Microscopic inner ear structures reveal why major groups of bats
echolocate differently

Date:
January 26, 2022
Source:
Field Museum
Summary:
A new article compares the inner ear structures of the two main
groups of bats. By examining the microscopic inner ears of bats
from 19 of the 21 known bat families, the researchers were able
to show that the presence of extra neurons and specialized ear
structures align with a split in bat evolution revealed by DNA.



FULL STORY ==========================================================================
Lots of bats echolocate -- they emit high-pitched squeaks, and based
on how those sound waves bounce off their surroundings, they're able
to navigate in the dark and find insects to eat. But a lot about how
bats evolved these sonar capabilities has been a mystery to scientists,
until now. A new paper in Nature is the world's first study to compare
the inner ear structures of the two main groups of bats. By examining
the microscopic inner ears of bats from 19 of the 21 known bat families,
the researchers were able to show that the presence of extra neurons and specialized ear structures align with a split in bat evolution revealed
by DNA.


========================================================================== "This is the first physical evidence we have to support what genetic
data tell us about the split of bats into two major groups," says Bruce Patterson, the Field Museum's MacArthur Curator of Mammals and one of
the study's authors. "It represents the greatest structural innovation in mammalian ears since the Jurassic, and it helps solve the mystery of how
bats evolved the echolocating abilities we see today." "Biologists have
always speculated that two major groups of bats have different ways of
seeing the world through sound," says the study's lead author Benjamin
Sulser, a PhD student at the American Museum of Natural History and
graduate of the University of Chicago, where he worked with Patterson and UChicago Professor of Organismal Biology and Anatomy Zhe-Xi Luo. "This
is the first time we found different neuroanatomies in the inner ear,
which give these bats different ways of processing the echolocating
signal." For a long time, scientists divided bats into Megachiroptera,
the big fruit bats reliant on vision, and Microchiroptera, the little
bats that use echolocation to find bugs. Genetic analyses in the past
couple decades showed that this categorization needed an update, because
there were some little echolocating bats that were more closely related
to big fruit bats than to their fellow echolocaters. So researchers
proposed a new way to classify bats, based on how closely related they
are to each other. Bats have since been split into two main groups: Yinpterochiroptera and Yangochiroptera -- the yin and yang of bats.

"It's become clear that genetic analysis is the best way for us to
reconstruct the evolutionary history of these bats, but we had such a
clear genetic signal that there were two groups, we thought there had to
be some physical traits that aligned with this striking genetic split,"
says Patterson.

The researchers set about looking for physical traits that separated
the yin bats from the yang. One clue lay in the bats' methods of
echolocation. "Yin and yang bats echolocate differently," explains
Patterson. "Not all yin bats echolocate, but the ones that do use a
constant wavelength frequency, and they make a lot of these calls --
about a third of the time, they're actively emitting sounds in hopes of something bouncing back," says Patterson, "whereas Yangochiroptera will
emit a signal and be silent for a long period of time, but the sounds
they make vary in pitch and frequency." Since ears are the organ that
processes those echolocation signals, the researchers' first stop was
the bats' inner ears.

The researchers CT scanned 31 bat skulls from the Field Museum's
collections, along with several from the American Museum of Natural
History in New York. The bats represented 39 different species from 19
of the 21 known bat families.

Most of the bat skulls the scientists examined were about the size of a
large blueberry, meaning that the tiny organ in the inner ear responsible
for hearing, the cochlea, was about the size of a poppy seed. And deep
inside this tiny poppy seed-sized cavity lay the key physical differences between yin and yang bats.

All mammals, including bats, are able to hear thanks to tiny hairs deep
inside their cochlea. When sound waves cause these hairs to vibrate,
swirled masses of nerve cells connected to those hairs pick up the signal
and translate it into an electrical impulse that gets transported to
the brain.

CT scans of the bats' brains revealed huge differences in these tiny structures. The yin bats' ears were a lot like ours, including a thick
bony canal wall packed with nerve endings to protect the spiral nerve
cells. The yang bats, one the other hand, had extra neurons for processing sound waves and were missing the protective bony canal. The extra space afforded by the missing canal walls gives the nerve cells room to evolve
into increasingly complex shapes.

"All mammals going back to the mid-Jurassic have these bony canal walls,
but yang bats are missing them," says Patterson. "The evolution of yang
bats without this canal is the greatest structural innovation in all
mammalian ears that we've ever seen." "We hypothesize that by developing
this new configuration, without the space constraint on the inner ear
ganglion, the yang bats have a greater capacity for the ganglion cells
to multiply and different ways to connect to the brain, unlike most other mammals," says Luo. "A greater size of the ganglion and a greater number
of neurons may have contributed to this big evolutionary diversification
of bats relying more on frequency modulating echolocation." The hypothesis might account for why there are so many more species of yang bats than
yin: the ability to evolve more specialized inner ears could open them
up to a wider variety of habitats and prey.

Patterson says that the study is important because it deepens our
understanding of how bats came to be such a diverse group. "In
the 52 million years since the earliest known bats lived,
they've exploded into one of the most successful mammals on
Earth. Twenty percent of mammal species are bats, and they're
crucial for our planet and for human activities like agriculture,
since they eat so many pest insects." says Patterson. "This study
helps explain how they were able to diversify so much and so rapidly." ========================================================================== Story Source: Materials provided by Field_Museum. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. R. Benjamin Sulser, Bruce D. Patterson, Daniel J. Urban, April I.

Neander, Zhe-Xi Luo. Evolution of inner ear neuroanatomy of
bats and implications for echolocation. Nature, 2022; DOI:
10.1038/s41586-021- 04335-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220126122432.htm

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