Alternate name: Mexican Free-tailed Bat
Family: Molossidae, Free-tailed Bats view all from this family
Description The smallest free-tailed bat. Dark brown or dark gray above, with hairs whitish at base. Ears separated at base. Calcar pointed backward. Tail extends beyond interfemoral membrane for more than half its length. L 3 1/2–4 1/4" (90–110 mm); T 1 1/4–1 3/4" (33–44 mm); HF 1/4–1/2" (7–14 mm); FA 1 3/8–1 3/4" (36–46 mm); Wt 3/8-1/2 oz (11–14 g).
Warning Bats are susceptible to rabies, a serious viral disease that results in death if untreated. Rabid bats rarely attack humans or other animals, but bats found lying on the ground may be rabid. Never touch or pick up any bat. Stay away from any animal that seems to be acting strangely and report it to animal-control officers. If you are bitten by a possibly rabid animal, you must immediately consult a doctor for a series of injections; there is no cure once symptoms emerge.
Similar Species Except for Little Free-tailed, which occurs only in Florida Keys and has ears joined at the base, all other free-tailed bats are considerably larger.
Breeding Mates February–March; ovulates in March. Females form very large maternity colonies usually in caves or man-made structures. 1–3 (usually 1) young born in June. Female hangs head downward during birth, but flight membrane is not used to receive young.
Habitat Deserts, canyons, farmlands, and other habitats. Roosts in buildings on West Coast and in Southeast, and in caves from Texas to Arizona.
Range Throughout s U.S.; in West, south from s Oregon and s Nebraska; in East, south from n Louisiana, Alabama, and South Carolina. A few are scattered farther north.
Discussion The Brazilian Free-tailed Bat is by far the most common bat in the Southwest; with a U.S. population previously estimated at a minimum of 100 million, it is also one of the most numerous mammals in the country. In the East and on the West Coast, it hibernates in winter rather than migrate. From Texas through the Southwest, it lives in huge colonies in caves, packed 250 per square foot (2,700 per sq m); a few of the Southwestern bats hibernate, but most migrate to Mexico for the winter, usually toward the end of October, returning northward in March to mate. The young hang, sometimes among millions of others, in a nursery, yet pups and mothers are capable of finding one another by their calls and probably odor. Mothers make no attempt to save young that lose their grip on the ceiling, however; such pups perish on the cave floor, where they are consumed by tenebrionid beetles. The Carlsbad Caverns in New Mexico were discovered when these bats were seen emerging from them. Although the cavern population has declined from an estimated 8 to 9 million in the 1930s to several hundred thousand, the bats’ daily emergence is still a major tourist attraction. At sunset, bats begin flitting about inside the cave, causing a slight rise in temperature and humidity. After circling for several minutes, they begin to emerge from the depths of the caverns in a counterclockwise spiral, ascending 150 to 180 feet (45–55 m) into the night air. They emerge in various ways: as one continuous wave; split into two groups with an interval of half an hour in between; or in bursts of several hundred to several thousand bats that give way after 15 to 20 minutes to a continuous stream. They make a great roar and form a dark cloud visible miles away; when the egression is at its heaviest, 5,000 to 10,000 bats emerge each minute. While they may roam up to 150 miles (240 km), most Carlsbad bats feed within a 50-mile (80-km) radius. Generally they fly throughout the night, at 10 to 15 mph (15–25 km/h), feeding on a variety of small insects, especially moths, ants, beetles, and leafhoppers captured in the tail membrane. Each night, a bat eats up to one-third its own weight; 250,000 bats can consume half a ton of insects. The return to the caves, at sunrise, is even more spectacular than the emergence, as the bats plummet straight down from heights of 600 to 1,000 feet (180–300 m) at speeds of more than 25 mph (40 km/h) to a reported maximum speed of 60 mph (96 km/h). Bat droppings in Carlsbad Caverns over the past 17,000 years have formed guano deposits covering several thousand square feet to a depth of almost 50 feet (15 m). Guano was used during the Civil War as a source of sodium nitrate for gunpowder and mined as fertilizer from the turn of the century through the 1940s. A few small-scale guano mines are still in operation. These bats may have a life span of up to 18 years. Hawks and owls sometimes sit at cave entrances and prey on them as they emerge. Black snakes, Common Raccoons, house cats, and other predators sometimes manage to gain access to their roosts. A hazard of entering free-tailed bat caves in the Southwest is the possibility of contracting rabies, which can be transmitted by a bat bite or by the airborne virus. People have also contracted histoplasmosis, a fungal infection of the respiratory tract, from bat caves of the Southwest.
