A Tiny, Partially Missing Bone Structure in Bat Ears May Have Cleared the Way for Echolocation to Evolve

Nearly 90 percent of the nighttime hunters use sound to find prey

An image of a big brown bat flying against a black background
Big brown bats Eptesicus fuscus (pictured) are a Yangochiroptera species that uses complex sounds to echolocate.  Sherri and Brock Fenton

No matter what's for dinner, many different species of bats hunt using sound. Some bats use echolocation to target mosquitos, while others seek out cattle for blood-sucking or search for agave flowers to sip nectar from. Now, researchers suspect a tiny ear bone may reveal details of how they evolved their specialized echolocation abilities, according to a study published this week in Nature.

For the first time, scientists identified a key anatomical difference between the two major bat groups: Yinpterochiroptera (Yin), made up of mostly large fruit bats, and Yangochiroptera (Yang), which contains small bats that use echolocation. However, in 2000, genetic analysis showed some small insect-eating bats actually belong to the Yin group, but a physical difference between the groups had not been identified until now, reports Warren Cornwall for Science.

Previously, researchers thought bat ears had fine-tuned versions of mammalian ears. With nearly 1,500 species, bats are very diverse and make up nearly 20 percent of all mammal species on the planet.

In mammal ears, signals are carried from the ear to the brain through a network of neurons that run through an inner ear bone known as Rosenthal's canal. The bony canal, located within the spiral-shaped inner ear ganglion, is usually riddled with tiny openings for nerve fibers. The only mammals with auditory nerves not encased in bone are monotremes, which are egg-laying mammals like echidnas or platypuses, per Science.

While 3-D scanning a bat skull in 2015, biologist Benjamin Sulser of University of Chicago found this particular species, a Yang group bat, was missing part of these inner ear structures. After using computer tomography (CT) scanning on several more related species, Sulser noticed a pattern and showed his lab supervisor, University of Chicago evolutionary biologist Zhe-Xi Luo.

For the new study, Sulser and his colleagues scanned a total of 39 bat skulls from museum specimens of varying species. When comparing Yin and Yang inner ear bones, the Rosenthal's canal was missing or had large holes in Yang bats that rely on echolocation to hunt, whereas the structure in Yin ears was thick and dotted with mini holes as normal.

Because Yang bat's nerves were not encased in bone, this change may have allowed the mammals to unleash new hearing capabilities or created more room for a more beefed-up auditory nerve, per Science. Yang bats, in general, show diversified hunting methods and variation in the anatomy of their inner ears. In contrast, the few species of Yin bats that use sound to hunt make single note calls tuned to locate insects' fluttering wings and look like most mammals' ears, reports Science

 "These are different ways of achieving the same goal. It's like these two types of bats are speaking different dialects of a language," Luo said in a statement.