Like a Bat into the Desert

by Ron Harton


For most people, an experience with a bat occurs at the extremity of its life. Both Joseph Wood Krutch and Ann Zwinger tell of their experiences with bats near death. It is so difficult for humans to enter into the bat’s world. Perhaps that is one reason for the many strange stories associated with bats. My only close up experience with a bat came early in the morning along a canyon trail. I was leading the way and stepped right past a grounded bat. Cindy saw it and called me back to look. The bat was lying on the side of the trail, frightened and angry, wings outstretched, head raised, eyes glaring, hissing in warning. Its tiny white teeth looked razor sharp. Instinctively we backed up, but the bat was not going anywhere. Suffering from some invisible injury or disease, it apparently was hopelessly grounded in that spot.

A bat’s “otherworldliness” comes from several unique attributes. First, it’s the only mammal with wings that work for real flight. Bat wings are thin, hairless layers of skin that extend from the entire length of the sides of the body incorporating the hand, front legs, back legs, and tail. The fingers of the bat’s hand form the ribs of the wing. The wing muscles are attached to the shoulder blades. The wings are very effective, enabling bats to reach speeds of sixty miles an hour. Second, a bat’s hind leg joints flex backward like a bird’s legs instead of forward. The backward fold helps them to attach to the sides of caves and crevices. In addition, a bat’s small head is dominated by its large ears, eyes and wide mouth. Because of their unusual looks and their nocturnal habits, many people assume bats are rare. However, although some species are declining, bats are a common life form on planet Earth.

Range and Habitat

In fact, bats make up one fourth of all mammal species and are second in diversity only to rodents. They are found worldwide except in the Arctic, Antarctica and a few isolated islands. In the U.S., they are most common in the Southwest. There are over 20 species of bats in the California deserts.

The pallid bat (Murcielago palido or Antrozous pallidus) is a common desert species. It is named for its blond fur. Murcielago is the Spanish word for bat. Adults are about 3 inches long with a 15 inch wingspan. This bat lives below 6,000 feet from Mexico north through the arid West to Canada, in the open desert and low mountains, roosting in caves, rock crevices, and old buildings. The pallid bat hunts for crawling insects rather than flying ones.

The little western pipistrelle bat (Pipistrellus hesperus) is another desert species. It has a 2.5 inch pale silvery body with black wings and ears. It stays active in the cold desert winters, even in temperatures to – 5 C.

The Mexican free-tailed bat (Tadarida brasiliensis) is the most famous bat-resident of Carlsbad Caverns, New Mexico. Visitors watch the bats spiral counterclockwise from the mouth of the cave every evening. The tail of this reddish-brown bat extends beyond the membrane attaching it to the wing.

In contrast to the previous bats, whose ranges extend to non-desert areas, the California leafnose bat (macrotus californicus) lives only in the hottest parts of the California, Nevada, Arizona, and Mexican deserts. It gets its name from a leaflike flap of skin projecting upward from the tip of the nose.

The unusual spotted bat (Euderma maculata) is another species restricted to the arid West. It has three white spots on its back, one on each shoulder and one just above its tail. The spotted bat’s huge ears are half as long as its 4.5 inch body.

Echolocation

Although bats are not blind, they are famous for their use of ultrasonic sound waves to navigate and to locate food. Echolocation in bats was discovered by Donald Griffith in 1940. Bats produce ultrasonic sound waves through their nose or mouth. The waves reflect from insects in flight and other objects and are received by the bat’s large, flexible ears. Muscles in the ear and the shape of the inner ear control reception of echo signals and block interference from outbound signals. The bat uses sound waves to determine the size, shape, texture, and movement of objects. One of the few things a bat can’t perceive with sound is color.

Echolocation, of course, enables bats to be active at night. Nocturnal activity gives the bat several advantages. Bat’s thin wing membranes generate a lot of heat. Cooler night temperatures facilitate body temperature regulation. Night also greatly reduces moisture loss. At night bats also avoid most predators. Occasionally an owl may capture a bat, but not often.

Life Cycle

Although bats worldwide eat a wide variety of food, bats native to the United States are all insectivores. Bats become active around sunset. They awaken and fly out to drink and hunt for food for a few hours. Bats often drink when they first awaken in the evening before they feed. They drink while flying, sailing low over water and scooping up a drink with its lower jaw. They catch insects in their mouth, or, in some species, a basket-like pouch formed from the tail membrane. Studies on the little brown bat (Myotis lucifugus), found in cooler desert regions, show a capture rate of 7 insects per second. Then they fly back to the roost to rest. Most desert species roost in caves, abandoned mines, and rock crevices. Toward dawn they emerge for a second feeding time. They return to socialize and groom, communicating with each other before resting through the day. Bats are extremely clean–licking, scratching, and grooming themselves for hours, much like a cat does.

In many species, male and female have separate lives except for mating. Males and females even roost separately. They mate sometime during the fall and winter. The female stores the sperm until spring when she emerges from hibernation. In May and June the female bats congregate to give birth. Most bats give birth to 1 or 2 young. Bats are like primates and only have two mammae with which to feed their young. The mother hangs with her head up to give birth, catching the newborn in a membrane pouch. The baby crawls up to his mother’s mammae and attaches itself to feed. Remaining there for the first few days, the baby may ride with the mother on her nocturnal foraging. Soon, however, the baby attaches itself to the roost and the mother brings back food for it several times a night. Thousands of baby bats may be hanging in a cave, but the mother locates her own offspring by a pattern of audible and ultrasonic sounds. Infant bat mortality is very high. Some bats weaken and fall from their roost while they await their mother’s return. Others die from disease and parasites. At about three weeks old the young bats are ready to forage for themselves. They become members of the colony into which they were born, using the same roosts and foraging territories.

When winter comes and the insects are no longer plentiful, bats hibernate. In hibernation, their heart rates may drop to one beat per minute and their internal body temperature lowers. Hibernation is a critical time for bats. Humans entering caves and mines can unknowingly disturb hibernating bats, causing them to use up precious body fat resources. The awakened bats may not be able to renew their resources during the winter and may die before spring. Desert explorers should be careful not to disturb sleeping bats anytime, but particularly in the winter.