Mexican Free-tailed Bat

Mexican Free-tailed Bat
Closeup of Mexican free-tailed bat
Conservation status
Least Concern (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Chiroptera
Family:	Molossidae
Genus:	Tadarida
Species:	T. brasiliensis
Binomial name
Tadarida brasiliensis (I. Geoffroy, 1824)
Subspecies
T. b. antillularum T. b. bahamensis T. b. brasiliensis T. b. constanzae T. b. cynocephali T. b. intermedia T. b. mexicana T. b. murina T. b. muscula

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. The bat is widely regarded as one of the most abundant mammals in North America and is not on any federal lists. However, its proclivity towards roosting in large numbers in relatively few roosts makes it especially vulnerable to human disturbance and habitat destruction. Documented declines at some roosts are cause for concern. It is considered a Species of Special Concern because of declining populations and limited distribution in Utah. While being one of the most numerous mammals in North America, the whereabouts and status of winter populations of these animals is still largely unknown.^[1] The species is named as the official flying mammal (state bat) of Oklahoma and Texas. Its immage is the icon for the Bacardi rum brand.

Physical description

Mexican free-tailed bats are about 9 cm (3.5 in) in length, and they weigh about 12.3 g (0.43 oz). The tails makes up almost half the length. Their ears are wide and set apart to help them find prey with echolocation. The fur color varies from dark brown to gray. This species also has distinctive short snouts and wrinkled upper lips. The tail of the bat extends beyond the uropatagium, hence why they are named “free-tailed” bats.The wings are long, narrow and pointed, making them well-equipped for rapid, direct flight.

Range and ecology

Bats flying near Frio Cave in Concan, TX.

The Mexican free-tailed bat is one of the most widespread mammals in the Western Hemisphere. The northern limit of its range extends from southern Oregon though Nevada northern Utah, northern Nebraska, Arkansas, northern Alabama, Mississippi, Georgia and into southern North Carolina.^[2] It has been claimed that bats in North Carolina are residents that have recently invaded the coastal plain and are not post-breeding vagrants.^[2] Extra-limital northern records of this bat have been documented Illinois, Iowa and South Dakota.^[3] From these northern limits, the range of the bat extends through most of Mexico, and through most of Central America. The range of the Mexican free-tailed bat in South America is less understood. It lives in four of seven of South America’s faunal provinces, including the eastern Brazilian highlands and coast, the eastern slopes of the Andes and the Pacific coast of Peru and northern Chile.^[4] It does not occur in much of the Amazon rainforest. The bat ranges widely across the Caribbean, and is native to all of the Greater Antilles and to 11 of the Lesser Antilles.^[5] The largest known colony is found at Bracken Cave, north of San Antonio, Texas, with nearly 20 million bats; research indicates that 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).

Habitat

Mexican free-tailed bats roost primarily in caves. However they will also roost in buildings. Any type of building is suitable for roosting as long as the bats have access to openings, to dark recesses in ceilings or walls.^[6] The age, height, architecture, construction materials, occupancy by humans and compass orientation apparently do not affect suitability for roosting.^[6] Caves occupied by this species need to be large enough to provide adequate wall and ceiling space for millions of bats.^[6] Hollows of trees such as red mangrove, black mangrove, white mangrove and cypress where probably the natural roosts for free-tailed bats in the southeastern United States. However most bats in Florida seem to prefer building and other man-made structures over natural roosts.^[6] 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 requires less relative humidity than the southeastern myotis.^[6]

Migration

Mexican free-tailed bats, emerging from Carlsbad Caverns, Carlsbad Caverns National Park, New Mexico.

Mexican free-tailed bats in southeastern Nevada, southwestern Utah, western Arizonia and southeastern California will form a unit and migrate westward and southward into southern California and Baja California.^[6] Bats in southeastern Utah, southwestern Colorado, western New Mexico and eastern Arizona migrate along the western side 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^[3] and into eastern central and perhaps western Mexico.^[6]

Emergence of the bats of the Congress Avenue Bridge in Austin, Texas at dusk.

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 state capitol. It is the largest urban colony in North America with an estimated 1,500,000 bats.^[7] Each night they eat 10,000 to 30,000 lb (4,500 to 14,000 kg) of insects. Each year they attract 100,000 tourists who come to watch them. In Houston, Texas, there is a colony 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.^[8]

Bats ranging eastward from eastern Texas do not migrate but local shifts in roost usage often occur seasonally.^[6] Also, a regional population that ranges from Oregon to California, has a year round residence.

Diet

Mexican free-tailed bats are primarily insectivores. They hunt their prey using echolocation. The bats eat moths, beetles, dragonflies, flies, true bugs, wasps, beetles, and ants. Bats usually prey on flying insects when they themselves are in flight.^[9] Large numbers of Mexican free-tailed bats fly tens of meters above the ground in Texas to feed on migrating insects.^[10]

Health and mortality

The longest living individual bat lived for more than eight years, based on dental records.^[11] Predators of the bat include large bird such as red-tailed hawk, American kestrels, great horned owls, barn owls, and Mississippi kites.^[6] Mammalian predators include Virginia opossums, striped skunks, and raccoons.^[6] Snakes such as eastern coachwhips and eastern coral snakes may also prey on them, but predation is low. Certain types of beetles are important agents of mortality for neonate and juvenile bats that have fallen to the ground.^[6] This species seems to have a low amount of rabies, at least in the United States.^[6] They do however contain certain pesticides.^[6]

Behavior

Mexican free-tailed bats begin foraging at sunset and continue feeding throughout the night. When feeding they will fly over 50 km to a foraging area. They have straight, quick flights. This species has the highest recorded flight altitude among bats, flying at around 3300m.^[12] Bats in a Colorado mine have been documented being most active, mostly feeding and roosting, in the late morning and afternoon during the months of June through September.^[13] Free-tailed bats are more active in warm weather.^[14] The bats call, squeak, and move around during waking hours.

Mexican free-tailed bats are also great pollinators and dangerous insect eaters. Their pollination of sugar cane as well as their consumption of insects that damage sugar cane are the reasons why Bacardi rum features the Mexican free-tailed bat as its icon.

Echolocation

Mexican free-tailed bats use echolocation for navigation and detecting prey. They emit brief constant frequency calls while traveling. However, they transfer to modulated frequency calls between 75–40 kHz when they detect food or another object.^[15] The normal frequency range of their echolocation tends to be between 49–70 kHz, but can drop to around 25–40 kHz.^[15]

Mating and reproduction

Free-tailed bats roosting at a cave in the Bahamas.

During the breeding season, male bats vocalize and mark territories in order to attract potential mates. Females gather in large groups at maternity roosts in caves. Smaller groups can be found in bridges, buildings, and other man-made structures as well as trees. Male and female call to one other and when they single out a mate they move away from the group. Mating can come in an aggressive or passive form. During aggressive copulation, the female is separated from a roost cluster by the male who restricts her movements during mating and produces characteristic calls.^[16] During passive copulation, a male will move very slowly onto a female that is roosting in a dense cluster. With passive copulations, there no resistance from the female and the males does not vocalize. Mating is likely promiscuous as both males and females mate with multiple partners.^[16] Female free-tailed bats become sexually mature at about nine months while males take even longer, at two years. Females have one annual estrous cycle that lasts around five weeks during ovulation. The sexual activity of males coincides with female receptivity in the spring. The gestation period of the bat last 11–12 weeks with a single young being born. Mothers leave their young with a larger cluster of other young, known as creches, rather than roosting with them. The female identifies her young through a series of calls and odors produced by the pup. The scent of the mother in imprinted on the young from an early age.^[17] However, young try to latch on to any female that passes in the cluster to try to get fed. Young are nursed daily and at 4–7 weeks old they reach adult size, are weaned, and are independent.^[18]

Protection

Though abundant and widespread, there are some local populations which 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 individuals population, as a result of vandalism, pollution, and uncontrolled tourism, the organization decided to buy the property in order to place it under conservation. Other species of high ecological value that inhabit the cavern are also being protected.

References

• Barquez, R., Diaz, M., Gonzalez, E., Rodriguez, A., Incháustegui, S. & Arroyo-Cabrales, J. (2008). Tadarida brasiliensis. In: IUCN 2008. IUCN Red List of Threatened Species. Downloaded on 04 March 2009.
• "Tadarida brasiliensis". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=180088. Retrieved 23 March 2006. 

1. ^ "Carlsbad Caverns National Park – Bats' Wintering Sites (U.S. National Park Service)". http://www.nps.gov/cave/naturescience/wintering_bats.htm. 
2. ^ ^{a b} Lee, DS, and C Marsh. 1978. "Range extension of the Brazilian free-tailed bat into North Carolina ". Amer. Midland Nat., 100:240-241
3. ^ ^{a b} Glass BP (1982) Seasonal movements of Mexican free- tail bats Tadarida brasiliensis mextcana banded in the Great Plains. Southwestern Nat., 27:127-133.
4. ^ Koopman, K. F. 1982. Biogeography of the bats of South America. Pp. 273-302 in Mammalian biology in South America, M. A. Mares and H. H. Genoways (eds.), Pymatuning Laboratory of Ecology, University of Pittsburgh, Special Publication Series, Volume 6.
5. ^ Baker, R. J., Genoways, H. H. 1978. Zoogeography of Antillean bats. In. Zoogeography in the Caribbean, ed. F. B. Gill pp. 53-97. Philadelphia: Acad
6. ^ ^{a b c d e f g h i j k l m} Wilkins, K. (1989). "Tadarida brasiliensis ". Mammalian Species , 331: 1-10.
7. ^ "Bat Conservation International page on the Congress Avenue Bridge Bat Colony". http://www.batcon.org/home/index.asp?idPage=122. 
8. ^ "Texas State Symbols, Texas State Library and Archives Commission". http://www.tsl.state.tx.us/ref/abouttx/symbols.html. 
9. ^ McWilliams, L. 2005. Variation in diet of the Mexican free-tailed bat (Tadarida brasiliensis mexicana). Journal of Mammalogy, 86/3: 599-605.
10. ^ Gary F. McCracken, Erin H. Gillam, John K. Westbrook, Ya-Fu Lee, Michael L. Jensen and Ben B. Balsley (2008) "Brazilian free-tailed bats (Tadarida brasiliensis: Molossidae, Chiroptera) at high altitude: links to migratory insect populations", Integrative and Comparative Biology 48(1):107-118.
11. ^ Gannon, M., A. Kurta, A. Rodriquez-Duran, M. Willig. 2005. Bats of Puerto Rico. Jamaica: The University of the West Indies Press.
12. ^ Williams, T., L. Ireland, J. Williams. 1973. High altitude flights of the free-tailed bat, Tadarida brasiliensis, observed with radar. Journal of Mammalogy, 54/4: 807-821.
13. ^ Svoboda, P., J. Choate. 1987. Natural history of the Brazilian free-tailed bat in the San Luis Valley of Colorado. Journal of Mammalogy, 68/2: 224-234.
14. ^ Allen, L., A. Turmelle, M. Mendonca, K. Navara, T. Kunz, G. McCracken. 2009. Roosting ecology and variation in adaptive and innate immune system function in the Brazilian free-tailed bat (Tadarida brasiliensis). Journal of Comparative Physiology, 179: 315–323.
15. ^ ^{a b} Gillam, E., G. McCracken. 2007. Variability in the echolocation of Tadarida brasiliensis: effects. Animal Behavior, 74: 277-286.
16. ^ ^{a b} Keeley, A., B. Keeley. 2004. The Mating System of Tadarida brasiliensis (Chiroptera: Molossidae) in a Large Highway Bridge Colony. Journal of Mammalogy, 85/1: 113-1.
17. ^ Loughry, W., G. McCracken. 1991. Factors influencing female-pup scent recognition in Mexican free-tailed bats. Journal of Mammalogy, 72/3: 624-626.
18. ^ Kunz, T., S. Robson. 1995. Postnatal growth and development in the Mexican free-tailed bat (Tadarida brasiliensis mexicana): birth size, growth rates, and age estimation. Journal of Mammalogy, 76/3: 769-783.

External links

• 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