Support for a controversial evolutionary classification of bats, based on the anatomy of their inner ears, is presented in a paper published in Nature this week. Understanding how structures in the inner ear vary among bats offers insights into the evolution of their diverse echolocating strategies.
The phylogenomics — evolutionary classification incorporating analysis of genome data — of bats suggests that their echolocation either evolved separately in the bat suborders Yinpterochiroptera (a subset of microbats and megabats, which navigate mainly by sight) and Yangochiroptera (microbats, which are echolocating species), or evolved once in bat ancestors and was later lost in some of the Yinpterochiropterans. However, this classification is controversial as it has previously only been supported on the molecular level, and anatomical support has been lacking.
Zhe-Xi Luo, Benjamin Sulser and colleagues used minute dissection and CT scans to examine the inner ear structures of 39 species of bats from 19 different families. They observed highly derived structures of the spiral ganglion — a group of neuron cell bodies in the auditory system — in Yangochiropteran bats, structures that are unique to present-day and ancestral Yangochiropterans. This particular anatomical arrangement allows for a larger ganglion with more neurons, higher innervation density of neurons and denser clustering of auditory nerve fibre bundles, which differs from the anatomy of Yinpterochiropterans and non-bat mammals.
The authors conclude that these features may be the evolutionary driver for a range of echolocating strategies and are associated with the diversification of Yangochiropterans, which account for 82% of the species of all echolocating bats.
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