Tadarida brasiliensis: The Mexican Free-Tailed Bat
Tadarida brasiliensis, also known as the Mexican free-tailed bat, belongs to the Molossidae family and the Vespertilionoidea superfamily in the suborder Microchiroptera. They are relatively small bats, weighing about 15 grams each with a head and body length approximately 40mm to 130mm in length (Wilson 33, Nowak 230). One third of the tail extends beyond the legs and tail membrane, hence the name ³free-tailed bats² (Tuttle 61). The head is thick with a broad muzzle, the eyes are small, and there is no nose leaf (Nowak 231). They have short, strong legs and broad feet with curved bristles on the outer toe of each foot; these bristles are used for grooming their short, velvet-like body hair (Nowak 230,231). Free-tailed bats eat mostly hard-shelled insects, and their teeth are of the normal cuspidate insectivorous type (Nowak 213,232).
Tadarida wings are long, thick, and leathery, as well as being narrower than most bats outside of the Molossidae family (Nowak 231,232). This wing design aids speed and makes possible the long-distance migrations of these bats (Tuttle 62). Free-tails beat their wings at a faster rate than most other insectivorous bats and their flight is swift and relatively straight (Nowak 231). A pattern of free falling flight has been observed at Carlsbad Caverns, New Mexico when the Tadarida bats return to roost. The bats will approach the cave at an altitude of 300 to 400 meters, then fold their wings and plummet to about 2 meters off the ground before snapping their wings back open and shooting into the cave (Hill 48).
The tadarida brasiliensis species is found primarily in the Southwest United States and Mexico, although populations range from as far north as Oregon and as far east as Nebraska and Kansas to Central and Southern America. (Hill 147) It is the only free-tail species common in the U.S. (Tuttle 61). Hill and Smith recognize four general population groups. The first is a small group in southern Oregon and California which is mainly non-migratory. Some of the bats here may become torpid for brief periods while others remain active (Hill 147). The second group described is also non-migratory and remains in parts of California, eastern Nevada, and Western Arizona. Instead of hibernating, these bats move from shelter to shelter seeking out warmth in tunnels or near chimneys (Hill 147). Another population which occupies a region spanning Utah, Colorado, Arizona, and New Mexico migrates seasonally to and from parts of western Mexico (Hill 147). A fourth group of bats which spans Oklahoma, New Mexico, Texas, and the Southeastern U.S. is probably the most well known due to the extensive studies of the Carlsbad Caverns. This group also migrates south each season to caves in eastern Mexico and other southern locations (Hill 147).
Mexican free-tailed bats live primarily in caves and in some man-made structures. They form the largest colonies of any mammals, with most of the species living in roughly a dozen caves (Tuttle 61). The Bracken Cave in San Antonio, Texas houses 40 million bats and is owned and preserved by Bat Conservation International (Sherrow 73,74). The bats generally prefer roosting temperatures of 90-110 degrees Fahrenheit and heights of 20-40 feet (Tuttle 45,46). Carlsbad Caverns in New Mexico is home to another large colony of tadarida brasiliensis which may have been in residence there for approximately 17,000 years (Schober 88). This cave has become a primary location for both tourism and study of these bats.
The Congress Avenue Bridge in Austin, Texas is home to the largest urban colony of Mexican free-tails in existence (Tuttle 62). Repairs on the bridge in 1980 created many small crevices which attracted more than a million bats, mostly female, looking for roosting sites (Sherrow 81). The human residents of Austin were disturbed by this at first and asked the government to get rid of the bats, but due to the cloud-like spectacle exhibited when the bats depart every evening, the bridge has become a tourist attraction (Sherrow 81). Bat Conservation International built an educational kiosk near the bridge, and similar bat friendly bridge designs have been suggested to other building companies (Sherrow 81).
Mating of the tadarida brasiliensis begins in late February and early March, ovulation occurs in late March, and then pups are born in June (Tuttle 62). Sperm production begins in the fall and is completed by the end of February; sperm is then absent from the testes and epididymides from April or May until early the next January (Krutzsch). Free-tails typically produce one young per litter and one litter per year, with gestation lasting between 50 and 120 days (Nowak 232, Hill 99). The young learn to fly in about five weeks (Tuttle 67).
Large masses of young bats are left behind in ³crèches² while their mothers leave to hunt for food, often covering a hunting territory of up to 75 kilometers (Wilson 48, Schober 33). It was thought for a while that when the mother bats returned, they would feed any pup that they found, but more recent studies have shown that mother free-tails are very particular when it comes to finding their own pup (Wilson 48). Baby bats will try and attach themselves to any mother bat, but she will beat off the other young with her wings until she can find and nurse her own (Wilson 48). Mothers who have lost their own pups, however, will help to nurse others (Wilson48).
Although Mexican free-tails are not endangered, their numbers have been significantly reduced by humans. In 1963, the Eagle Creek Cave in Arizona was estimated to be home to about 30 million bats, a colony which consumed roughly 350,000 pounds of insects per night (Sherrow 44). However, by 1969, the colony had been reduced by 99.9% to only 300,000 bats (Tuttle 50, Sherrow 44). Shotgun casings were found at the caveıs entrance, implying that the bats may have been attacked by shooters (Sherrow 44).
Studies have shown that the pesticide DDT often used by farmers in the 1950ıs and 1960ıs may also have led to the depletion of large numbers of Mexican free-tailed bats (Clark). The Carlsbad Caverns colony decreased steadily in size from nearly 20 million down to only a couple hundred thousand during the 1960ıs due to DDT use (Wilson 110). A study in 1974 documented levels of the toxin in fat stores the bats would accumulate before migration, and found that when those fat stores were metabolized during the long flight, DDT levels were high enough to kill many of the bats (Wilson 110). In addition, DDT ingested by mother bats was passed along to their young causing most of them to die before reaching maturity (Wilson 110). Clarkıs follow up study in 2001 also showed levels of DDT in bat specimens from the 1950ıs and 1960ıs to be considerably higher than in specimens from later decades (Clark). These kinds of toxin levels would account for the dramatic decrease in the Carlsbad bat population.
Clark Jr, D R ³DDT and the Decline of Free-Tailed Bats (Tadarida brasiliensis) at
Carlsbad Cavern, New Mexico.² Archives of environmental contamination and
toxicology. 40 (2001): 537.
Hill, John E. and Smith, James D. Bats: A Natural History. Austin: University of Texas
Krutzsch, Philip H. ³Reproductive Biology of Male Mexican Free-tailed Bats (Tadarida
brasiliensis mexicana)² Journal of Mammalogy. 83 (2002): 489-500
Nowak, Ronald M. Walkerıs Bats of the World. Baltimore: The Johns Hopkins
University Press, 1991.
Schober, Wilifred. The Lives of Bats. New York: Arco Publishing, Inc., 1984.
Sherrow, Victoria. Bats. San Diego, CA: Lucent Books, Inc., 2001.
Tuttle, Merlin D. Americaıs Neighborhood Bats. Austin: University of Texas Press,
Whitaker, Jr., John O. ³Dietary variation in the Mexican free-tailed Bat (Tadarida
brasiliensis mexicana).² Journal of Mammalogy. 77 (1996): 716.
Wilson, Don E. Bats in Question: The Smithsonian Answer Book. Washington, DC:
Smithsonian Institution Press