Thousands of toque macaques died mysteriously in 1987. Still unexplained. Possibly from a novel pathogen or toxic algal bloom carried by monsoon rains.
Carlos Delgado, CC BY-SA 4.0, via Wikimedia Commons
An unsolved epizootic
In 1987, a sudden and mysterious event devastated the population of toque macaques in Sri Lanka's Horton Plains National Park. Over several months, a significant portion of the highland toque macaque (Macaca sinica opisthomelas), a subspecies found only in this high-altitude region, perished without a clear cause. The die-off was alarmingly swift, wiping out entire troops of these monkeys and leaving a lasting impact on the local primate population. Decades later, the exact cause remains a puzzle in the field of disease ecology.
Horton Plains is a montane ecosystem, a plateau of cloud forests and wet grasslands sitting at an elevation of 2,100–2,300 meters (6,900–7,500 feet). This cool, misty environment hosts many endemic species. The affected monkeys, the highland toque macaque, are one of three subspecies endemic to Sri Lanka. Primatologist Wolfgang Dittus, who has studied Sri Lanka's macaques for over 50 years, has documented the long-term population dynamics of these animals, providing the context for understanding the severity of such an event. While his primary research site is in Polonnaruwa, his work shows the vulnerabilities of these primate populations.
The leading hypotheses
Without preserved tissue samples from 1987, scientists cannot definitively identify the cause. However, two primary hypotheses persist. The first suggests a novel pathogen, such as a virus or bacterium, swept through the immunologically naive macaque population. Sudden die-offs in primate populations have been documented elsewhere, sometimes caused by pathogens like simian hemorrhagic fever or other viruses that can be highly lethal. The closed nature of the Horton Plains population could have made them particularly susceptible to a newly introduced disease.
A second compelling theory involves environmental toxins, specifically from an algal bloom. Horton Plains is the source of three major Sri Lankan rivers and contains many streams and pools. The hypothesis suggests that unusual weather patterns, perhaps linked to the monsoon, could have triggered a bloom of toxic cyanobacteria in the water sources. These toxins could have been ingested by the macaques, leading to rapid and widespread death. Cases of sudden death in monkeys due to poisoning, often from pesticides in agricultural areas, show that primates are vulnerable to environmental contaminants. The lack of a definitive answer makes the Horton Plains die-off a critical, if frustrating, case study in the complex factors that can trigger sudden wildlife mortality events.
💡Fun Facts
Horton Plains is a UNESCO World Heritage Site, recognized for its unique high-altitude ecosystem and biodiversity.
The toque macaque (*Macaca sinica*) is one of the smallest species of macaque and is endemic to Sri Lanka.
The park is the source of three of Sri Lanka's largest rivers: the Mahaweli, Kelani, and Walawe.
The highland subspecies of toque macaque (*M. s. opisthomelas*) is the largest of the three subspecies and has a noticeably thicker coat to survive the cool climate.
Park gates are open daily from 6:00 AM to 6:00 PM. Ticket counters close at 4:00 PM.
Admission
For foreign adults, the fee is approximately $25 USD plus service charges, totaling around $30-$35 USD. Payment must be in Sri Lankan Rupees.
Accessibility
The main attraction is a 9 km circular trek on foot over undulating terrain, including grasslands and forest paths. The trail is often misty and can be cool, especially in the early morning.
