The genetic diversity loss of the Javan bull makes it increasingly difficult for this Indonesian endemic mammal to adapt to climate change, habitat loss, and disease threats.

Pasuruan, East Java (Indonesia Window) — Threats to the survival of the Javan bull (Bos javanicus) are no longer limited to hunting and habitat loss. In the long term, declining genetic diversity is a crucial issue that could make it increasingly difficult for Indonesia's endemic bull to adapt to climate change, habitat loss, and disease threats.

Eko Windarto, a mammal curator at the Prigen Conservation Breeding Ark (PCBA), made the remark during a media trip to PCBA (Prigen Conservation Breeding Ark) in Indonesia’s Pasuruan district, East Java province, on Thursday (Dec. 18).

"One of the long-term threats to bull is losing genetic diversity. If the genetic diversity is limited, they would have difficulty adapting to environmental changes or facing disease," Eko pointed out.

According to him, the bull population in conservation institutions (LK) currently faces serious limitations in obtaining fresh blood from the wild, and as a result, the choice of breeding stock is very limited.

"If a zoo only has one pair of bull, there's a high risk of inbreeding. This is different from an institution like Taman Safari Indonesia (the Indonesia Safari Park), which has several individuals, so breeding rotation is still possible," he explained.

Eko said that the release of a bull into the wild should not be in a hurry. "We don't want their genetics to be poor when they're released. In the wild, they'll be more susceptible to disease, their bodies would shrink, and their population would weaken," he noted.

Bulls in conservation

Currently, the ex-situ (outside their natural habitat) population of Javan bulls in Indonesia is recorded at 67 individuals, consisting of 23 males and 39 females, spread across seven conservation institutions.

Birth data shows an increasing trend, although it has slowed down. In 2019, there were eight births, 2021 (3), 2022 (1), 2023 (3), and 2024 (5).

"On the one hand, we want to increase the population, but on the other hand, we're concerned about mating due to the risk of inbreeding," Eko said.

By the end of 2025, bull births had nearly reached the annual target of 10, with a total of nine births spread across Bogor Safari Park (West Java province), Prigen Park, Bali Safari Park, Ragunan Zoo (Jakarta), and Pananjung Nature Reserve in Pangandaran district, West Java province.

Eko explained that to address genetic issues, bull conservation is being carried out cooperatively among conservation institutions, including the transfer of individuals.

Six institutions are recommended to exchange female and male bulls to expand genetic variation. Two bulls, Dewa and Titus, are even scheduled to be transferred to Surabaya Zoo soon because the males there are no longer productive.

"Bulls can be productive at 15-16 years old, but in conservation institutions they can alive up to 26 years as most of them are F-0 (first generation) from the wild," he noted.

Challenges of reintroduction

Some in situ conservation facilities for the Javan bulls can be found in some areas in East Java province, namely Alas Purwo in Banyuwangi district, Meru Betiri (southern coast), and Baluran in Situbondo district. The total number is estimated at 150–200 bulls.

Eko noted that one of the most challenging reintroduction efforts was carried out in the Pananjung Nature Reserve, West Java province. The bull population in that area was declared extinct after the only female died in 2023. Through collaboration with the Ministry of Forestry and the Natural Resources Conservation Agency (locally known as its abbreviation BKSDA), four bulls were released in stages on December 25, 2024, following a strict habituation process, temporary electric fencing, and several days of feeding.

"We can't immediately release them into the wild. If they get sick after release, we could loss. So there's an adaptation period, including observations of mating behaviors and health," Eko said.

These steps have showed results. The bulls demonstrated natural behaviors such as grazing, foraging, establishing territories, and even mating.

In fact, the bull's presence has attracted other animals, such as deer, to the area. This process resulted in the birth of two calves in 2025, named Exploitasia on July 27, 2025, and Haruni on August 7, 2025.

Genetics key to the future

The PCBA, along with other conservation institutions, is currently collecting bulls’ genetic data. Blood samples from 31 bulls have been collected for DNA analysis, which is planned to be sequenced by BRIN (the National Research and Innovation Agency). Until now, lineage tracing has relied on manual records among zoos in the country.

Furthermore, PCBA is the only location in Indonesia with a laboratory and sperm bank for Javan bulls, with a reserve of around 150 straws. "If a natural disaster occurs or animal migration becomes more difficult, we can still transfer genetic materials through artificial insemination, just like we do for cattle," Eko said.

He emphasized that the challenges of Javan bull conservation would become increasingly complex due to limited pens, high migration costs and complicated permits, large animal logistics, and the risk of diseases such as foot-and-mouth disease. However, maintaining genetic diversity remains the key for Javan bulls.

Reporting by Indonesia Window