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A Rare Find: Yakushima Weasel Dissection

Updated: Apr 26

Important scientific data can weasel its way into the most unlikely of places. Earlier this year we were alerted that a Japanese weasel was found as roadkill near the side of the road in Yakushima. Roadkill incidents are always hard for conservationists to hear about, but thankfully it seems that weasels are rarely involved in vehicular collisions on Yakushima. And, importantly for us, this specimen could give us a rare insight into the inner workings of this cryptic endemic subspecies found only on Yakushima island.

This adult male was involved in a car accident mid March in Yakushima. Female weasels are smaller than males. Photo credit: Kasia Majewski


The Japanese weasel (Mustela itatsi) is listed as near threatened by the IUCN as its populations across mainland Japan have been consistently shrinking. This is due largely to habitat loss, and the introduction of the invasive Siberian weasel (Mustela siberica), which has already colonized 20 of the 46 prefectures of the three main islands of Japan and now competes with the native weasel populations for space and resources. Japanese weasels are carnivores, with their diet comprised of invertebrates, reptiles, amphibians, and small mammals.


The Yakushima Japanese weasel (Mustela itatsi sho) is a distinct subspecies which is a bit different from its mainland cousin. A publication by Suzuki and Matsumoto (2020) describes the skull variation between the mainland species of weasel and the Yakushima population. They indicate that different ecological conditions on Yakushima island, such as prey size and abundance, might have led to slightly different body morphology resulting in this unique subspecies of weasel.


In short, Yakushima weasels are a unique population, likely adapted to the specific conditions found on Yakushima island. But, their cryptic behaviour makes them a difficult species to study. Little is known about their specific dietary preferences on the island, their habitat preferences, the parasitological communities associated with their populations, and of course, a major focus of the Tanuki Project, their direct and indirect competition with the invasive raccoon dogs on the island through diet and parasite infections. This is why a roadkill specimen of this species, while tragic, is also like biological gold dust. It can start to shed some light on many of our important questions about this species.

The team assembles for introductions and to discuss the strategy for sampling. Photo credit: Kasia Majewski

Ikeda-san and Yato-san also showed us their impressive collection of specimens they have collected for research. The skill required to prepare specimen mounts like these can take years to master. Photo credit: Kasia Majewski

Ikeda-san focuses on researching the bat populations in Japan. Here is one of his bat skulls used for a morphological study. Photo credit: Kasia Majewski

Ikeda-san was kind enough to show us the bat specimens he had prepared for his work on bats in Japan. Interestingly, bat specimens are preserved similarly to insect specimens. Their wings are pinned out until they dry into the desired shape. The big difference however is in the body. Fluid within most insects will dry out during the pinning process. However for mammals, the organs and body fluids must be removed. Smaller specimens have their organs removed through the mouth, while larger specimens have their organs removed through a small incission in their abdomen. Fascinating stuff! Although maybe best not contemplated while eating a meal.. Photo credit: Kasia Majewski

We were kindly invited to the Kyoto University Museum where we met with some skilled KU PhD students belonging to Dr. Masaharu Motokawa's lab, who have experience in preparing specimens. With the help of Yugo Ikeda-san, Takashi Yato-san and Yuka Yato-san as well as Annegret Liederbach, we were able to remove the skin of the weasel and proceed with the dissection. We were excited to learn that this is the first Yakushima weasel subspecies to be prepared for the Kyoto University Museum collection. Different parasite species colonize different parts of a host body and can also be found migrating through organs or tissues on their way to their target area. We have a few choice parasites of interest we are looking for, so we collected the organs of the weasel, as well as the contents of the digestive tract to look over for these species of parasites.

Yato-san works on preparing the skin of the weasel specimen. Photo credit: Kasia Majewski


Kenneth assists with the skin removal. Yato-san instructs how to carefully work around the area most impacted by the vehicular damage. Photo credit: Yuka Yato.

Yato-san brings life back to the specimen through his careful work. Now this specimen can be appreciated for decades to come as part of the museum collection. Photo credit: Yugo Ikeda.


We're also interested in the diet of this species. Understanding the specific diet of weasels on Yakushima can help us understand if the invasive tanuki are eating the same things, thereby competing with the weasels through their diet. Examining the diets of weasels may also give us some more clues about what kind of parasites might be present within their population.


Kasia carefully cuts open the thin walls of the intestinal tract to expose the gut content and collect them for DNA analysis and parasite morphology. Photo credit: Yuka Yato.

Kasia, Andrew and Kenneth examine the digestive tract for the presence of any parasites. Photo credit: Yuka Yato. Stay tuned with our lab updates as we continue to analyze our samples and see what we find!

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