A climate-change-fueled cyclone in November 2025 killed approximately 58 Tapanuli orangutans, representing 7% of the total species population, according to research published June 10 in the journal Current Biology. The four-day storm triggered catastrophic landslides across the Batang Toru forest in Sumatra, Indonesia, creating an immediate, life-threatening crisis for the world’s rarest great ape.
Did You Know? Tapanuli orangutans are the world’s most recently identified great ape species, having been classified as distinct from their Bornean and Sumatran cousins only in 2017.
The Impact of Cyclone Senyar
During the four-day event in November 2025, 21.8 inches (556 millimeters) of rain fell on the west block of the Batang Toru forest, according to the study. This extreme rainfall led to over 50,000 individual landslides, which destroyed critical habitat and caused the deaths of the apes through drowning, trauma from falling trees, or burial in debris. Before the storm, the west block was 99.3% forested, providing the necessary arboreal environment for the species.
Why the Species Remains Vulnerable
The Tapanuli orangutan population is particularly susceptible to environmental shocks due to a slow reproductive rate, with intervals of six to nine years between births, as noted in the research. Because the species relies heavily on specific tree cover and food sources found in topsoil, the destruction of the forest floor—which contains vital plant-feeding fungi—poses a long-term threat to the remaining population. Serge Wich, a professor of primate biology at Liverpool John Moores University, described the loss as “extremely worrying” for the future of the species.
Expert Insight: The convergence of this specific cyclone with a negative Indian Ocean Dipole and La Niña suggests that the species is facing a compounding threat. When habitat destruction—already a primary driver of their endangerment—is accelerated by intensified, climate-driven weather events, the window for natural recovery narrows significantly due to the apes’ low reproductive turnover.
Future Climate Risks
Analysts expect that climate change will continue to increase the frequency and intensity of heavy rainfall in Indonesia, creating potential for further habitat loss. With the onset of a significant El Niño event, which officials at NOAA indicated in a June 11 update could rank among the largest on record since 1950, weather conditions are likely to remain volatile. U.N. Secretary-General António Guterres stated on June 2 that such climate conditions serve as an urgent warning that the world must address the warming climate.
Frequently Asked Questions
How many Tapanuli orangutans were killed in the cyclone?
Approximately 58 individuals died, which accounts for 11% of the west block population and 7% of the entire species, according to the study.
What caused the landslides in the Batang Toru forest?
The landslides were caused by 21.8 inches of rain falling over four days, an event intensified by human-induced climate change, the negative Indian Ocean Dipole, and La Niña, according to the research.
Why are these orangutans at higher risk than other species?
They are the rarest great ape species, with a total population of only 767 individuals as of 2019, and they have a very slow reproductive cycle that limits their ability to recover from sudden population losses.
What measures could be prioritized to protect remaining habitats from similar climate-driven disasters?
