A creature the size of a rabbit, nocturnal and so elusive that Singapore's scientists had written it off as locally extinct, is now thriving on a 10-square-kilometer island in the Johor Strait. The greater mouse-deer — a tiny ungulate that hadn't been spotted in the city-state for more than 80 years — has made an unexpected comeback, and the numbers are staggering.
When researchers first confirmed sightings on Pulau Ubin in 2008, they set up a monitoring program with modest expectations. Forest restoration work was underway on the island, so they figured the population would slowly climb as new habitat became available. What they found instead left them double-checking their data.
Between 2009 and 2024, the mouse-deer population density increased fivefold. By last year, it had reached three times higher than any recorded population of the species anywhere in the world. "I had to perform the analyses multiple times and check my data to be sure there was no mistake," said Marcus Chua, curator of mammals at the Lee Kong Chian Natural History Museum at the National University of Singapore.
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The forest restoration clearly helped. But the real driver of this explosion came from an unexpected direction: African swine fever. When the virus swept through Singapore's wild pig population in 2023, it wiped out more than 98% of them. With their main predator essentially gone, the mouse-deer population responded by thriving.
It's a reminder of how ecosystems work — tightly interconnected, full of dependencies that aren't always obvious until something shifts. The loss of pigs created space for the deer. And that matters more than it might sound. Mouse-deer are key seed dispersers in tropical forests. As their numbers recover, they're helping regenerate the island's woodland in ways that support the entire ecosystem.
"This is an unexpected but welcome conservation outcome," Chua said in the study, published in Biological Conservation. "The greater mouse-deer is a key seed disperser, so its recovery will be crucial for the long-term health of Singapore's forests."
The story doesn't fit neatly into any narrative we're used to hearing — it's not a triumph of protection or a cautionary tale of loss. It's something messier and more honest: a disrupted ecosystem finding a new balance, one species declining while another, thought gone forever, comes roaring back. On a crowded island where nature has been squeezed into fragments, that kind of recovery deserves attention.
