The second possibility is the most likely, that centuries ago there were other openings to the cave through which the bats entered and spent the daylight hours. As old age overtook them they would die and fall to the floor. Then, as subsequent earth movements closed the openings, the bats would find new doorways to their daytime abode. This also accounts for large quantities of guano which were found in portions of the cave where no bats were to be seen sleeping on the ceiling above.
There are eighteen species of bats to be found in New Mexico, and thirteen are known in the Carlsbad region. In the United States as a whole, scientists have classified over 251 varieties of these winged mammals.
Of the thirteen found near the Caverns, eight are known to have made use of the cave at one time or another. Old skeletons have been found in remote sections of the cave which have been identified as those of the Large Pale Bat (Antrozous pallidus), wing bones of which were found in one of the innermost rooms; the Big Brown Bat (Eptesicus fuscus), a skull and wing bones having been discovered in an inner room; House Bat (Myotis incautus), several skull fragments of which were found in one of the lower rooms of the cave; Red Bat (Nycteris borealis), two old skulls of which were found on the floor of one of the deepest rooms; Cave Bat (Myotis velifer), evidenced by imperfect skull fragments found deep in the cave; Great Hoary Bat (Nycteris cinerea), largest of the northern bats, possessing silvery grey fur and not normally a cave dweller, but wing bones were found in the cave, indicating they may have lived there at one time, and the Little California Bat (Myotis californicus pallidus), whose presence was indicated by a skull picked up amid the dust and debris of a low, inner room.
The chief resident of the caves for centuries has been the Mexican Free-tailed Bat (Tadarida mexicana), also sometimes called the guano bat because its species is especially noted for the huge quantities of commercially excellent guano it produces.
The Free-tailed Bat is distinguishable from the northern bats by its extended tail which reaches about an inch beyond the interleg membrane, by its short fur, and by a strong odor not found with other species. They also possess short, stubby ears, will weigh between 10 and 12 grams, and prefer caves more than most any other species.
The life of the bat is somewhat of a mystery to the average person, partly because they seclude themselves during the day in places relatively safe from discovery, and venture out at night when humans are not around and couldn't very well see them if they were.
Bats resemble the ordinary field mouse in appearance, except for their large, expansive wings. They are mostly insectivorous, living almost exclusively on insects they catch while in flight, chewing their prey into the finest bits with their sharp teeth. A bat will eat enormous quantities of food between sunset and sunrise, and on the basis of only two full meals a night will consume an amount almost half its own weight.
Their diet consists of beetles, flies, and other flying insects, most of which are enemies of civilization; thus the bat is economically a valuable animal.
They leave the mouth of the cave at dusk, heading first for a drink of water at the nearest available spot, then spend the night in search of their food. As the sun's rays begin to appear in the east they fly back to the cave where they dig their tiny claws into the ceiling and remain there sleeping until the sun sets in the west and the cycle starts all over again.
Due to their habits they have few enemies, the great horned owl being one. Very wide awake at night, he often catches an unsuspecting bat, chews the flesh, letting the bones drop to the ground where they remain as evidence of the owl's dinner.