here.
Please contact me if you would like to be sent a reprint copy of this paper.