Similarities and differences in neural structures involved in echolocation in echolocating bats and toothed whales

Abstract

Aim of this thesis is to examine possible differences in neural structures involved in echolocation caused by different ecologies of two mammalian groups. Echolocation is the process of emitting sounds and deriving information from the returning echoes to be able to navigate and forage. Bats and toothed whales possess vastly different ecologies, but have evolved sophisticated echolocation to perceive their environment. Echolocation calls for complex processing of sounds and the integration of auditory information into the motor systems in the brain and differences in ecology may be evident in involved brain structures in echolocating bats and toothed whales. In general, the auditory system is highly enlarged, with important echolocation sound parameters overrepresented. Species with a higher dependence on echolocation often show higher hypertrophy of auditory neural structures. Some remarkable adaptations are observed in bats. The MSO, normally involved in processing ITD’s in mammals, has evolved a different function in bats, most likely in prey detection and classification. The MSO in toothed whales is lacking in most species, due to the lack of detectable ITD’s because of high frequency echolocation sounds and faster sound carrying properties of water. Another remarkable adaptation is the VNLLc in bats, an adaptation for exact localization of objects. Similar neural substrates have been found in the VNLL and IC of toothed whales, and they possibly possess a similar function. The sensorimotor system also shows some remarkable adaptations according to ecology. The cerebellum is enlarged for movements in a 3D environment and, at least in toothed whales, for the rapid integration of acoustic stimuli into motor functions in the aquatic environment. Different nuclei are involved in the sensorimotor system in bats and toothed whales, which can be explained through morphological differences in sound generation and sound perception. This thesis shows differences in neural structures are evident in and between echolocating bats and toothed whales and can be related to their respective ecologies.