Tadarida brasiliensis
Brazilian free-tailed bat

Tadarida brasiliensis is a member of one of the widely distributed genera of bats in North and South America. Extensive studies on their range have yet to be completed, especially within South America; however they have been found throughout the much of the United States, Mexico, Central America, and southwestern South America, including Brazil, Chile, and Argentina. In the United States Tadarida brasiliensis is found from southern Oregon to Nevada and eastward to North Carolina and southwestern Virginia. In the last 50 to 100 years, Tadarida brasiliensis populations have declined, possibly due to a decrease in habitat, damage to roosts, and indirect consumption of pesticides. (Cranford and Fortune, 1994; Gannon, et al., 2005; Wilkins, 1989)

Brazilian free-tailed bats use a variety of different roost sites, including caves and man-made structures, such as bridges and attics. Caves with large rooms and high ceilings are the primary roosting habitats, although roosts also occur in hollow trees. Roosts are used for nesting, breeding, and interaction between individuals. (Wilkins, 1989)

With brown fur and large ears, Brazilian free-tailed bats are medium-sized, with distinctive short snouts and wrinkled upper lips. The free-tailed bats, which include the genera Tadarida, Eumops, and Nycintomops, are most easily recognized by their “free-tail,” which extends well beyond the uropatagium. They have powerful legs and can climb well. Their long, narrow, pointed wings make them well-suited for rapid, direct flight. Like other temperate bat species, Brazilian free-tailed bats take advantage of daily torpor to conserve energy and may hibernate.

Adults range in size from 79 to 98 mm in length, with a tail almost half the size (31 to 41 mm). Their body mass varies seasonally and depending on maturity, adults typically weigh 7 to 12 g. Ear length is 8 to 15 mm, forearm length ranges from 37 to 41 mm, and their hindfoot measures 6 to 9 mm.

The dental formula is the same as other members of the genus Tadarida: Incisors= 1/3, canines= 1/1, premolars= 2/2, molars= 3/3, with a total of 32 teeth. (Gannon, et al., 2005; Jones Jr. and Manning, 1992; Wilkins, 1989)

Male Brazilian free-tailed bat behavior and scent-marking changes throughout the year based on the breeding season. Females gather in large numbers at maternity roosts in caves, while smaller groups can be found in tree, bridges, buildings, and other man-made structures. Males vocalize and mark territories in order to attract potential mates. Male and female free-tailed bats call to each other, singling out a mate. Once found, they move away from the group. Males aggressively mate with the female, restricting her movement by grabbing her neck, jaw, or ear. He moves onto her back, biting her neck to keep her in place. The female and male call to each other during mating. Some free-tailed bats mate multiple times, moving from mate to mate. (Gannon, et al., 2005; Keeley and Keeley, 2004; Krutzsch, et al., 2002; Wilkins, 1989)

Brazilian free-tailed bat males mature at about two years, while females mature at nine months. They are monestrous, with females having one annual estrous cycle lasting roughly five weeks during ovulation, which occurs in the spring. Male sexual activity coincides with spring female receptivity, suitable timing for mating interactions to occur. A female usually gives birth to a single offspring after an 11 to 12 week gestation period. Births occurs upside down and last roughly 90 seconds. It takes an additional 10 to 15 minutes for the newborn to find a nipple for feeding. The sex-ratio is typically 1:1 in pups. (Gannon, et al., 2005; Krutzsch, et al., 2002; Wilkins, 1989; de Magalhaes and Costa, 2009)

Because mothers do not roost with their offspring, but rather leave them with a larger cluster of pups (a creche), she has to identify her own young through a series of calls and odors produced by the pup. Loughry and McCracken (1991) found that the scent of mothers is imprinted during early stages of development; however, pups will try latching onto any female that passes in the cluster to get fed. The young are nursed daily. They reach adult size, are weaned, and are independent in 4 to 7 weeks after birth. Brazilian free-tailed bat females have the highest milk fat content of any bat, over 28% fat, which allows their pups to grow relatively quickly. (Gannon, et al., 2005; Kunz and Robson, 1995; Loughry and McCracken, 1991; Wilkins, 1989)

Dental studies (Gannon et al., 2005) determined that the longest-living individual was over eight years old. Most adults have a survival rate of 70 to 80% each year, with the rate decreasing with age. Males and females have roughly equal lifespan and mortality rates. Weigl (2005) reported a live free-tailed bat in captivity that was 12 years old. It is therefore expected that this species can live beyond 12 years in captivity. (de Magalhaes and Costa, 2009; Gannon, et al., 2005; Weigl, 2005; Wilkins, 1989; de Magalhaes and Costa, 2009)

Brazilian free-tailed bats start foraging after sunset and feed through the night. They may fly over 50 kilometers to get to a foraging area. Their flight is quick and straight. At over 3300 meters, this species has the highest recorded flight altitude among bats. Brazilian free-tailed bats are also capable of long-distance flight, allowing them to forage over large areas and migrate seasonally. Observations in a Colorado mine documented that Brazilian free-tailed bats were most active, through feeding and searching for roost sites, in the late morning and afternoon during the months of June through September. Weather can affect their activity; warm weather stimulates greater activity and there is less activity in cold weather. During their waking hours, Brazilian free-tailed bats call, squeak, and move around. They are social animals that live in large colonies. (Allen, et al., 2009; Svoboda and Choate, 1987; Williams, et al., 1973)

Radar studies have tracked this species, and estimate large groups to cover areas of 400 square km and reach altitudes of 3300 m. With a range this size, it is not surprising that this species will travel 25 km to forage in an evening. Indeed, records report travels up up to 65 km from roosts. (Williams, et al., 1973)

Brazilian free-tailed bats use echolocation as their primary mode of perception for navigation and detecting prey. They emit brief constant frequency calls as they travel, unless food or another object is detected, then they transfer to modulated frequency calls between 75 and 40 kHz. Their normal frequency ranges from 49 to 70 kHz, but can drop to 25 to 40 kHz when objects cross their flight path. Mate and intra-specific recognition is determined through the use of echolocation and through chemical, visual, and audible vocalizations. Females do not roost with their offspring, they must find their young through scent and sound recognition. (Gannon, et al., 2005; Gillam and McCracken, 2007)

Brazilian free-tailed bats are aerial insectivores that use echolocation to find and consume their prey. Their diet varies based on geographical range, but mainly includes moths (Lepidoptera), beetles (Coleoptera), dragonflies (Odonata),flies (Diptera), true bugs (Hemiptera), and wasps, bees, and ants (Hymenoptera). Diet is influenced by the abundance of prey, weather conditions, metabolic demands, and lunar illumination, which can alter food availability. Brazilian free-tailed bats prey on flying insects while they are, themselves, in flight. (McWilliams, 2005)

Predators of Brazilian free-tailed bats include a number of raptors, such as red-tailed hawks (Buteo jamaicensis), American kestrels (Falco sparverius), great horned owls (Bubo virginianus), barn owls (Tyto alba), and Mississippi kites (Ictinia mississippiensis). Virginia opossums (Didelphis virginiana), striped skunks (Mephitis mephitis), and raccoons (Procyon lotor are among the mammalian roost predators. Snakes also prey on these bats in roosts, including eastern coachwhips (Masticophis flagellum) and eastern coral snakes (Micrurus fulviusprey). However, predation on Tadarida brasiliensis is rare; the number of bats lost due to predation is very low compared to their total population of around 100 million individuals. ("Brazilian Free-tailed Bat", 1997; Davis, et al., 1962; Wilkins, 1989)

Brazilian free-tailed bats play host to both ecto- and endoparasites. Individuals that are part of a colony are at higher risk for being parasitized than individuals in smaller roosts. Mite, tick, chigger, flea, and beetle infections are common among Brazilian free-tailed bats, and may act as vectors for other diseases. For example, the chigger Microtrombicula merrihewi affects the nasal passages of Brazilian free-tailed bats, while other parasites affect the blood stream and digestive system. Brazilian free-tailed bats, like other mammals, are also hosts for the rabies virus and at least five other known viruses, such as the Rio Bravo virus, St. Louis encephalitis virus, Eastern equine encephalitis, Western equine encephalitis, and Japanese B encephalitis.

Brazilian free-tailed bats are also known as guano bats. Excrement (guano) in roosts can build-up and result in tons of guano. Guano can be harvested as fertilizer and can pose a health risk in spreading diseases that are transmitted through the air (e.g., histoplasmosis). During the summer, disease transmission risk worsens, with higher temperatures and movement from within the caves generating dust clouds. Higher temperatures are also ideal for parasites and pathogens.

More studies are needed on associations with other bat species. In Texas, Myotis velifer are most commonly seen in roosts with T. brasiliensis. Although segregated, individuals can be seen within the other’s colony especially if the roost is crowded. The flight paths of M. velifer and T. brasiliensis differ, which helps avoid competition between the two species while exiting the roost.

Deer mice (Peromyscus species) and squirrels can also be seen using cave roosts as shelters. However, they have no known impact on T. brasiliensis.

During the summer months, these bats significantly affect local insect populations, which makes maintaining them important to agriculture and human health by eating agricultural pests and disease vectors. (Davis and Loomis, 1971; Davis, et al., 1962; Jameson, 1959; Wilkins, 1989)

Brazilian free-tailed bats eat large numbers of insects nightly, some of which are agricultural pests or disease vectors. Their positive economic impact on agriculture is substantial. However, agricultural pests are often exposed to pesticides through agricultural applications, which can indirectly. Population declines may be linked to toxicity from pesticides. In addition, the large amount of guano produced in Brazilian free-tailed bat colonies are used for fertilizer and as a component in gunpowder. (Clark Jr., et al., 1996; Davis, et al., 1962)

There is no known negative economic importance. However, histoplasmosis is a potential health concern in caves with large guano accumulations and, like other bats, Brazilian free-tailed bats can carry and transmit rabies. (Gannon, et al., 2005)

Populations of Tadarida brasiliensis have declined over the last century. Some suggest this decline has been caused by disturbance and destruction of roost sites and indirect poisoning by pesticides. Tadarida brasiliensis is labeled as “near threatened” by the International Union for Conservation of Nature, with a Species Action Plan created. (Arita, 1993; Gannon, et al., 2005)