Effects of altitudinal change on the auditory bulla in Ochotona daurica (Mammalia, Lagomorpha)

The lengths and widths of body parts are influenced to a considerable extent by the environment. Along with increasing altitude, wings of arthropods degenerated or even disappeared so that a trend of degeneration on their acoustic systems appeared. We measured 131 adult skulls of daurian pikas, Ochotona daurica , in order to know whether the degeneration will happen in mammals. Our results show that both the size and relative size of auditory bulla of daurian pikas were negatively correlated with altitude and this change was significant. However, the ear length showed an opposite trend. We suggest that lower oxygen concentration, constantly strong winds and lack of resources at high altitude are responsible for the decrease in the size of the auditory bulla. Besides thermoregulation for large different temperatures of day and night in high altitude, larger external ears of the pika help detect the direction of the predator and could compensate for a relatively small bulla.