Natural History of the
Mexican Free-Tailed Bat
Authors: Sarah Winstanley & Kathy Coulombe
Classification:
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Chiroptera
Suborder: Microchiroptera
Family: Molossidae
Genus: Tadarida
Species: braziliensis
Subspecies : mexicana
Common name: Mexican free-tailed bat
Bats first appear in the fossil record in the early Eocene, about 60 million years ago [1]. Icaronycteris index, who lived in what is now Wyoming during the early Eocene, was probably the first insectivorous bat [1, 2]. However, all of the known fossils from the Eocene are well-differentiated microchiropterans, and probably were not the ancestors of the Megachiroptera [1]. Further, these fossils are so well-differentiated that they provide few clues as to which might have been the earliest ancestor of bats: a tree- or land-dwelling mammal [2]. In fact, it appears that bats evolved and differentiated early in their history, and have not changed a great deal since [1, 2]. It is suspected that bats originated much earlier -possibly as early as the Paleocene or late Cretaceous period (70-100 million years ago), at a time when the angiosperms (flowering plants) were diversifying and thus pollinating insects (such as beetles, butterflies, and flies) were emerging [1, 2].
For these reasons, the origin of bats is still somewhat mysterious. The Chiroptera (bats) have some characters similar to those of primates and the Dermoptera (gliders), but it is thought that the Microchiroptera evolved from small, quadrupedal, insectivores that possessed gliding membranes much like the extant flying foxes and flying squirrels [1, 2]. As these ancestors became more dependent on insectivory, it is conceivable that their gliding membranes slowly became more elaborate (elongated phalanges and forearm, and shortened upper arm bones), eventually into a modern day bat wing (see figure 1) [1]. However, some researchers now believe that due to similarities in tooth morphology, bats are mostly likely to be descended from a Tupaia (tree shrew)-like ancestor, and not from the insectivores [2].
Specifically, the Molossidae ("free-tailed" bats) family had evolved by the middle Oligocene, about 30 million years ago [2]. These bats are considered to be highly specialized for flight [2]. They have evolved three synapomorphies, or characters that are unique to this family: 1) the greater trochanter of the femur has a hook-like process, 2) there is a fringe of "spoon-shaped" bristles present on the first and fifth toes, and 3) the uropatagium is freely moveable along the ("free") tail [3].
Tadarida braziliensis is a member of the suborder Microchiroptera. Individuals
of the genus Tadarida tend to be small, averaging 12.3g, and have long,
narrow wings spanning 29-35cm. Their fur ranges from dark brown to gray in color,
but ammonia fumes from the accumulated guano in their caves can sometimes bleach
it to a lighter shade. T. braziliensis has broad, black ears that
are not attached in the midline, as well as characteristic wrinkled lips. The
free-tailed bats get their name from their prominent tail, the distal half of
which extends beyond the interfemoral membrane [4, 5].
T. braziliensis has a wide range, which spans southern North America,
Central America, and northern South America [4, 5, 6]. Free-tailed bats inhabit
diverse environments varying from desert to woodland and forest, and up to elevations
of 9,000 feet above sea level [5, 6].
The subspecies T. braziliensis mexicana is found predominantly
in the south western United States and Mexico, and many individuals undertake
seasonal migrations that can extend 1200 kilometers or more [2]. After mating
in Northern Mexico during February or March, these bats migrate as far north
as Oklahoma, then return south in mid-summer through October, when the young
are able to fly [2].
T. braziliensis is nocturnal and active year-round [4, 5, 6].
In huge colonies of up to ten million or more, they roost primarily in caves,
but also in other protected areas such as under bridges or in buildings [4,
5, 6]. The social T. braziliensis forms segregated colonies of
male groups (tens to several thousand individuals) and maternity groups numbering
in the millions, larger than any other warm-blooded animal [5]. These bats use
echolocation to navigate, and it has been suggested that males use vocalizations
to attract a mate [7].
Flight:
Among the fastest flying bats, T. braziliensis flies at an average
speed of 40 km per hour, and can reach speeds of up to 100 km per hour [4, 5].
They can also reach altitudes of 3.5 km, which is higher than those recorded
for any other bat [5]. Their long, narrow wings allow for high-speed, long-distance
travel, but low manoeuvrability [5]. T. braziliensis has short fur to reduce
drag, and their ear orientation aids in lift during flight [5].
Diet:
An insectivorous species, T. braziliensis uses echolocation to
find its prey [1, 4]. Feeding exclusively on flying insects such as moths, flying
ants and beetles, an individual can consume an amount equal to its body weight
each night [1].
Reproduction and Ontogeny:
As mentioned above, many T. braziliensis mexicana migrate north
in spring to give birth to their offspring [2, 5]. Males reach reproductive
age at around two years, whereas females can become fertile after one year [5].
Each female can produce one pup per year, each a quarter of the mother's weight,
and roughly half as long [5]. The pups are usually born with their eyes open
and their deciduous dentition erupted [5]. The mother will spend the first hour
after birth holding the offspring and learning its unique scent and voice, which
she will later use to recognize her pup [5]. Mothers roost separately from their
pups, which are born in close proximity to each other, returning to nurse them
for up to five weeks [5].
Guano:
T. braziliensis are known in Central America as the "guano
bats" due to the large quantities of droppings, or guano, they produce
[4, 5]. Many human communities currently use guano as a natural fertilizer,
and it has recently been found to be a source of new antibiotics [5].
1. Hill, J.E., & Smith, J.D. 1984. Bats: A Natural History. University of Texas Press, Austin
2. Neuweiler, G. 2000. The Biology of Bats, E. Covey (tr.). Oxford University Press, New York.
3. Simmons, N.B. 1998. "A reappraisal of interfamilial relationships of bats", pp. 3-13 in Kunz, T.H., & Racey, P.A. (eds.) Bat Biology and Conservation. Smithsonian Institution Press, Washington.
4. Barbour, R.W., & Davis, W.H. 1969. Bats of America. University Press of Kentucky, Lexington.
5. McCracken, G. 1996. Bats aloft: A study of high altitude feeding. Bats. 14: 7-10.
6. Long, F.R., Simpson, T., Tzung-Su, D., Heydon, S., & Reil, W. 1998. Bats feed on crop pests in Sacramento Valley. California Agriculture. 52: 8-11.
7. French, B. 1995. The song of the Mexican free-tail. Bats. 13: 7-9.