The echolocating Vietnamese pygmy dormouse

This week's paper is lead authored by Aleksandra A. Panyutina and is in the journal Integrative Zoology. It is a documentation of the use of echolocation by the Vietnamese pygmy dormouse, Typhlomys chapensis. Echolocation has been best demonstrated in bats and the toothed whales (especially dolphins). Beyond these groups the authors state that "no other mammal has been shown to rely on echolocation as the main means of echolocation". But there are some other mammals that use something like echolocation. For example, shrews "twitter" to assess their habitat, but it does not appear to be an advanced bat-like echolocation. What makes these Vietnamese pygmy dormice so unusual is that they have very reduced eyes (they are nocturnal) but are fast and nimble tree climbers. This study has two components. They 1) dissected the eyes of dormice to assess their vision, and 2) they recorded dormice exploring their cages using high-speed video and ultrasound recordings. They show that the eyes of these pygmy dormice are highly reduced, with the retinas actually have irregular folds in them. They state that therefore the pygmy dormouse "has no other means for rapid long-range orientation among tree branches other than echolocation". They then recorded the behavior of these pygmy dormice when they are exploring branches in their cages. In the video above you can see them exploring and hear their echolocation calls.

The echolocation calls made by these dormice are so faint that bat detectors cannot even pick them up. Additionally their echolocation calls are such high frequency (93 kHz) that they are above the hearing of most small rodents. This paper is exciting because there are so few species that we know to echolocate. Additionally, evidence of an arboreal mammal that echolocates adds an interesting point to the debate about whether echolocation or flight evolved first in bats. A non-volant echolocating mammal indicates that it is possible echolocation evolved first. It was long ago debunked that bats are rodent relatives, so they would not have evolved from something similar to the Vietnamese pygmy dormouse, but rather a very different arboreal mammal. It is particularly exciting that their echolocation is so quiet and directional that it is very hard to detect. This means that there may be other echolocating mammals out there that we have overlooked.