Spotlight

Blind and Bitten in the Bat Cave

By Dr Simon D. Pollard

I sat on a rock outcrop in north 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 five kilometers 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 80 km 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 tonnes 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 -- an average of 500 cm a year. Rapid weathering of the limestone by water flowing through it has created a massive cave system. Deer Cave, over two kilometers long and ranging up to 170 meters wide and 220 meters high, could hold five cathedrals the size of Saint Paul's in London.

Deer Cave and its bats are a prime attraction of Gunung Mulu National Park, an almost 53,000 hectare zone close to the border between Sarawak and Brunei. I visited the park with Robert Jackson, jumping spider expert and colleague from the Zoology Department at Canterbury University. One thing that drew us was a report from a 1977-78 expedition 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.

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 five kilometers up the twisting Melinau River. We would then walk three kilometers on a walkway through the rainforest to the west entrance of Deer Cave.

The huge entrance, from which flows a three-meter-wide stream, is at the base of a theater of limestone cliffs, many of them covered with vegetation. A couple of hundred meters inside, the ground beside the stream is a seething mass of cockroaches, beetles 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 byproduct of the breakdown of proteins. It is warm inside the cave, about 26^oC, 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 at Queensland University, estimates that they number between two and five million individuals. He has observed at least 12 species, probably the greatest variety known for a single cave. Most prominent are wrinkle-lipped, horse-shoe and naked bats. I rarely saw bat bodies on the ground, however -- further testimony to how quickly scavengers absorb whatever falls to the cave floor.

About half a kilometer into the cave, the stream disappears below ground and the passage narrows. Here, the bat roosts are only 60-90 metres 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 2km-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 metres high. From the air, it looks as if a gigantic finger had poked a hole in the rainforest.

The vegetation in the Garden of Eden is beautifully backlit when seen from within the cave, but to reach it you have to surmount one last obstacle, a deep, dark pool in the stream. Swimming through the pool, I could not help feeling that some monster would nibble at me or yank me beneath the surface, but fortunately, as in the garden itself, no large animals lurk there.

The most obvious signs of creatures that inhabit the cave, other than mounds of guano, are spider webs. Thousands of amaurobiid cobweb spiders build flat, circular sheet webs on the moss and soil around the entrances and on high mounds of collapsed limestone farther into the cave, where light penetrates.

In darker parts of the cave and in dim regions close to the entrance, psechrid cobweb spiders with very long legs bounce across the silk of their huge trampoline-like webs to grab trapped insects. Their webs are littered with the hollow exoskeletons of the cockroaches, flies, earwigs and beetles, that feed on bat guano. Hairy earwigs, which live among naked bats and suck oil from their bodies are also common victims. Assassin bugs, disguised as bits of fungus-riddled detritus, scavenge the remains of prey, moving very slowly across the silk to avoid being detected.

On the limestone walls, where the psechrids cannot attach their trampolines, small hersiliid or two-tailed hunting spiders sit motionless, with their long pairs of spinnerets sticking up behind them. Hersilids do not build webs, but sit in ambush and wait for prey to land within striking distance. When a victim is close enough, the spider behaves like an out-of-control wind-up toy, wrapping its meal in a bag of silk. Other hunting spiders common on the pale brown limestone walls are two-tone brown heteropoids or huntsman spiders, the size of human hands. They wait in the darkness for tactile and vibratory cues from prey before leaping on them. Their victims include camel crickets, which hang out in large groups, their long antennae "listening" for predators.

Many spider species inhabit Gunung Mulu National Park, including some eighty jumping spiders (salticids). One thing we were on the lookout for inside Deer Cave was Portia fimbriata, a jumping spider recorded as living on the nearby limestone cliffs. Spotting these brown, 8-mm creatures is no easy task, because they mimic bits of detritus. My colleague is an unrivaled jumping spider detector, however, and he soon discovered them living among the spider webs inside the cave and dining on their preferred prey -- other spiders, including some particularly dangerous species.

We found that when it had sufficient light, P. fimbriata used its normal tactics, which depend on its eyesight. Typically, from a distance it sees and identifies a spider in a web. It then mimics prey trapped in the web by making vibratory signals and watches how the other spider responds. Because it can see well, P. fimbriata can time its attack with precision. Our subsequent experiments have confirmed that they can also capture prey in total darkness. Exactly how they do this is still being investigated.

Scavenging among the spiders and crickets are tailless whip scorpions and 15cm centipedes. The centipedes, whose bodies are dwarfed by large legs and long antennae, move backward and forward equally quickly and, when disturbed, cover 10-20 metres before pausing. Seeing all these invertebrates on the cave walls reminded me of the African savanna, where the herbivores seek safety in numbers and carnivores and scavengers forage opportunistically.

Unlike most of the cave's residents, the bats have to leave to feed. The mass exodus is spectacular. At first, the bats are hesitant, perhaps mindful that bat hawks on surrounding cliffs are prepared to pick them off. Some circle around the roof of the entrance, like penguins trying to avoid being the first one in the water. Eventually, however, some spiral up the cliff-face and into the sky. Others soon follow in continuous streams.

Like the bats, we too spent the days in Deer Cave and left in the evening. As we walked back through the rainforest to our boat, frogs and insects sang out in a cacophony. Cicadas and crickets rasped industrial techno beats. One species of frog croaked like an antique car horn, while another spent the night singing "what, what, what." Most unsettling was the frog that sounded like a cuckoo clock running in reverse. Our headlamps picked up the eye shine off 10-cm huntsman spiders. A light show was also provided by the fireflies, whose continual flashes made me feel as if I were being pursued by insect paparazzi ("Alien Found in Bat Cave!").

Bat hawks aren't the only ones waiting for the bats to leave Deer Cave. I would often emerge from my forays into the cave's deeper recesses, covered in bat guano and with my mud-caked camera equipment around my neck, to find a hundred people of all ages, some carrying picnic baskets, waiting around the west entrance. Since Gunung Mulu National Park was opened to the public in 1985, the number of visitors coming to see the bats has risen to about 1,600 a month.

The park can be reached only by a 35-minute flight in a small plane or an eight-hour boat journey. The nightly bat exodus is becoming one of Sarawak's biggest tourist attractions. Park officials hope they can implement a management plan that ensures the bats are not overwhelmed. They are worried that the noise levels from visitors inside the cave could stress the bats and cause them to seek other caves. Another concern is the destruction of habitats on the coast, where some of the bats fly to feed.

As outside technology, wealth and culture infiltrate this isolated region, wildlife has made some ingenious adaptations. On our boat trips, bats would dip in and out of my headlamp beam to grab insects attracted to the light. In nearby Clearwater Cave, I saw a 2-metre cave racer snake coiled next to one of the lights used to illuminate the cave during visiting hours. The light was off, but according to our guide, the snake stakes out a position next to the light which attracts flying insects when it is switched on. I wondered if the snake salivated Pavlovian drool when the lights come on.

With the rapid development of Mulu as a tourist destination, I asked senior guide Tom Damit how long it would be before cellphones would be as common on the hips of locals as they are in the city of Miri. He laughed and said he could imagine a scenario where he would be hunting monkeys in the forest with a blowpipe and just as he was about to blow the dart, his cellphone would go off and scare the monkey away.

Indirectly, any such development would depend on the continued daily fall of tons of bat guano to the floor of Deer Cave.

Dr Simon D Pollard is Curator of Invertebrate Zoology at Canterbury Museum.