I was talking with a grandson about troglodytes. It was a word he did not know. In a dictionary he was amused to read, “reclusive cave dweller; unacquainted with affairs of the world; one who creeps into holes.” Thereupon our conversation took one tack, my thoughts another.

Caves don’t figure much in my experience, although I’ve been in some large unexplored ones. I’m not entirely comfortable in such surroundings, despite finding remarkable creatures hidden deep in the darkness.

I entered an ancient lava tube in Hawai’i, a seven-mile-long subterranean tunnel that had been superficially entered only once in the previous century. I clambered among great fallen rocks in cloying red dust that rose in choking clouds to obscure my headlamp’s beam. Water dripped from overhead. Abrasive dust penetrated my clothes and I worried that my camera gear was insufficiently protected. Evidence of prehistoric native explorers was revealed in ashes from illuminating fires on top of huge blocks that had crashed from the ceiling. I wondered about the earthquake-prone island, how often rocks broke free from overhead, and whether or not there might be a squashed human skeleton nearby. I didn’t want to be added to the exhibits.

Once I got a grip on myself and returned to biological inclinations, I began looking for signs of life. Terrors of the unknown dark and discomfort receded. I thought about what living in a cave must be like.

A cave is a genuine habitat, albeit one unfamiliar to most of us. Its mouth is the sunlit world’s edge. After only a matter of yards, a cave dims into a zone of perpetual twilight, of limited temperature fluctuation, of freedom from rain and snow, a place where only a few plants are able to grow. By plants alone you can determine diminishing light. They range from shrubs outside, to ferns at the mouth, to mosses a little way inside, then to algae on the walls, finally no greenery at all, just pale fungi.

While this zone of transition is worthy of study, it is the deep cave’s utter darkness that I find fascinating, where temperature hovers around 55ºF (15ºC), and the air is saturated at 100% humidity. It is a constant environment, dependable in its characteristics for those creatures able to live there, and hostile to others. Within the first few hundred yards caves may be littered with skeletons of unlucky animals that blundered in and could not escape. Inside a cave on the Galapagos I found the ghostly bones of huge tortoises that had tumbled in and were unable to return to life-sustaining sunlight. And in a Virginia cave my great-great-grandfather discovered bones of a giant ground sloth that Thomas Jefferson “borrowed” from him and did not return, insisting they were those of a lion.

With no plants available, vegetarians are absent and the interactions of deep cave animals are restricted to predation and scavenging detritus and remains of the dead. Nevertheless, no matter how deep an animal lives, it is ultimately dependent upon the outside world. Food doesn’t appear spontaneously in a cave, but must be imported, even if indirectly.

Hawai’i’s hidden lava tubes run for miles only about thirty feet (9 metres) beneath the surface, so their roofs are penetrated by roots of o’hia trees overhead. Dangling in the dank air, they are unpleasant surprises when you run into them, face-on. Examined closely, they are found to support populations of tiny mites and springtails, and their decay gives sustenance to fungi, and various small animals that eat decomposing vegetation.

A confirmed plant eater must live at least in the twilight zone, where it is preyed upon by animals emerging at night from deeper in the cave, and these in turn are eaten either by still deeper ones, or their carcasses are consumed by dark-dwelling scavengers. Thus the chain of nutrition reaches into the blackness, until all connection with the outer world seems lost.

If a cave has been formed by a stream, and water flows from outside into the depths, a continuing source of food may be transported as plant fragments and terrestrial animals are caught in the current. A study of cave fishes reveals a sequence of eye degeneration that makes all blind troglodytic crayfish, shrimps, flatworms, millipedes, spiders, harvestmen, and skinny salamanders easier to understand.

If an eyed fish enters a cave, its vision draws it back to the lighted mouth and the outside world. If, however, an occasional mutation weakens eyesight, competition in the light may be too severe, so life in the twilight zone could be just the place. Additional mutations leading to complete blindness mean a fish incapable of seeing light can live well in the utter dark of a cave, using its senses of scent and pressure-detection to locate food and water movement. Such fishes can be found miles from the mouth of a cave, yet in the laboratory they are capable of interbreeding with their cousins from the light. They are, in effect, the same species, for cave animals have a history dating back at most to the last ice age, about 100,000 years ago.

Some troglodytes have a more recent origin. The lava tubes of Hawai’i can be dated precisely. I studied one only a few centuries old, and already it has eyeless spiders and millipedes. The rate of mutation, therefore evolution of troglodytes, can be rapid indeed.

Once an animal is a confirmed cave-dweller, eyelessness is only one characteristic. Another is a lack of camouflaging pigmentation, for in darkness and without eyes, all animals are invisible. Only in my headlamp’s beam is a pure white flatworm or millipede glaringly obvious. Other senses of scent and touch and vibration are pronounced and are all that is needed for survival.

Cave animals cannot be specialists. A previously vegetarian snail from the outer world must become an omnivore and a scavenger, and like all other creatures around it must be adaptable when it comes to diet.

When bats are numerous, they provide a major source of food for other cave-dwellers. The ceiling of a bat cave on the Caribbean island of Barbuda was packed with millions of bats, not an inch between them. Unpleasant to remember, there was a constant rain of urine, guano, and the occasional fall of a dying bat. Each bat carried parasitic mites, ticks, and blood-sucking flies that dropped off to become part of the larder below. The floor of the cave was seething with hungry beetles, insect larvae, mites, harvestmen, millipedes, flatworms, impossible to avoid. A nightmarish scene not for the faint of heart. To make matters worse, a dangerous mold grew on the organic deposits and if inhaled by humans, its spores cause histoplasmosis, a serious disease some of my colleagues contracted on the spot.

The microscopic world was even more crowded, not only with fungi beyond accounting, but flagellates, ciliates, and ameba-like creatures. Nematodes were abundant, and of course the moist squirming masses on the floor of the cave established a culture medium for more bacteria than I’ve ever seen under any other condition. Having a battery-illuminated field microscope (Nikon Model H) with me allowed glimpses into this hidden world that offered a lifetime study I had neither the time nor inclination to accept. Has anyone?

No wonder caves evoked legends of an underworld among superstitious folk, a purgatory where the unfortunate were condemned to suffer, and a troll or two might caper in glee. Too bad a bit of biology did not explain that life arises and dies as rapidly and normally in caves as anywhere else—although darkness carries its own demons. As for suffering, it’s all in the mind, but spending long hours in a humid, dusty, bat-ridden, insect-infested, spooky cave is a pretty good case for misery.

More happily, my favorite candidates for troglodytes are pale long-legged cave crickets, snowy millipedes, spidery white crayfish, and blind wolf spiders.

What was my grandson’s candidate for a troglodyte? His well-being may be in jeopardy as I reveal this, but he thought his older brother fitted the bill very well.

 

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All comments to the author Bill Amos are welcomed.

© 2002 William H. Amos