Every time field biologist An Nguyen finds a mammal in the wild that he's never seen before, he adds a line to the tally count tattoo on his left wrist.

The silver-backed chevrotain, a tiny "mouse-deer" native to Vietnam, is a sighting significant for more than just Nguyen's personal tally. There has been only one confirmed record of the elusive mammal since 1910—a specimen obtained from a hunter in 1990—until Nguyen and his team set camera traps that recorded 280 sightings within nine months.

The news, reported this week in Nature Ecology & Evolution, is more than just confirmation that the silver-backed chevrotain is not yet extinct. It means that researchers can start studying it more comprehensively, trying to get a sense of how many are left and what kinds of protections it needs. And protecting the chevrotain also means protecting the less cute, but equally essential, species that share its habitat.

Most wanted

The silver-backed chevrotain is one of the world's "most wanted" mammals. Along with other elusive species like the Pondicherry shark, Attenborough's long-beaked echidna, and the Wondiwoi tree kangaroo, it hadn't been spotted for decades. Despite the lack of sightings, conservationists have had an inkling that these species might not yet be extinct.

For all of these lost species, there's a huge gap in what conservationists know about them. They're not sure where to find them, how many might be left, what threats they face, and how best to save them. The silver-backed chevrotain is listed by the International Union for the Conservation of Nature as "Data Deficient," meaning that there hasn't been enough field research for anyone to know whether they're extinct, endangered, or doing OK—or whether populations are stable, shrinking, or recovering.

Rediscovering lost species means filling in those knowledge gaps, which will allow conservationists to start figuring out what needs to be done to protect these fragile species and their habitats. The organization Global Wildlife Conservation lists 1,200 lost species, highlighting 25 of the most charismatic—including the silver-backed chevrotain—as flagship "most wanted" species that can carry the torch for the efforts to find them all.

Silent forests

In eastern Vietnam, Laos, and Cambodia, the mountains and forests of the Greater Annamites Ecoregion are home to an incredibly diverse range of mammal species. But many of these animals face severe threats from humans. The threat comes from habitat destruction as well as intensive hunting that supplies the thriving wildlife trade in the region.

This hunting mostly uses wire snares, which indiscriminately catch any unlucky animal that happens to stumble through them. The trapping is so pervasive that it has led to "empty forest syndrome"—an intact forest, eerily devoid of its inhabitants.

Pervasive snaring is what left conservationists unsure whether there were any silver-backed chevrotains remaining in the region. The beasts have been lost twice. After British zoologist Oldfield Thomas first described them in 1910 based on four dead specimens, there were no more reports of their existence for 80 years. In 1990, Russian researchers obtained a specimen from Vietnamese hunters, confirming that the silver-backed chevrotains did still exist.

Then once again, nothing for nearly 30 years. "Given the considerable increase in hunting pressure that has occurred in Vietnam since the early 1990s, it was unclear whether the species still existed," write Nguyen and his co-authors.

The team started out by interviewing villagers and rangers in regions where the silver-backed chevrotain might still be found. A few people described seeing animals that sounded like they fit the bill, and everyone the researchers interviewed pointed out that all chevrotain species were becoming increasingly rare thanks to intensive snaring. Based on the interviews, the researchers set three camera traps in November 2017 and went to retrieve them in April 2018.

Rediscovered

The silver-backed chevrotain had shown up at all three camera trap stations—the first pictures of silver-backed chevrotains in the wild. To avoid inflating the numbers, the team counted photos taken within an hour to be a single "event"; the three camera traps had recorded 72 events.

Following up on this success, the researchers set up plenty more camera traps and captured 208 events between April and July 2018. It's impossible to say how many individual animals this represented, but the new information does mean that the researchers could start sketching out some notes about the species' behavior: it seems to move around during the day, either alone or in pairs.

But there's a huge amount of information still missing. It's essential, the researchers write, to work out how this newly recorded population is doing, search for other populations, and figure out how to combat the threats that the animals are facing: "Together, this information can provide the foundation for an evidence-based conservation plan for the silver-backed chevrotain."

A crucial question is whether this information could put the remaining silver-backed chevrotains in further danger, although there is no specific location information about the sightings. "We carefully weighed the risk of increased hunting owing to an increase in attention to the species," the researchers write.

But that risk needed to be weighed against the benefit of building support for more research and new conservation measures, they point out. "In an age of mass extinctions, confirming the survival of lost species provides rare second chances for biodiversity conservation."

Nature Ecology & Evolution, 2018. DOI: 10.1038/s41559-019-1027-7 (About DOIs).