Different size of tympanic membranes and its functional and ecomorphological meaning in Crocidurinae and Soricinae (Lipotyphla: Soricidae)

Abstract

Adult skulls of Crocidura russula and Sorex araneus have been studied by μCT and by histology. The virtual representations of these skulls in ventral view display the middle ear structures very clearly. It is evident that the ectotympanic rings, which frame the tympanic membrane, are significantly larger in Crocidura than in Sorex; simple measurements and calculations reveal that the tympanic area in the two chosen skulls is significantly larger in the former taxon than in the latter (4.3 mm² vs. 2.4 mm2). When a few more specimens from these two species as well as further taxa are added to the data set, a regression analysis shows that crocidurines have distinctly larger tympanic membranes than soricines – but there are a few exceptional species (Suncus murinus and Anourosorex planipes). Preliminary hypotheses concerning the meaning of these differences with regard to hearing performance (optimal range of frequencies) and ecomorphological adaptation (open or dense micro-habitat) are put forward. A histological cross section shows the anatomy of the ectotympanic and the tympanic membrane in Sorex araneus. It is hypothesized that the ectotympanic ring with the eardrum, which are loosely attached to the basicranium by a fibrous membrane, is not a primitive, but a derived character of soricids. The loss of a processus tympanicus could have facilitated the easy evolutionary change between ‘auditory adaptation zones’ – and perhaps explains the enormous number of soricid species.