In the dense forests of East Asia’s mountainous regions, the Chinese pygmy dormice (Typhlomys cinereus) move along branches and the ground in search of food like berries, seeds, and insects. These tiny creatures are nearly blind, but a 2021 study published in the journal Science revealed that they use echolocation to navigate their environment. They emit high-pitched squeaks and rely on the echoes that bounce back from objects around them to map their surroundings and move safely. Earlier studies had indicated that the Vietnamese pygmy dormice, a related species, might also use echolocation. However, this study is the first to combine multiple sources of data to confirm that all four species in the genus Typhlomys share this ability. Peng Shi, the lead author of the study and a researcher at the Kunming Institute of Zoology in China, said that the finding surprised the team. "We didn’t expect that all species of the genus would have this ability," she said. Echolocation has been well-documented in two groups of mammals: bats and cetaceans, such as whales and dolphins. Some evidence also suggests that shrews and tenrecs, small mammals found only in Madagascar, may use echolocation as well. According to Ms. Shi, this ability has likely evolved independently in five different mammalian lineages. Some birds, like the cave swiftlet (Steatornis caripensis) and the oilbird, also use a simpler form of echolocation. In 2016, Aleksandra Panyutina, a biologist at the Severtsov Institute of Ecology and Evolution in Moscow, demonstrated that Vietnamese pygmy dormice could avoid obstacles in the dark. She recorded their calls, which were similar in frequency and rhythm to those of bats using echolocation. However, she noted that the task was difficult. "I didn’t have the right equipment to record these signals because my bat detector wasn’t sensitive enough for the soft sounds these rodents make," she said. Panyutina worked with Ilya Volodin, a biologist at Moscow State University, to further study the dormice’s vocalizations and eye structure. They found that the animals have very small eyes with few photoreceptor cells, suggesting poor vision. In the recent study, Ms. Shi and her team captured four species of pygmy dormice in China’s mountains. Each species is only a few centimeters long and has brown-gray fur. The researchers tested the animals’ echolocation abilities in complete darkness. They found that the dormice made more and more frequent ultrasonic calls in cluttered environments compared to open ones. The animals were also able to navigate through narrow holes in a wooden board after emitting a series of squeaks. When the scientists placed earplugs on the dormice and repeated the experiment, the animals could not find the path and made fewer calls. The researchers also compared the dormice’s skeletal structures with those of echolocating bats. They found surprising similarities in the pharyngeal region, where the calls are produced, and in the fusion of certain bones near the ears. These anatomical features are also found in bats. Rebecca Whiley, a researcher at York University who was not involved in the study, explained that these similarities suggest homoplasy, or convergent evolution, where unrelated species develop similar traits independently. The study authors believe that these features help the dormice better process their outgoing signals and compare them to the echoes they receive, giving them a clearer mental map of their surroundings. The researchers also sequenced the genome of the Chinese pygmy dormice and compared it to that of dolphins and two species of bats. They found many similarities in the genes related to hearing that could not be explained by chance. They also noticed that the main gene responsible for vision was missing in the three dormice species, further supporting the idea that these animals have poor eyesight. Ms. Shi and her team continue to study these rodents and their relatives, suspecting that more species in the genus remain undiscovered. They also believe that other animals outside this genus may have the ability to navigate in the dark using echolocation. "Our study shows that there is more biodiversity in adaptive traits than we ever imagined," Ms. Shi said. "We are almost certain we will discover many more animals that use echolocation."