Under the rainforest

Natural History, April, 1998 by Simon D. Pollard

I sat on a rock outcrop in northern Sarawak, the principal Malaysian state on the island of Borneo, looking east across the Melinau River. The rainforest spread out before me and gently rose like a rumpled quilt over a small range of mountains three miles away. As dusk descended and rain clouds gathered, black streaks rose from the base of the mountains like floating scarves. The scarves were millions of bats pouring out of Deer Cave and beginning their almost daily foraging commute to distant sites, including the coast fifty miles away. They would return to their roosts before daylight, after not only feeding themselves but also securing a source of energy for other inhabitants of the cave. Just as life on earth depends on the sun continuing to shine, a host of invertebrates depend, directly and indirectly, on the three tons of bat guano that fall to the cave floor each day.

Deer Cave owes its existence to the region's very thick limestone deposits and heavy rainfall - about 200 inches a year. Rapid weathering of the limestone by water flowing through it has created a massive cave complex. A travel brochure boasts that Deer Cave, more than a mile long and ranging up to 600 feet wide and 700 feet high, could hold five cathedrals the size of Saint Paul's in London. Long known to the local inhabitants, Deer Cave got its name because people used to hunt deer that came to drink water flowing out the west entrance. Not far from Deer Cave is the Sarawak Chamber, the world's largest natural chamber, not discovered until 1984. It is nearly 2,000 feet long, 1,500 feet wide, and more than 300 feet high.

Deer Cave and its bats are a prime attraction of Gunung Mulu National Park, a 210-square-mile zone close to the border between Sarawak and Brunei. The park is named for Gunung (Mount) Mulu, a 7,800-foot sandstone peak that looms nine miles east of Deer Cave. I visited the park with Robert R. Jackson, a jumping-spider expert and my colleague from the zoology department at New Zealand's University of Canterbury. One thing that drew us was a report, from a 1977-78 expedition conducted by the Royal Geographical Society and the Sarawak government, that jumping spiders were living around Deer Cave. We wondered if these creatures, which depend on keen eyesight to catch prey, could be living inside a cave. In any case, the habitat was bound to be intriguing.

Despite the remote location, we did not have to rough it. Our base camp was the recently constructed Royal Mulu Resort, just outside the park boundary. Each morning, our guide, Richard Jalong, would arrive in a long, wooden boat with an outboard motor and take us three miles up the twisting Melinau River. We would then proceed two miles on a walkway through the rainforest to the west entrance of Deer Cave. For the benefit of tour groups, a concrete path leads about two-thirds of the way through the cave. Even some generator-powered lighting is installed, but the tour guides we saw never turned it on, preferring to rely on their flashlights.

The huge entrance, from which flows a ten-foot-wide stream, is at the base of a theater of limestone cliffs, many of them covered with vegetation. A couple of hundred yards inside, the ground beside the stream is a seething mass of cockroaches, beeties, and flies crawling over bat guano. These creatures extract nutrients from the partly digested or undigested remains of insect prey that have passed through the bats. A cloud of mist often hangs over the guano, filling the air with an ammonia stench, a by-product of the breakdown of proteins. It is warm inside the cave, about 78 [degrees] F, and the air is saturated with moisture.

Hundreds of feet above, visible only as pools of blackness, vast roosts of bats emit a sound like rustling cellophane. Les Hall, a bat biologist in University of Queensland's Department of Veterinary Pathology, estimates that the bats number between two and five million. He has observed at least twelve species, probably the greatest variety known for a single cave. Most prominent are wrinkle-lipped bats and several species of horseshoe bats. I rarely saw bat bodies on the ground, however - further testimony as to how quickly scavengers absorb whatever falls to the cave floor.

About a half mile into the cave, the stream disappears below ground and the passage narrows. Here, the bat roosts are only two or three hundred feet above the floor. Looking into this blackness with my headlamp was like looking into the night sky during a light snowfall, except these flakes were guano. Beyond the narrow passage, another stream appears, and the cave expands into a large chamber. At the east end is an opening to the Garden of Eden, a mile-wide, circular sinkhole filled with jungle vegetation. The sinkhole was created when part of the cave roof collapsed many thousands of years ago, dropping the rainforest down to the level of the cave floor and leaving sheer cliffs hundreds of feet high. From the air, it looks as if a gigantic finger had poked a hole into the rainforest.