Life in caves

   For students of evolution, caves are ideal natu­ral laboratories in which to study adaptations. Not only do they offer an environment where heat and cold, humidity and light remain fixed throughout the year, but the unique conditions found in caves produce dramatically exaggerated effects on plant and animal populations. And from their entrances through a twilight zone extending as far back as light can penetrate to the region of perpetual night, they harbor a surprising number of habitats.

   A partial cave dweller of the interior zone is the Venezuelan oilbird, or guácharo, the only bird known to live in caves. Though it roosts and nests in the sheltering cave, it finds its food outside—in nocturnal darkness. The guácharo's blue eyes can see, but in the dark the bird navigates effectively, like a bat, by bouncing echoes off objects in its path.

   True cave dwellers like the other animals never leave their dark dwellings, but spend their entire lives in blackness. In such surroundings, these animals show many similar traits, sharing certain features unknown in related forms in the outer world. There are, for instance, no large animals that are permanent residents in caves; rather cave creatures tend to be small and slender, with thin, pale body coverings. Surrounded by high humidity, they need no special hair or scales to hold or repel moisture. They have no use for eyes, evolving instead long organs of touch and a sharp sense of smell for locating sparse food. Scavengers or hunters, they follow their own cycles, unrelated to day and night outside. But while most of these blind animalos bear sightless progeny, young Ozark cave salamanders are born able to see. If their larvae are raised in the light, they develop a dark pigment, but lose it later in dark caves. Apparently the ability to produce pigment has been retained de­spite countless generations spent in darkness.