These blind mice can ‘see’ with their ears, a primary amongst rodents

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Under cover of darkness in the mountain forests of East Asia, Chinese harvest mice emerge from the trees to scurry over the branches and the forest floor, collecting berries, seeds and insects. The remarkable thing about it is that these animals are almost completely blind.

So how do they get around? New research published today in the journal Science conclusively shows that it is echoing: The mice get a feel for their surroundings and navigate by emitting high-frequency squeaks and then paying attention to the echoes that are reflected from nearby objects.

Previous work suggested that another tree-climbing relative of the same genus, the Vietnamese dwarf dormouse, could likely echo-localize. But this is the first study to bring together various pieces of evidence and unequivocally prove that the ability is present in all four species of the genus Typhlomys, also known as soft fur tree mice.

“The echolocation in all species in the genus surprises us indeed,” says Peng Shi, lead author of the study, researcher at the Kunming Institute of Zoology at the Chinese Academy of Sciences.

To date, there are only two well-studied groups of echolocalizing mammals: bats and whales, which include whales, dolphins, and porpoises. There is some evidence that shrews and tenreks – a diverse group of small mammals endemic to Madagascar – can perform echolocation, although almost certainly not as effectively as bats and whales. Shi says this ability likely evolved independently in five different lines of mammals.

Several species of birds, including oil birds and swiftlets, use a more rudimentary form of echolocation. (More information: Echolocation is nature’s built-in sonar. This is how it works.)

Previous evidence of echolocation

In 2016, Aleksandra Panyutina, a biologist at the Severtsov Institute of Ecology and Evolution in Moscow, provided evidence that Vietnamese dormice can dodge obstacles inside the laboratory in complete darkness. She recorded some of her calls that were similar in frequency and cadence to those of echolocating bats: very high and repeated, in some cases dozens of times per second.

But it wasn’t easy to record. “We didn’t have the necessary equipment to record the echolocation signals because my bat detector was too insensitive to this quietly voiced rodent.”

She teamed up with Ilya Volodin, a biologist at Lomonosov University in Moscow, and other colleagues. Together they learned more about the sounds made by dormice and studied their eyes. These are “not only very small, but also have very few light-sensing cells,” says Volodin.

Bring together

For the current study, Shi and colleagues collected four types of dwarf sleepers from mountains across China; each species is only a few inches long and covered with soft gray-brown fur. In the laboratory, they carried out a large number of experiments in complete darkness to test the ability of their subjects to echolocation.

First, the researchers compared the behavior of dwarf sleepers in a tidy room and the same animals in a tidy room. They found that the animals in the former environment significantly increased the frequency and number of ultrasound calls compared to the latter. Next, they showed that the animals can find their way through small holes in a clapboard, but only after a series of squeaks.

The scientists also inserted the mice into a raised disk and had them explored. Below this platform they placed a narrow ramp that led to a meal reward. All of the mice raised their shouts and were able to drop onto the ramp in total darkness. The researchers also plugged the mice into earplugs and had them try again. This time they couldn’t find the ramp and did less ultrasound vocalizations.

The scientists compared the bone structure of the tree mice with that of echo-localizing bats and found surprising similarities in the structure of the pharynx behind the oral and nasal cavities, where the calls are made. They also found that the stylus bone of the tree mouse was fused to the eardrum near the ears. The only other mammals with this structure: bats.

These anatomical similarities suggest homoplasia, a type of convergent evolution in which similar traits develop in different, unrelated species, says Rebecca Whiley, a researcher and master’s student at York University’s Sensory Biophysics Lab who was not involved in the work. The study authors suggest that this anatomy allows the animals “a more effective neural representation of the outgoing signals for comparison with returning echoes” – in other words, a better way of mentally mapping their surroundings.

Next, the researchers sequenced the genome of the Chinese dwarf dormouse and compared it to that of dolphins and two species of bats. They found a higher number of similarities in hearing-related genes than chance would explain. They also found that the one important vision-related gene that helps cells within retinal function was inoperable in all four species of tree mice – further evidence that the animals can barely see.

Shi and colleagues hope to be able to continue studying these animals and perhaps their relatives. Little is known about these mice and there are likely more than four species within the genus. Shi also suspects that there are other animals outside of this genus that can navigate in the dark.

“Our study indicates a greater biodiversity of adaptive traits than we would ever have thought,” she says. “We are almost certain that there are more echolocation animals waiting to be discovered.”