Echo Positioning: Blind Mouse players have special skills

Pictured here is a Chinese pygmy dormouse at the Moscow Zoo. This animal can effectively echolocate, making extremely high-pitched squeaks and sensing its surroundings by listening to echoes. Photo by JOEL SARTORE, NATIONAL GEOGRAPHIC PHOTO ARK

Written by: DOUGLAS MAIN

　　In a forest in a mountainous region of East Asia, under the cover of darkness, several pig-tailed rats climbed out of trees and explored the branches and ground, competing for berries, seeds and insects. Surprisingly, these animals can barely see anything.

　　So how do they move around? A new study recently published in the journal Science concluded that pig-tailed rats navigate by making high-frequency squeaks and then listening to echoes reflected by nearby objects, and then perceiving their surroundings.

　　Previous research has shown that another subspecies of the pigtail rat, the Vietnamese pygmy dormouse, is likely to also have echo localization capabilities. However, this study is the first time that researchers have gathered evidence that there is no doubt that four species of pigtail rats (also known as soft-haired tree rats) have echolocation capabilities.

　　Shi Peng, lead author of the study and a researcher at the Kunming Institute of Zoology at the Chinese Academy of Sciences, said: "We were really surprised that all species in this genus have echo localization capabilities. ”

　　So far, the existence of only two species of mammals with echolocation capabilities has been fully studied: bats and cetaceans, including whales, dolphins and porpoises. There is evidence that shrews and horse hedgehogs – a small mammal endemic to Madagascar – can be echo-localized, but it is almost certain that they are less effective than bats and cetaceans. This ability may have evolved independently in five different mammalian lineages, Shi said.

　　Several other birds use a more basic method of echolocation, including the oil plover and swiftlet.

Previous echo localization studies

　　In 2016, Aleksandra Panyutina, a biologist at the Sefertsov Institute for Ecology and Evolution in Moscow, demonstrated evidence that the Sabah pigtail rat could evade laboratory obstacles in complete darkness. She recorded some of their calls, which were similar in frequency and rhythm to bats capable of echolocation: the tone and frequency were very high, repeating dozens of times per second in some cases.

　　Recording is not easy, though. "We lacked the necessary equipment to record echolocation signals because my bat detector was not sensitive enough to record the cries of this rodent."

　　She collaborated with Ilya Volodin, a biologist at Moscow State University, and other colleagues. Together, they learned more about the pigs' tail rats and studied their eyes. Their eyes are "very small and have very few photosensitive cells," Volodin said.

Comprehensive research

　　In the current study, Shi Peng and colleagues captured four species of pigtail rats from mountainous areas across China. Each species is only a dozen centimeters long and covered with soft gray-brown fur. In the lab, they conducted a series of experiments in complete darkness to test the echo localization ability of pigtail rats.

　　First, the researchers compared the behavior of pigtail rats in a cluttered environment and in a tidy environment. They found that the number and frequency of ultrasonic calls made by pigtail rats increased significantly in the former environment compared to the latter. Next, they showed that the pigtail rat could find its way through a small hole in the partition, but only after making a series of squeaks.

　　The scientists also placed pigtail rats on an elevated disk to give them the freedom to explore. Underneath this platform, they placed a narrow ramp leading to food rewards. All pigtail rats raised their calls and were able to jump down the ramp in complete darkness. The researchers also put earplugs on the pigtail mice and asked them to try again. This time, they couldn't find the ramp and made fewer ultrasonic calls.

　　The scientists compared the skeletal structure of the pigtail rat with the skeletal structure of the bat and found that the structure of its pharyngeal region was strikingly similar. The pharynx is located behind the mouth and nasal cavity, which is where the call is made. Similarly, they found that the symboid bone of the pigtail rat was fused with the drum bone near the ear. The only other mammals with this structure are bats.

　　Rebecca Whiley, a researcher and master student at the University of York's Sensory Biophysics Laboratory, said these anatomical similarities suggest that there is homogeneity between the two, that is, the characteristics of convergent evolution, that is, similar characteristics present and develop in different or even unrelated species. She was not involved in the study. The study authors argue that this anatomy allows the pigtail rat to "perform more effective neuronal characterization of the signals emitted in comparison to the echoes returned," in other words, a better way to characterize the surrounding environment in the brain.

　　Next, the researchers sequenced the genomes of the Chinese pigtail rat and compared them to those of dolphins and two species of bats. They found that in these animals, the genetic similarity associated with hearing was higher than random chance could explain. They also found an important visually-related gene that helps cells in the retina function, which none of the four species of pigtail mice worked — further evidence that the animals had little visual ability.

　　Shi Peng and his colleagues hope to continue studying these animals, even their close relatives. These pig-tailed rats are still little known, probably with more than four subspecies. Shi Peng also speculated that in addition to the animals of this species, there are other animals that also have the ability to navigate in the dark.

　　"Our study shows that biodiversity with adaptive traits is greater than we think," she said. "We're almost certain that there are more echolocation animals waiting for us to discover."

(Translator: Stray Dog)