- Once thought extinct in Singapore, a little-known species of miniature deer has reemerged in unprecedented numbers on a small island reserve in the Johor Strait.
- Researchers documented the greater mouse-deer thriving on Pulau Ubin at the highest population density recorded anywhere in the species’ range.
- The team put the surge down to increased availability of prime habitat following a decade of forest restoration, as well as reduced competition for food after the collapse of the island’s wild pig population due to African swine fever.
- Experts say the dramatic “ecological cascade” underscores the need for long-term, ecosystem-wide monitoring throughout Southeast Asia, particularly at sites impacted by sudden shifts triggered by disease.
Rabbit-sized, nocturnal and elusive, the greater mouse-deer hadn’t been seen in Singapore for more than 80 years. Researchers long presumed it was locally extinct.
In 2008, however, researchers confirmed sightings of the pint-sized ungulates, Tragulus napu, on Pulau Ubin, a 10-square-kilometer (4-square-mile) island in the Johor Strait that separates Singapore’s main island from Peninsular Malaysia to the north. The team quickly set up a monitoring program to learn more about the small population.
They expected to see numbers slowly recover as new suitable habitats became available thanks to ongoing forest restoration programs on the island. But the scale of what they recorded over the next 15 years astonished them.
“I had to perform the analyses multiple times and check my data to be sure there was no mistake,” Marcus Chua, curator of mammals at the Lee Kong Chian Natural History Museum at the National University of Singapore, told Mongabay.
In a new study published in Biological Conservation, Chua and his colleagues document a fivefold increase in the population density of greater mouse-deer on Pulau Ubin between 2009 and 2024. By 2024, the mouse-deer population density was three times higher than previously recorded anywhere in the species’ range.
While the rehabilitation of forest habitats will have given the mouse-deer an initial boost, the researchers conclude the most dramatic surge was triggered by the outbreak of African swine fever (ASF) in 2023.
The virus, which is infectious and deadly to wild and domestic pigs, wiped out more than 98% of Singapore’s wild pigs (Sus scrofa) by 2024. “There was an urgency to find out about the ecological consequence of such a major event,” Chua noted, especially given the paucity of information about how wild pigs and greater mouse-deer interact with one another.
Eased competition and predation pressure
Using historical monitoring data and new data collected in 2024 by both line-transect and camera-trap sampling, the team calculated the population densities of Pulau Ubin’s two resident ungulates.
By 2024, there were an estimated 293 mouse-deer per square kilometer, about 759 per square mile, spiking significantly just after the onset of ASF and the pig mortality in 2023. In contrast, they estimated the wild pig density was no more than 30 individuals per square kilometer (78 per square mile) in 2024.
The team attribute the skyrocketing mouse-deer numbers to a one-two easing of ecological pressure on the greater mouse-deer, the only ungulate species besides the pigs on the island: reduced competition for food, and less predation pressure.
“There is competitive overlap in their diet,” Chua said. While wild pigs are generalist omnivores and greater mouse-deer are herbivorous, both feed on understory vegetation and fallen fruits and flowers. Both species were photographed during the study feasting on durian blossoms scattered across the forest floor, he said.
Wild pigs might also exert predation pressure on greater mouse-deer. While there are no records of them eating mouse-deer, studies document wild pigs preying on rabbits and other species of deer like chital (Axis axis) in India and pampas deer (Ozotoceros bezoarticus) in Argentina. “The diminutive mouse-deer could certainly end up as food for them,” Chua said.
According to the study, the mammal dynamics are “clear evidence” of a dramatic ecological cascade — a phenomenon in which a dramatic shift in one species triggers ripple effects throughout the ecosystem — set in motion by the ASF outbreak. The findings underscore the need for long-term, ecosystem-wide monitoring at sites impacted by sudden shifts triggered by disease, the authors say.
Poised for population explosion
Greater mouse-deer, which range from southern Thailand to the islands of Borneo and Sumatra,belong to an evolutionarily distinct lineage of miniature deer, also known as chevrotains, that defend their territories not by clashing antlers, but with fang-like canines.
They’re also extraordinarily prolific breeders. They reach sexual maturity at just 5 months, and have a relatively short gestation period of also roughly five months. Females can also conceive again within hours of giving birth.
The researchers note in their study that this reproductive efficiency equips the species for population explosion should a sudden abundance of resources and suitable habitat become available, such as when their competitors died off in 2023.
Globally, the species is listed as least concern on the IUCN Red List. However, the diminutive ungulate is so elusive and physically similar to other species in its genus that few studies have focused on assessing its conservation status. (The last Red List assessment was carried out in 2014.) Moreover, very little is known about its sensitivity to forest degradation and poaching, leaving isolated subpopulations vulnerable to local extirpation.
This makes Pulau Ubin a critical site for the species, according to Robert Teo, director of conservation at the National Parks Board of Singapore (NParks) and a co-author of the study. “Pulau Ubin remains the primary known habitat for the greater mouse-deer in Singapore,” he said, adding that NParks plans to continue with the forest restoration efforts to further enhance habitats on Pulau Ubin.
Pulau Ubin wasn’t always swathed in prime mouse-deer habitat. In fact, up until the late 1990s it had been largely cleared for granite quarrying, rubber plantations and aquaculture ponds. Since the quarries were shut down and agricultural communities relocated to Singapore’s main island at the turn of the century, habitats began to naturally revegetate.
“The rehabilitation work carried out by NParks aimed to improve the availability of habitats for wildlife,” Teo said. Abandoned quarries were converted into wetlands, shrimp ponds rehabilitated into mangroves, and forest areas enriched with more than 40,000 native tree seedlings, such as Cordia subcordata and the critically endangered Kopsia singapurensis, Teo added.
Besides the greater mouse-deer boom, other signs are emerging that show forest restoration is giving local wildlife a boost. Camera traps recorded an array of other species in 2024, including a female Malayan tapir (Tapirus indicus), an endangered species only occasionally sighted in Singapore.
Dramatic ecological cascade
The steady increase in greater mouse-deer on the island over the past 15 years is “an exceptional example of Singapore’s re-greening and reforestation efforts succeeding in recreating habitat for wildlife,” according to Therese Lamperty, a biologist at the University of Florida, who wasn’t involved in the study.
Lamperty, who led a study in 2023 that documented how various mammals naturally recolonized Singapore, said the mouse-deer recovery is “the most dramatic” case she’s ever seen of a sharp drop in one species correlating with an explosive rise in another.
“Somewhat related research on ecological cascades has connected defaunation to rodent increases for similar reasons but has not found such dramatic results,” she noted. Research in 2015, for instance, linked a 45% surge in rodent numbers in Brazil’s Atlantic Forest to crashing populations of white-lipped peccaries (Tayassu pecari), likely due to reduced competition for food.
Lamperty said she would be interested to see how mammal population dynamics are playing out in other parts of Southeast Asia reeling from African swine fever. However, the magnitude of Pulau Ubin’s ecological cascade might not be seen elsewhere, she said. Singapore has a specific ecological context not typically seen in other parts of the region, such as low poaching pressure and few natural mammal predators.
“It might be a lot more subtle and perhaps not really a substantial ecological cascade in other parts of Southeast Asia, [but] a point that I think managers should monitor,” she said.
With mouse-deer numbers now so high on Pulau Ubin, the authors recommend further research on their potential to reshape forest ecology. Long-term monitoring of their herbivory on plants and seedlings, and their role in seed dispersal will be vital.
The team also plans to continue tracking both mouse-deer and wild pigs as they continue to recover and interact. The camera traps photographed piglets frolicking in front of the lens in 2024 — a sure sign wild pigs are breeding and could bounce back from the brink. Whether they regain dominance or settle into a new balance with the considerably larger mouse-deer population, however, remains unknown.
For Chua, the remarkable recovery of the tiny deer on Pulau Ubin is reassuring proof that habitat restoration is a conservation strategy that pays off in the end.
“Such efforts can build resilience and facilitate species recovery,” he said, “even in heavily modified habitats.”
Banner image: A greater mouse-deer photographed in captivity in Washington D.C. Image by Clyde Nishimura/Smithsonian’s National Zoo via Flickr (CC BY-NC-ND 2.0).
Carolyn Cowanis a staff writer for Mongabay.
Citations:
Chua, M. A., Thomas, N., Teo, R. C., & Lim, H. C. (2026). Rapid population growth of greater mouse-deer after active forest restoration and African swine fever outbreak. Biological Conservation, 313, 111564. doi:10.1016/j.biocon.2025.111564
Chua, M. A., Sivasothi, N., & Teo, R. C. (2009) Rediscovery of greater mouse deer (Tragulus napu) in Pulau Ubin, Singapore. Nature in Singapore, 2, 373-378. Retrieved from https://lkcnhm.nus.edu.sg/wp-content/uploads/sites/11/app/uploads/2017/04/2009nis373-378.pdf
Behera, S., & Gupta, R. P. (2007). Predation on chital Axis axis by wild pig Sus scrofa in
Bandhavgarh National Park. Journal of the Bombay Natural History Society, 104(3), 345-346. Retrieved from https://ia601402.us.archive.org/35/items/biostor-150941/biostor-150941.pdf
McDonough, M. T., Ditchkoff, S. S., Smith, M. D., & Vercauteren, K. C. (2022). A review of the impacts of invasive wild pigs on native vertebrates. Mammalian Biology, 102(2), 279-290. doi:10.1007/s42991-022-00234-6
Chua, M. A., Thomas, N., Teo, R. C., & Lim, H. C. (2024) Biodiversity record: Malayan tapir on Pulau Ubin. Nature in Singapore, 17, e2024118. doi:10.26107/NIS-2024-0118
Lamperty, T., Chiok, W. X., Khoo, M. D., Amir, Z., Baker, N., Chua, M. A., … Luskin, M. S. (2023). Rewilding in Southeast Asia: Singapore as a case study. Conservation Science and Practice, 5(3), e12899. doi:10.1111/csp2.12899
Galetti, M., Guevara, R., Neves, C. L., Rodarte, R. R., Bovendorp, R. S., Moreira, M., … Yeakel, J. D. (2015). Defaunation affects the populations and diets of rodents in Neotropical rainforests. Biological Conservation, 190, 2-7. doi:10.1016/j.biocon.2015.04.032
See related story:
Southeast Asian wild pigs confront deadly African swine fever epidemic
FEEDBACK: Use this form to send a message to the author of this post. If you want to post a public comment, you can do that at the bottom of the page.
Credits
