The Mexican free-tailed bat (Tadarida brasiliensis), also known as the Brazilian free-tailed bat, is a medium-sized bat that is native to the Americas, regarded as one of the most abundant mammals in North America. Its proclivity towards roosting in huge numbers at relatively few locations makes it vulnerable to habitat destruction in spite of its abundance. The bat is considered a species of special concern in California as a result of declining populations.
Molecular sequence data indicates T. brasiliensis’s closest relatives are Chaerephon jobimena of Madagascar and Tadarida aegyptiaca of Africa and south Asia; the latter two are sister species. These three species form aclade believed to be about 9.8 million years old.
Mexican free-tailed bats are on average 9 cm (3.5 in) in length and weigh approximately 12.3 g (0.43 oz). Their tail is almost half their total length and stretches beyond the uropatagium, giving them the name “free-tailed” bats. Their ears are relatively close behind the muzzle and eyes; they are wide and set apart to help them find prey using echolocation. The muzzle is condensed, with wrinkled upper lips. The wings are elongated and narrow with pointed tips, making them well-equipped for quick, straight flight patterns. Their fur color ranges from dark brown to gray.
Range and ecology
The Mexican free-tailed bat 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 3,280 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.
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.
Additionally, Mexican free-tailed bats are also efficient pollinators. Their pollination of sugarcane 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.
On 6 November 2014, Aaron Corcoran, a biologist at Wake Forest University, North Carolina, reported online in Science that he and his team had detected Mexican free-tailed bats emitting ultrasonic vocalizations which had the effect of jamming the echolocation calls of a rival bat species hunting moths. The ‘jamming’ call led to an increased chance of the rival missing its prey, which the Mexican free-tailed bat was then able to eat itself. Earlier researchers had discovered some 15 types of social calls made by Mexican free-tailed bats and reported that they could adjust their calls to avoid interfering with others in range of their calls.
Mating and reproduction
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.