Zoological Museum, M.V.Lomonosov Moscow State University, Bol.Nikitskaya ul. 6, Moscow 103009, Russia

N.A.Severtsov Institute of Ecology and Evolution, Leninsky pr. 33, Moscow 117071, Russia

Abstract

Relation between size of pinna and auditory bulla in specialized desert rodents (families Allactagidae, Dipodidae, Gerbillidae, Heteromyidae, and genus Selevinia) is statistically significant and described by hyperbola. The empirical distribution of respective size can be interpreted as a total of two main trends. one is enlargement of the pinna, the bulla remains small (most of Allactagidae), another is enlargement of the bulla, the pinna remains small (Gerbillidae, Heteromyidae, Selevinia, most Dipodidae). A few species display moderate enlargement of both pinna and bulla, while in Euchoreutes they both are hypertrophied. Biomechanical preconditions are supposed to be following. Enlargement of the pinna is responsible for increasing of both sensivity and locatory abilities of the hearing organ, while enlargement of the bulla is responsible for the first function only. So, the two main trends are results of selection of development of the hearing organ at the level of either pinna or bulla. It is shown by dispersion and correlation analysis that size of the parts of the hearing organ under investigation are correlated mostly with mobility and escaping mode. It is concluded therefor that the rodents characterized by active and speedy escaping from predation display the first of the above trends, while the rodents characterized by passive escaping from predation display the second trend.