The dark winged inhabitants form swirling black waves in the evening sky in a phenomenon known as the bat exodus, which takes place almost every evening, as they emerge out of Deer Cave here.
It is not just a sight to behold but an extraordinary experience, akin to watching a perfectly orchestrated symphony of nature.
The Mulu National Park was recognised as a Unesco World Heritage Site in 2000 for its geological features and rich biodiversity. Deer Cave is said to be one of the largest limestone caves in the world – and home to millions of bats.
To reach the viewing area, visitors must traverse a 3km plank walk that crosses a peat swamp, stream, and limestone outcrop. It takes about 45 minutes before you arrive at an open area near the entrance to the cave.
According to park guide Richie Philip, the cave is home to an estimated two to three million bats across 12 species, with the wrinkled-lipped bat colony being the most dominant, creating striking formations as they leave the cave in search of food.
“They usually come out between 5pm and 6pm,” he told Bernama. “Sometimes they don’t emerge if it rains heavily. But when conditions are right, the bats pour out in an unbroken stream, swirling through the sky. It is truly an incredible sight.”
Before their mass exit, the bats circle the cave entrance in spiral or S-shaped formations as a natural strategy to confuse predators such as eagles, as well as to wait for other groups to join them for safety in numbers.
Richie said the bat exodus occurs only at Deer Cave thanks to its main chamber, which rises more than 100m high and is partly illuminated by sunlight, unobstructed by surrounding trees.
The bat stream can last several minutes, forming what looks like black clouds across the sky, with the winged mammals flying at speeds of up to 70km per hour for as far as 50km before returning to their habitat the next morning.
Richie added that the bats are not only a major tourist attraction but are vital to the cave’s ecosystem, as their guano is a food source for numerous insect species and small fish. “Without the bats, the cave’s food chain would collapse,” he said.
He also advises visitors to remain silent while watching the phenomenon as noise can cause the bats to fly at higher altitudes.
“In silence, they fly lower, allowing the sound of their flapping wings – like the gentle patter of applause – to be heard clearly,” he concluded.