Mexican free-tailed bat
|Mexican free-tailed bat
|Closeup of Mexican free-tailed bat|
(I. Geoffroy, 1824)
The Mexican free-tailed bat (Tadarida brasiliensis), also known as the Brazilian free-tailed bat and Austonian bridge bat, is a medium-sized bat that is native to the Americas and is widely regarded as one of the most abundant mammals in North America.
However, its proclivity towards roosting in large numbers in relatively few roosts makes it especially vulnerable to human disturbance and habitat destruction, and declining numbers at some roosts such as in the western state of Utah have been documented. In the western coastal state of California, the bat is considered a species of special concern as a result of declining populations. The species' winter migratory habits and destination points are still relatively unknown.
Mexican free-tailed bats are about 9 cm (3.5 in) in length, and they weigh about 12.3 g (0.43 oz). Their tails make up almost half their lengths. Their ears are wide and set apart to help them find prey with echolocation. They are also fairly close behind the snout and eyes. Their fur color varies from dark brown to gray. Their muzzles are condensed, with wrinkled upper lips. The tail of these bats stretches further than the uropatagium, hence they are named "free-tailed" bats. The wings are elongated and narrow with pointed tips, making them well-equipped for quick, straight flight patterns.
Range and ecology
The Mexican free-tailed bat is one of the most widespread mammals in the Western Hemisphere. It ranges from the southern half of the continental United States through most of Mexico, and through most of Central America into South America. The range of the Mexican free-tailed bat in South America is less understood where it lives in the eastern Brazilian highlands and coast, the northeastern Andes and the coast of Peru and northern Chile. It is absent in much of the Amazon rainforest. The bat is also found in the Caribbean, and is native to all of the Greater Antilles and 11 of the Lesser Antilles. The largest known colony is found at Bracken Cave, north of San Antonio, Texas, with nearly 20 million bats; research indicates the bats from this colony congregate in huge numbers at altitudes between 180 and 1,000 m (590 and 3,300 ft), and even as high as 3,000 m (9,800 ft).
Mexican free-tailed bats roost primarily in caves. However, they will also roost in buildings of any type as long as they have access to openings and dark recesses in ceilings or walls. The bats can make roosting sites of buildings regardless of "age, height, architecture, construction materials, occupancy by humans and compass orientation". Caves, on the other hand, need to have enough wall and ceiling space to fit millions of bats. Before buildings, free-tailed bats in the southeastern United States probably roosted in the hollows of trees such as red mangrove, black mangrove, white mangrove and cypress. However, most bats in Florida seem to prefer buildings and other man-made structures over natural roosts. Caves in Florida tend to be occupied mostly by the southeastern myotis. Caves in Florida tend to have pools of water on the floor and the free-tailed bats do not need as much relative humidity as the southeastern myotis.
Mexican free-tailed bats in southeastern Nevada, southwestern Utah, western Arizona and southeastern California come together to migrate southwest to southern California and Baja California. Bats in southeastern Utah, southwestern Colorado, western New Mexico and eastern Arizona travel though western edge of the Sierra Madre Oriental into Jalisco, Sinaloa and Sonora. Some bats that summer in Kansas, Oklahoma, eastern New Mexico and Texas will migrate southward to southern Texas and Mexico. Some bat populations in other areas of North America do not migrate, but are residents and may make seasonal changes in roost sites.
In Austin, Texas, a colony of Mexican free-tailed bats summers (they winter in Mexico) under the Congress Avenue Bridge ten blocks south of the Texas State Capitol. It is the largest urban colony in North America, with an estimated 1,500,000 bats. Each night they eat 10,000 to 30,000 lb (4,500 to 13,600 kg) of insects. Each year they attract 100,000 tourists who come to watch them. In Houston, Texas, a colony is living under the Waugh Street Bridge over Buffalo Bayou. It is the home to 250,000 bats and also attracts viewers. The Mexican free-tailed bat is the official "flying mammal" of the state of Texas.
Bats ranging eastward from East Texas do not migrate, but local shifts in roost usage often occur seasonally. Also, a regional population that ranges from Oregon to California, has a year-round residence.
Mexican free-tailed bats are primarily insectivores. They hunt their prey using echolocation. The bats eat moths, beetles, dragonflies, flies, true bugs, wasps, and ants. Bats usually catch flying prey in flight. Large numbers of Mexican free-tailed bats fly hundreds of meters above the ground in Texas to feed on migrating insects. The consumption of insects by these bats can be quite significant.
Additionally, Mexican free-tailed bats are also efficient pollinators. Their pollination of sugar cane as well as their consumption of insects that damage sugar cane may be among the reasons why Bacardi rum features the Mexican free-tailed bat as its icon. Bacardi Ltd. themselves attribute the use of the bat in the logo to, "... Don Facundo’s wife, Amalia, who suggested using a bat for the company logo. It was an insightful choice, because according to Cuban and Spanish lore, bats symbolize good health, good fortune and family unity."
Health and mortality
One individual bat was recorded to have lived eight years, based on dentition. Predators of the bat include large birds such as red-tailed hawk, American kestrels, great horned owls, barn owls, and Mississippi kites. Mammal predators include Virginia opossums, striped skunks, and raccoons. Snakes such as eastern coachwhips and eastern coral snakes may also prey on them, but at a lesser extent. Certain types of beetles prey on neonate and juvenile bats that have fallen to the ground. This species seems to have a low incidence of rabies, at least in the United States. They do, however, contain certain pesticides.
Mexican free-tailed bats are nocturnal foragers and begin feeding after dusk. They travel 50 km in a quick, direct flight pattern to feed. This species flies the highest among bats, at altitudes around 3300 m. Bats appears to be most active in late morning and afternoon between June and September. Free-tailed bats are more active in warm weather.
Mexican free-tailed bats use echolocation for navigation and detecting prey. Traveling calls are of a brief but constant frequency. However, they switch modulated frequency calls between 40 and 75 kHz if they detect something. Typically, the frequency range of their echolocation is between 49 and 70 kHz, but can be between 25 and 40 kHz if something crosses their path while in flight.
Mating and reproduction
During the breeding season, females aggregate into maternity roosts. The size of these roosts depends on the environment, with caves having the larger roosts. Mating can occur in an aggressive or passive form. In the aggressive form, the male controls the female's movements, keeping her away from the other bats in the roost. He also tends to vocalize when mating. During passive copulation, the males simply flies to a female in her roost and quietly mounts her with no resistance. This species is a promiscuous breeder and both sexes copulate with multiple partners. Females become sexually mature at about 9 months, while males take even longer, at two years. Females enter estrus once a year, which typically lasts five weeks in the spring. The gestation period of the bat lasts 11–12 weeks, with only one young being born. A number of pups are left in "creches", while their mothers roost elsewhere. The female uses vocalizations and scent to identify her pup. The mother imprints her scent on the young early on. However, young try to steal a suckle from any female that passes through the cluster. A mother will nurse her young daily, and by 4–7 weeks old they are full grown, fully weaned, and independent.
Though abundant and widespread, some local populations have prompted protection and conservation efforts. For instance, during the spring and summer, one of the largest Mexican free-tailed bat populations inhabits Cueva de la Boca, a cave near Monterrey, Mexico. In 2006, the Mexican environmental conservation NGO, Pronatura Noreste, purchased the property. Because of a reduction of more than 95% of the original 20 million bat population, as a result of vandalism, pollution, and uncontrolled tourism, the organization decided to buy the property to place it under conservation. Other species of high ecological value that inhabit the cavern are also being protected.
Bat bomb, an experimental incendiary weapon that used Mexican free-tailed bats as a dispersal mechanism
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|Wikimedia Commons has media related to Tadarida brasiliensis.|
|Wikispecies has information related to: Tadarida brasiliensis|
- ARKive - images and movies of the Brazilian free-tailed bat (Tadarida brasiliensis)
- Untamed Science interview with a bat specialist at the Congress Street Bridge Colony
- Bat Conservation International website
- Mexican Bats Find Cross-Border Benefactors (Washington Post)
- Pronatura Noreste Announces the Purchase of Cueva de la Boca
- Bat Conservation International webpage about Mexican free-tailed bats
- Animal Diversity - Tadarida brasiliensis