Media & Education
Volume 12, Issue 3, Fall 1994
THE LIVES OF Mexican Free-tailed Bats
By Tuttle, Merlin D.
BY MERLIN D. TUTTLE
AS BATS GO, Mexican freetailed bats (Tadarida brasiliensis) may not be much to look at; they're drab in color, ranging from dark brown to grey, and they have the characteristic wrinkled lips that others of their genus share. Some have described them as looking like little gnomes with an overbite. They get their name from their tail, which protrudes freely beyond the tail membrane.
Despite their rather plain appearance, these are some of the world's most intriguing bats. Speedsters of the bat world, they have been clocked flying at 60 miles per hour using tail winds, and at altitudes over 10,000 feet, higher than any other bat. Free-tails can live in an atmosphere more like another planet than earth, one that can quickly kill most other creatures, including humans. And they form colonies larger than any other bat, larger, in fact, than any warm-blooded animal in the world.
The largest populations of Mexican free-tailed bats live in Central Texas and Mexico, but they are also common throughout much of western North America, southward through Central America, and into the arid and semi arid regions of western and southern South America. They live in many habitats, including urban areas, and range- from deserts to piñon-juniper woodlands and pine-oak forests. Although bachelor colonies of free-tails have been found at elevations over 9,000 feet, large nursery colonies tend to prefer relatively dry areas below 5,000 feet. Mexican free-tails typically live in caves, abandoned mines, or tunnels, and also roost in buildings, under bridges, in rock shelters, in hollow trees, and in cliff-face crevices.
Mexican free-tailed bats are also known as "guano bats" for the prodigious quantities of droppings that they produce. Extraction of guano for use as natural fertilizer was once big business, and some is still sold commercially. From 1903 to 1923, at least 100,000 tons were removed from Carlsbad Caverns alone and sold to fruit growers in California. According to Charles Campbell, Bracken and Frio caves in Central Texas on average each produced 75 to 80 tons annually in the early 1900s. Officials of the Southern Pacific Railroad estimated that, early this century, they annually transported 65 carloads of 30,000 pounds each from Texas, making bat guano the state's largest mineral export before oil. Bracken Cave, now owned and protected by BCI, was still producing from 80 to 85 tons per year in the late 1980s.
Each free-tail cave is also a potential treasure trove for biotechnologists. Microbiologist Bernie Steele examined guano from Bracken Cave, finding that a single ounce contains billions of bacteria. He concluded that the cave contains thousands of species of bacteria, many of which may live nowhere else, and most of which we know nothing about. Species he identified produce enzymes useful in detoxifying industrial wastes, producing natural insecticides, improving detergents, and converting waste byproducts into alcohol. A large proportion are also potential sources of new antibiotics. Stratified guano deposits in free-tail bat caves have also been used to monitor environmental pollution and to investigate prehistoric climatic changes.
Free-tailed bats have supported several American war efforts as well. When Confederacy ports were blockaded in the latter part of 1863, a gun powder factory was established near San Antonio. The powder's most valuable ingredient, saltpeter, was made from local bat guano. During World War 11, major free-tailed bat caves near San Antonio were carefully guarded during top-secret research coded "Project X-Ray."* The U.S. Air Force hoped to use bats as carriers of small incendiary bombs that would be dropped on Japan. The project began to lose favor when escaped bat bombardiers set fire to air base barracks and a general's car. After being passed on to the Navy, and finally the Marine Corps, the project was canceled.
WHILE MOST PEOPLE are unaware of the presence of these bats in their area, Mexican free-tails are very much a part of life in Central Texas, where the largest populations in the United States make their summer homes. These huge colonies, several numbering in the millions each, are where mothers congregate to give birth. The importance of these nursery sites is enormous; bats born here help replenish colonies throughout much of the Southwest and other areas.
Bats begin arriving in Central Texas in late February, having migrated from overwintering sites in Mexico. Active year-round, free-tails do not hibernate. just before their northward migration, they mate. Although young males apparently do not reach sexual maturity until their second year, females as young as a year old have been found pregnant.
By summer, male and female free-tails will have divided into bachelor and nursery colonies. Bachelor groups are relatively small, consisting of dozens to hundreds of individuals, but can number 100,000 or more. In contrast, most nursery colonies are large, numbering from the hundreds of thousands to millions. Bracken Cave is home to some 20 million free-tailed bats, a population that almost doubles when the bats give birth. This is the largest known bat colony in the world.
Typically, each female produces just one young, and virtually all give birth during a brief span of time, peaking between the first and third weeks of June. Birth periods may vary from year to year since weather differences can affect the length of gestation. Newborn young, called pups, weigh nearly a quarter of their mother's weight and are often more than half as long.
Mothers give birth while clinging to the roost with both thumbs and one or both feet. Babies are born naked, often with their eyes open. As soon as the baby is born, the mother carefully cleans and nurses it. For up to an hour, the newborn remains attached to its mother by the umbilical cord, safeguarding against falls and allowing time to learn one another's scent and voice before becoming separated.
Eventually, the mother pulls away to dislodge the placenta, which remains attached to the baby until it dries and falls off a day or two later. Pups have an instinctual tenacious clinging response, using their large feet and thumbs to hold on to walls and their tiny incisor teeth to cling to mothers or other bats. Richard Davis reported during his research that when a single baby was removed from a cave wall, as many as 15 could be pulled off as each clung to the next.
Each cave appears to have favored areas where young are deposited year after year. Gary McCracken and Mary Gustin, who conducted extensive research on the huge nursery colonies of Central Texas, found average roosting densities of 400 pups per square foot and sometimes as many as 500. As the thousands of pups squeak, jostle, and crawl over one another, the cave walls are alive with constant motion and sound.
With so much confusion, it had long been believed that mothers nursed the first pup they found. But McCracken postulated just the opposite. Using sophisticated genetic analysis of mothers with nursing young, he documented that nursing is not random. He and Gustin then used specially marked mother and young pairs, monitoring them with nightviewing devices attached to video cameras, to show that each mother finds and nurses her own pup multiple times daily.
They found that mothers roost apart in adult clusters, remembering the approximate locations of their pups. Since pups may move from a few inches to over a yard between feedings, locating them among the thousands of others is a remarkable feat. Mothers and pups recognize each other's unique voices at least three feet away and move toward one other despite the incredible confusion of calls emanating from countless thousands of other bats. Multiple landings are typically required to find a pup, each bracketing its location in a manner suggesting that a mother is triangulating her pup's voice. Finding her young can take as little as 12 seconds to nearly 10 minutes. She most commonly feeds her pup before she goes out to feed and again when she returns in the morning.
Final recognition is by scent, though it remains to be discovered whether the scent is placed on the pup from glands on the mother's face, or whether each pup has its own unique odor. A successful reunion ends with a mother touching the top of her pup's head with her muzzle, apparently smelling and exchanging vocalizations with it. Such exchanges can last for a minute or more before the mother raises her folded wing and nudges the pup toward one of her breasts.
Over a 24-hour period, she may produce as much as a quarter of her own body weight in milk. Young free-tails grow rapidly, benefitting from prodigious quantities of this extremely rich
milk. They reach adult mass and learn to fly when four to five weeks old and are weaned within approximately five to six weeks.
On its first attempt at flight, a young free-tail must avoid several mid-air collisions per second, relying on an as yet untested navigation system in a dark cave. Although amazingly few serious collisions occur, those that do can break wings or ground a bat long enough to be attacked by swarms of dermestid beetles and their larva that live on the floors of most free-tailed bat caves. As with other bats, the heaviest mortality probably occurs in the first year, perhaps as much as 50 percent.
Predation at entrances to nursery caves increases dramatically as the young bats learn to fly. Avian predators are many, with red-tailed hawks and owls the most common, catching flying bats during emergence and occasionally entering caves to catch those roosting near entrances. Raccoons, opossums, skunks, and other mammals also prey on the emerging bats, as well as several types of large snakes. Given the huge numbers of bats present, such predators likely have relatively little impact.
WITH COLONIES OF this size, cave temperatures are raised dramatically. In Bracken Cave, the 20 million mother bats, with a body mass roughly equal to 271 tons, generate an enormous amount of heat. During summer, the cave's temperature varies only one-sixth as much as the outside; without its bats, Bracken Cave's walls likely would be less than 68 F. Shared body heat raises average wall temperatures to 88 F, enabling the bats to maintain cluster temperatures of 100-105 with greatly reduced energy expenditure. As the summer progresses, however, bats may overheat the cave, forcing large numbers of roosting individuals to extend and
flap their wings or even take flight to cool down.
With fresh droppings and occasional dead bats falling to the floor in Bracken, dermestid beetles begin to multiply. By mid-summer, their numbers can be truly astronomical, causing the floor surface to be in constant seething motion with dermestids scurrying about looking for food. While young bats falling to the floor can be skeletonized in minutes, the greatest impact of dermestids comes from their waste byproducts, which, combined with water vapor, become ammonium hydroxide.
That free-tailed bats can thrive in this toxic atmosphere may be one of the most remarkable things about them. Concentrations of ammonia in free-tail caves can quickly build to levels that are lethal to humans, but the bats survive by lowering their metabolic rates. Carbon dioxide then accumulates, both in the bats' blood and in respiratory mucous, directly proportional to increases in ammonia inhalation. The carbon dioxide neutralizes the ammonia in a buffering mechanism that protects the lungs.
Although concentrations of just 250 parts per million are highly hazardous to humans, free-tails can filter out more than 97 percent of the ammonia present when inhaled at 1,130 parts per million and can still eliminate 73 percent at over 5,000 parts per million. Levels in their roosts, however, rarely exceed 1,000 parts. Depending on the concentration of ammonia in a freetail roost, the bats' fur bleaches from its natural dark brown or grey to various shades of reddish brown. In caves where there are no dermestid beetles, ammonia buildup does not occur.
EACH NIGHT, colonies leave their roosts to feed, emerging in great, often spectacular, columns. The most impressive flights occur after the young begin to emerge with adults in August and September. Many have likened the sound of thousands and thousands of wings beating the air to that of a white-water river. Observers often feel a slight breeze created by the bats as they swirl higher and higher to gain altitude before forming vast undulating columns. Flights from Bracken Cave are so dense that they can be seen on both airport and weather radar screens miles away. Emergences of colonies of this size often go on for hours.
Mexican free-tailed bats are designed for rapid, long-distance travel. Their exceptionally long, narrow wings are geared for relatively highspeed, low-maneuverability flight in open areas. Even their short, velvety fur appears to be an adaptation to reduce drag, and their ear orientation appears to form airfoils that contribute lift during flight. They have been clocked at average flight speeds of 25 miles per hour and as high as 47 miles per hour in level flight, but they can also attain speeds of over 60 miles per hour using tail winds.
Mexican free-tails normally emerge by sundown. Researcher Timothy Williams observed Bracken Cave bats with radar, concluding that most feeding occurred within 528 feet of the ground. He and his research team observed dense, early-evening concentrations of flying insects within this range. Some scientists speculate that the bats from Bracken, which have been found flying at altitudes of 6,600 to 10,000 and more feet, may also be feeding on concentrations of migratory moths at these heights. And again, they may be simply catching high tail winds to speed travel to distant locations. Little is known about how far they travel to feed, but given how high and fast they can fly, many likely go more than 50 miles in one direction each night.
Free-tails spend more time traveling and feeding each night than most bats, in part due to competition from large numbers of roost mates. They typically are on the wing from dusk until dawn. Nursing mothers require at least twice as much food as nonreproductive bats, especially as their pups near fledging. At such times, researcher Thomas Kunz found that they may consume their body weight nightly.
If one assumes that the 20 million nursing mothers at Bracken Cave each eat their body weight of about 12.3 grams, a single night's consumption easily could exceed 250 tons of flying insects. Their total ecological and economic impact is probably enormous. One study conducted near Carlsbad Caverns, New Mexico, determined that about half of the insects eaten were pests that had fed on alfalfa and cotton crops, the nearest of which were grown some 40 miles away along the Pecos River.
Mexican free-tails feed exclusively on flying insects, mostly moths, flying ants, and beetles, according to samples thus far reported. At the turn of the century, Charles Campbell, the city bacteriologist for San Antonio, Texas, built large artificial bat roosts to "control mosquitoes" [BATS, Summer 1989]. Some of these tower-like structures were occupied by hundreds of thousands of bats, and many San Antonians swore by his success.
Although Campbell observed bats of unknown identity catching mosquitoes in the area, there is no documentation that the free-tailed bats from his artificial roosts actually ate them. Given the high-speed, relatively low-maneuverability flight of free-tails, it seems unlikely that they would prey extensively on mosquitoes. Bats, however, are highly opportunistic; the larger, also fast-flying, hoary bat (Lasiurus cinereus) is known to home in on mosquitoes when they are abundant.
At dawn, the free-tails return to their roost in an event sometimes said to be even more spectacular than evening emergences. Richard Davis and his fellow researchers observed flocks of thousands of bats each, first becoming visible 4,900 to 8,200 feet above Bracken Cave. These high-altitude flocks sometimes flew past the entrance at speeds of almost 60 miles per hour before turning around and diving toward the entrance. Beginning about two hours before sunrise, small groups built up into a continuous diving stream, reaching the greatest density about 30 minutes before dawn. The first arriving bats came in shallow, zigzagging glides, but as flight density increased, they formed a continuous stream of individuals dropping out of the sky into the mouth of the cave. Each was executing a rapid series of free falls with closed wings, alternating with abrupt, brief wing openings to control speed and direction. Some groups dropped nearly 10,000 feet at speeds estimated to exceed 80 miles per hour.
AS SOON AS their young have become proficient flyers, many free-tails leave the major nursery caves of Central Texas. Once thought to be migratory movements, these August departures apparently are only local and are correlated with weather patterns, combined with the stress of overheating and concentrated gas buildup in their caves. just before bats begin to leave Bracken Cave in early August, huge clusters roost within inches of direct sunlight in the cave entrance where fresh air is most available. These factors may also be combined with attempts to escape parasites that build up on roosts during the nursery period.
As large numbers of bats leave the cave, they begin appearing in groups of tens to hundreds of thousands under highway bridges and in almost any other available place. During 1993, an extremely dry year in Central Texas, so many free-tails attempted to move under Austin's Congress Avenue Bridge that tens of thousands were forced to hang out in the open on the concrete pillars. With three-quarters of a million bats of its own, the bridge is the site of the largest urban colony of bats in the world.
Additional groups of up to 500,000 each were reported beneath other bridges that year, and unprecedented numbers moved into parking garages, vacant buildings, and sports stadiums. But on the night when the first mild cool front passed in early September, many thousands of free-tails that had been roosting in exposed places apparently returned to Bracken Cave, which had by then been purged of hot gases by the cool air. Although the emergence from Bracken had been surprisingly small for several weeks, it was extraordinarily large on the evening following the disappearance of the excess bats from the Congress Avenue Bridge, some 60 miles away.
True southward migration of the free-tails appears not to begin until October. The vast majority of the U.S. population spends the winter mostly in large caves of northern and Central Mexico. Populations living in California, western Arizona, Oregon, Nevada, and southwestern Utah apparently live in roughly the same areas year-round, though seasonal movements among roosts are common. There are two main migrations. Most of those from the Southwest migrate south along the Sierra Madre Occidental and the West Coast of Mexico at least as far south as the state of Sinaloa. Free-tails from the Great Plains typically travel southward through
Texas and along the Sierra Madre Oriental into eastern and south-central Mexico, some perhaps farther.
It is clear that major migratory departures in the fall are triggered by the passage of strong cold fronts from the north. Large departures from Bracken are typically correlated with passage of extra-strong cold fronts arriving in late October or early November. Departure dates can vary by several weeks in different years, according to changing weather patterns. Not all of the bats leave at once, instead departing in several large groups at different times.
Even among populations that migrate, not all bats leave. Several thousand have been observed overwintering in Bracken Cave, as well as in concrete crevices beneath the Congress Avenue Bridge, and in old buildings in Austin. Although free-tails can enter torpor during inclement winter weather, they are not true hibernators. During extremely cold weather, many die. It is unknown why some stay behind.
The longest proven migrations are of bats banded by Bryan Glass in northwestern Oklahoma and later recovered up to 1,104 miles south in Mexico. The northernmost area where he believed any of his bats could have overwintered was 480 miles south in Texas. The original bandings were made at four caves less than 48 miles apart, between which the bats intermingled. One bat was recaptured at its cave of birth in Oklahoma after having completed eight migratory circuits. Free-tails typically return to their home areas, but for these long distance travelers, a home area may include caves over 100 miles apart.
All available evidence suggests that free-tails typically travel in groups at all seasons. Richard Davis and fellow researchers recorded a particularly impressive spring arrival on April 22 at Frio Cave in Texas. At a time when few other bats had yet arrived, "several million bats hurtled down out of the night within the space of ten minutes." They arrived at about midnight. Denny Constantine, another researcher, believed that inexperienced travelers arriving at night could locate less familiar caves simply by listening for local bats and following them in. Traveling in groups certainly must increase the odds that some in the group will know the way.
Davis believed that migratory movements were rapid, crossing Texas in one or a few nonstop flights, covering at least 290 miles a night. Given knowledge of bat flight speeds with tail winds, migrating free-tails should be able to cover that distance in no more than five hours, perhaps substantially less, depending on wind velocity. Such timing would ensure arrival at stopover caves at optimal times for following other bats in, if necessary, and allow for unanticipated delays due to bad weather.
WHILE FREE-TAILED BATS are among the more studied, what remains to be discovered about them may be even more fascinating than what we already know. Why do so many fly so high? Are they simply catching tail winds to aid in rapid travel to distant locations, or are they actually feeding at such high altitudes? How do they navigate at high altitudes, given the fact that their echolocation signals reach little more than 100 feet and that cave entrances can be nearly impossible to see from even a few hundred yards? Bats are known to use celestial cues, but whatever cues they are relying on must work both night and day, since flights often arrive in midmorning.
Perhaps the most interesting questions of all involve the composition and role of flocks. How do they form? Who leads them, and how do they know where they are going, or how early to leave to ensure arrival at a time when they can maximize feeding success? Are groups composed of roostmates that hang in close proximity to each other by day, or do they have some other means of getting together prior to leaving the cave? With animals as fascinating as these, researchers will be pondering the answers to such questions for many years.
Merlin D. Tuttle is founder and Executive Director of BCI. Portions of this article are excerpted from his forthcoming book, Bats of North America, to be published by University of Texas Press.
* The project is thoroughly described in Bat Bomb, World War II's Other Secret Weapon by Jack Couffer, available in the BCI catalogue.
Like others of their genus, Mexican free-tailed bats have prominent wrinkled lips.
With their long, narrow wings, Mexican free-tailed bats are speedsters in the bat world, designed for fast, long-distance flight. They get their name from their tail, which extends freely beyond their tail membrane.
In his research Gary McCracken (left) marked pairs of mothers and nursing young, discovering that mother bats did not randomly nurse any pup, as had been previously believed. After her pup is born, mother and young spend up to an hour getting to know each other's scent and vocalizations (right). She roosts separately from her pup, returning only to nurse it. Remarkably, among the many thousands of other pups covering the cave walls (opposite page), she will find her own young. Note the other bat in the upper right corner, nearly buried by pups.
As the thousands of pups squeak, jostle, and crawl over one another, the cave walls are alive with constant motion and sound.
Millions of bats from Bracken Cave leave each summer night in an emergence flight that is truly one of the world's most spectacular sights. The bats circle above the cave entrance in order to gain height (opposite page), before forming dense, serpentine columns that go out over the surrounding landscape to feed (right).
So many bats roost in the great nursery caves of Central Texas that cave walls are virtually solid with bats, accommodating 200 or more adults per square foot.
Free-tails spend more time on the wing each night than most bats, consuming countless insects, including many agricultural pests. A large colony collectively can eat literally tons of insects.