The two major groups of bats diverged tens of millions of years ago, and the main difference between them lies in a tiny inner ear bone
A single bone in the inner ear marks the difference between the two major groups of bats.
The anatomical difference arose very early in the history of these animals. It may have led the two groups to evolve different forms of echolocation and even lifestyles.
Bats are the only mammals capable of powered flight and one of the most species-rich groups. Many of them navigate using echolocation: they make a loud noise and listen to the resulting echoes to determine what is around them. “Around 90 per cent of the bats can echolocate,” says Zhe-Xi Luo at the University of Chicago in Illinois.
Traditionally bats were divided into two groups: large ones called megabats and small ones called microbats. All the echolocating bats belonged to the microbats. However, in the early 2000s geneticists including Emma Teeling, now at University College Dublin in Ireland, showed that was incorrect. There really are two groups, but they both contain many echolocating bats. The groups are called the Yinpterochiroptera and the Yangochiroptera.
This created a puzzling question, says Luo. Either echolocation evolved twice, in each group, or it evolved just once in an ancestral bat species and was then lost in some lineages.
Luo and his team have now identified an anatomical difference between the Yinpterochiroptera and Yangochiroptera. In the heads of mammals, signals are carried from the ears to the brain by a specialised set of neurons that run through a bony tube called Rosenthal’s canal. This tube normally has small holes in it, but in Yangochiropteran bats the holes are much wider than normal, and in some species the wall is entirely absent on one side of the tube.
This change meant Yangochiropteran bats could evolve more neurons to carry sound information, and could reorganise them more freely, compared with Yinpterochiropterans. This may explain why the Yangochiroptera have diversified into so many species, and why they echolocate in a different way to the Yinpterochiroptera, argues Luo.
However, the finding doesn’t resolve the question of whether echolocation evolved once or twice. “It’s still possible that echolocation evolved in the bat common ancestor before the split of these evolutionary lineages,” says Luo. But if that is the case, echolocation must have changed a lot during the early stages of bat evolution.
Journal reference: Nature, DOI: 10.1038/s41586-021-04335-z
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