Chiba’s big agricultural headache comes in a tiny package

Katsuura, Chiba Pref. – Wiping sweat from his face, Takahiro Morosawa crouches to swap the data card in a camouflage-green trail camera attached to a tree trunk in the forested hills of Hirata, a rural district of Katsuura, Chiba Prefecture. The area is dotted with rice paddies and vast expanses of greenery, not to mention kyon — small deer native to China and Taiwan that have proliferated across the prefecture over the past few decades and are now turning up in neighboring prefectures.

“We’re trying to understand how they behave — what time of day they’re active, where they sleep, what they prefer to eat and so on — so we can better monitor their movements and develop effective countermeasures to prevent them from spreading,” says Morosawa, a lecturer at Tokyo University of Agriculture and Technology’s wildlife management center.

Kyon feed on a wide variety of vegetation and can cause considerable damage to agricultural crops, making their continued spread a growing concern for farmers.

On this balmy, windy autumn morning in mid-October, Morosawa took us along on one of his routine trail-camera maintenance rounds, retrieving SD cards from around 10 camera traps set up along forest paths and on the mountain slopes. Three of the cameras are also accompanied by recorders housed inside plastic bottles to protect them from the elements. The recorders capture the kyon’s calls, known for their high-pitched, screeching quality.

“Look!” Morosawa says, pointing as a small kyon trots away.

Takahiro Morosawa, a lecturer at Tokyo University of Agriculture and Technology’s wildlife management center, says officials in Chiba Prefecture are increasingly aware that they need to act before kyon spread much farther. | Johan Brooks

Hirata is just one of several areas where similar traps have been set in a research project supported by the government’s Environment Research and Technology Development Fund. We had initially planned to visit areas along the borders of the prefecture with smaller kyon populations — what Morosawa calls the “front lines” of the animals’ expansion — but torrential downpours and typhoons in August and September triggered landslides and damaged roads leading to these destinations, forcing us to change our plans. Instead, he took us to what he calls a “densely populated area.”

Reeves’s muntjac, or kyon, began spreading in southeastern Chiba Prefecture sometime between the 1960s and 1980s, after animals were believed to have escaped from a private tourist park in Katsuura, the now-defunct Namegawa Island. The region’s mild climate and abundant vegetation, combined with a lack of predators and the animals’ high reproductive rate, helped them flourish. Females can become pregnant as young as 6 months old and breed throughout the year. 

The estimated population grew more than sevenfold, from 13,264 in fiscal 2006 to 94,093 in fiscal 2024. By fiscal 2025, the animals had spread to 18 municipalities, causing annual crop damage of around ¥4 million to ¥9 million. Their range has also expanded northward, with sightings reported as far as Kashiwa, Chiba Prefecture, and across the border in Ibaraki Prefecture, where the first animal was recorded in 2017. In May this year, Ibaraki captured its first kyon, a male, in the city of Koga.

Kenji Suzuki, who manages a farm in Koga, says the captured animal had been spotted at a nearby farm, although he says he’s yet to see one with his own eyes.

Morosawa is testing whether kyon are attracted to mikan but has had mixed results so far. | Johan Brooks

Kyon can take a toll on both the natural environment and local livelihoods. They browse on a wide range of plants, including young trees and forest undergrowth, raising concerns about the effects on native vegetation and the wildlife that depends on it. They also raid crops and gardens, adding to the burden on farmers and residents. As their numbers grow and they spread into new areas, authorities worry that containing the damage will become increasingly difficult.

“Because Reeves’s muntjac is designated as an invasive alien species, our ultimate goal is to eradicate it,” says Atsuko Jonouchi, deputy director of Chiba Prefecture’s Environment and Lifestyle Department, which oversees wildlife management. “We want people to capture as many as possible.”

Male kyon age 2 or older weigh an average of 10 kilograms, compared with 8.9 kilograms for females. Males also tend to be slightly longer and taller, measuring an average of 75 centimeters from head to rump and 39 centimeters at the shoulder, compared with 73 centimeters and 36 centimeters for females.

Chiba Prefecture’s latest five-year control plan, which took effect in April, prioritizes preventing the species from spreading further north. Under the plan, municipalities are expected to increase trapping in areas where kyon are already established, while the prefecture concentrates on the fringes of their range. Most are caught using cage traps, with the prefecture providing subsidies to help cover the cost.

“It’s proving quite difficult. Their range is continuing to expand,” Jonouchi says.

Morosawa explains that males tend to arrive first, followed by females, as the species expands. Camera traps and audio recorders are deployed to track their spread. “Cameras cover a relatively small area, while calls can be heard much farther away,” he says. The challenge is that researchers still cannot distinguish between male and female calls.

A motion-activated video camera managed by Morosawa. | Johan Brooks

The team is also exploring the use of artificial intelligence to automate identification. “Foxes and owls make similar sounds, so we’re working on developing a system that can distinguish the calls automatically,” he says.

A separate kyon population took hold on Izu Oshima island after a typhoon in 1970 damaged fencing at the Oshima Park zoo, allowing animals to escape. With no natural predators, they spread across the island, feeding on crops such as ashitaba and endangered native plants, including the orchid kinran. Their population peaked at around 20,000, according to Tokyo’s estimates, before falling as the metropolitan government intensified its capture efforts, with the latest estimate putting the population at 12,707 at the end of 2025.

In September last year, the Tokyo government began paying island residents ¥8,000 for each animal recovered from traps on their property, provided they inspect the traps and report captures. It has also introduced a Line instant-messaging based system for reporting sightings, using the information to help determine where additional traps should be placed.

“One major difference from Chiba is that we can use firearms to capture the animals,” says Tamayo Haramizu of the Tokyo Metropolitan Government’s Bureau of Environment. In Chiba, forests and farmland are more often interspersed with homes, roads and other areas, where shooting would be unsafe or prohibited. Izu Oshima, by contrast, has extensive tracts of uninhabited woodland. “We conduct shooting operations in areas where there are no homes.”

Shooting operations are conducted by individual hunters as well as organized teams of around five people. The work is contracted to professional operators, including local businesses and companies from outside the island.

Under its Fourth Control Plan, Oshima has designated 2026–2028 as an intensive capture period, aiming to reduce the estimated population of 12,000 before eventually eradicating the species.

Understanding how kyon behave, eat and sleep is crucial for developing countermeasures to prevent their spread. | Johan Brooks

“While kyon’s numbers are down, we are still a long way from eradication,” Haramizu says. “Our immediate objective is to reduce the remaining numbers through this three-year intensive capture campaign.”

While the authorities’ goal is to eradicate the animal, Morosawa says kyon’s lean, red meat is surprisingly tasty, with a texture similar to young deer. But although eating an invasive species is not prohibited under Japanese law, keeping one without a permit, transporting it alive or releasing it into the wild are all forbidden. Authorities worry that turning kyon meat into a commercial product could give people an incentive to breed the animals or release them into the wild, making the problem even harder to contain.

“Deer tend to move around as they forage, whereas kyon are territorial and generally stay within a relatively small area,” Morosawa says. 

“Their behavior and movement patterns are quite different. We hope to use GPS tracking to better understand their movements and distribution, and apply that knowledge to prevent them from spreading further.”

As we make our way from one camera trap to another, Morosawa points to a small, upside-down basket secured to the ground near one of the cameras. Inside are pieces of mikan, or Japanese mandarin orange. “We’re testing whether kyon are attracted to the fruit,” he says, checking footage from a nearby camera. It shows a kyon wandering past the bait without stopping. “It seems some kyon are drawn to it, while others aren’t.”

Morosawa says it’s not yet clear how much damage kyon are doing to native vegetation. On Oshima, they have been known to eat rare orchids, while in Chiba, residents have reported damage to garden flowers and strawberry plants. If kyon continue moving north into Ibaraki, he says, farmers could face damage to crops such as sweet potatoes and lettuce.

The issue has attracted growing attention in Chiba, both in the media and within the prefectural government. The prefecture has dealt with other invasive species, including raccoons, and Morosawa says officials are increasingly aware that they need to act before kyon spread much farther.

“If kyon were to spread across Japan, people might ask why Chiba Prefecture didn’t do more to stop them when it had the chance,” he says.