The big mouse mystery

Let’s start from the beginning of this elusive rodent’s quirky natural history, with its discovery. Known from only two specimens collected over 130 years ago in the former German colonial capital of Bismarckburg, legend has it that the Togo Mouse was brought over from West Africa to Berlin in a bottle of rum - although researchers are unable to confirm this particular piece of the puzzle. These two individuals would be the only Togo Mice ever found, the species essentially disappearing from science. 

However, the information obtained from these two individuals suggests that the Togo Mouse may not be just any rodent. The definitive taxonomic treatise in 2005 placed the Togo Mouse as the only species within its subfamily, meaning its genus, and even its subfamily itself, is monotypic (represented by a single species).

About 40% of the world’s mammals are rodents, with over 2,270 species of rodents distributed under 75 subfamilies. To put it into perspective, one rodent subfamily, Murinae, has over 650 species under it. The Togo Mouse, on the other hand, is a completely unknown entity, standing within its own unique subfamily and evading attempts by researchers over the years to align it with any of the well-recognized groups. In the ever-growing phylogenetic tree of life, the Togo Mouse poses a challenge to reconstruct this unknown part of evolutionary history.

Since its discovery in 1890, numerous expeditions have sought to rediscover the mouse and get some answers to the growing list of questions surrounding little Leimacomys, including two recent expeditions, one in 2022, and another in February and March of 2023. To date, all have been unsuccessful, despite reports from local people saying that they have seen the mouse.

We may, however, be on the brink of a breakthrough. A combination of dedicated international scientific collaboration, ongoing monitoring, local knowledge, and the use of ancient DNA technology may bring the taxonomic truth behind the Togo Mouse to light. 

If there is one person who has been somewhat of a champion for getting to the bottom of the mouse mystery, it’s probably Julian Kerbis Peterhans, professor emeritus of natural science at Roosevelt University. Among his other specialties, Kerbis Peterhans has conducted small mammal surveys in the eastern half of Africa for over 30 years, contributing to the discovery of 19 species new to science. For Kerbis Peterhans, the mission to find the Togo Mouse has been a worthy pursuit.

“At least among the limited African small mammal specialist circles, all couple dozen of us, it’s a big deal,” Kerbis Peterhans says. “For anybody that works on mammals in Africa, it’s like this mythical animal that everybody’s heard of: the big mystery mouse.”

Kerbis Peterhans has co-led two expeditions in the search for the Togo Mouse, alongside Holly Lutz, wildlife and evolutionary biologist at the Field Museum of Natural History, postdoctoral affiliate in the department of immunology and microbiology at Scripps Research, and Kerbis Peterhans’s long-time collaborator. In February and March of this year, they traveled to Fazao-Malfakassa National Park and the protected Deguengue Forest for a month of surveying, accompanied by their research team and local scientists.

Togolese coordinator, Dr. Gabriel Hoinsoudé Segniagbeto, full professor at the University of Lome, played a crucial role in connecting and coordinating the involvement of local communities, working closely with Togolese students and local people in the field, like Brusunu Djinsa, son of the village chief in Yegue. Djinsa would not only provide critical local support even after the expedition but would also become somewhat of a steward of science himself.

“He started bringing in insects and showing me how cool the different wings were,” Lutz says. “Essentially, he’s a biodiversity person at heart, but maybe he’s never had the opportunity to share that interest with anybody. Now he’s started to call himself the ‘museum scientist of Yegue,’ and he’s started the first small museum of Yegue.”

Alongside people like Djinsa keeping the spirit and curiosity of the region’s biodiversity alive, ongoing monitoring has also been put in place thanks to the training and support of Amétépé Hounmavo, a PhD student at the University of Lomé. In order to search for the Togo Mouse outside of the dry season, when the expeditions have historically taken place, the 2023 team left live traps for Hounmavo, who will conduct monthly monitoring, giving the expedition team the best chance of finding any remaining Togo Mouse populations.

As ongoing fieldwork has yet to provide any answers to the mouse mystery, there is another parallel avenue of research that may be on the verge of a breakthrough: ancient DNA.

The study of ancient DNA, or aDNA, involves acquiring, extracting, and studying DNA sequences collected from ancient remains — think ice-frozen mummies and Siberian permafrost mammoths, and in our specific case, a not-so-ancient, potentially rum-preserved Togo Mouse.

Josef Bryja, senior researcher at the Institute of Vertebrate Biology of the Czech Academy of Sciences, and his wife, molecular biologist and lab manager, Ana Bryjová, are the first to use aDNA research in the case of the Togo Mouse. While the results are yet to be published, it seems their work is arriving at some exciting answers.

Bryja shares that the DNA extracted from one of the samples of the Togo Mouse was of relatively good quality, allowing him and Bryjová to sequence and assemble the entire mitochondrial genome of Leimacomys by using the most modern techniques of high-throughput DNA sequencing.

“We were quite sure it’s really the DNA from Leimacomys because no similar sequences from well-known species were found in available databases,” he says. “We were really careful to select which DNA we would include in the final assembly and we produced a good quality mitogenome, comparing it with other known rodents. We’re now finalizing a publication on results and the taxonomic position of this enigmatic species, but I can already say that Leimacomys is the only known representative of the unique evolutionary lineage that split from other murid rodents 15-17 million years ago.”

Bryja says that the Togo Mouse might be able to answer broader questions about the region, gaining interesting insight into which processes historically created patterns of biodiversity hotspots. Bryja says that it’s been recently discovered that the area between Ghana’s Volta River and the Niger River of Nigeria is home to a stretch of unique endemism in West Africa that the Togo Mouse is a part of. This begs the question: what kind of answers about the ecology and evolution of the region might lie in the legendary Leimacomys?

With the genetic truth behind this mystifying species seemingly just around the corner, it’s exciting to think about what a new understanding of the Togo Mouse might mean for future research and conservation in the region.

“In our culture, a mouse is a mouse,” Dr. Gabriel Hoinsoudé Segniagbeto says. “It’s something very common. Sometimes, the people here are surprised that we’ve just come here to look for a mouse. They don’t know how this species could possibly be so interesting as to bring people from faraway countries to their village. So I’ve said to them, if one day we find out about this mouse that everyone is focusing on, you will see how important your village is. I tell them it’s not just a mouse, it’s an enigmatic species that we can use to improve and inspire knowledge of this place.”