The mutually Toxic Relationship Between Mexican Free-Tailed Bats and Humans

A public science essay written and illustrated by Jessalyn Smith

The Mexican free-tailed bat looks like a gargoyle.  There is no sugar-coating it, this species pales in comparison to the puppy-like fruit bat or the world’s smallest mammal, the bumblebee bat.  However, they do not have the startling ears of Townsend’s big-eared bat or the reputation of the vampire bat that makes them stand out among the scientific order of gargoyles.  Their fur is typically on the muddy end of brown, and their lips are wrinkled like the ridges of a pecan.  Their large ears are stubby and rounded.  The tail that gives them their name extends past the tail membrane and almost looks like a third leg; the only thing it’s missing is a foot.  But despite all these unappealing features, they come together in an “it’s so ugly it’s cute” kind of way.  The fluffy gargoyles of the sky won’t win any beauty competitions, but they certainly make up for their looks in the talent portion.                                                 

Fig. 1. Taubert, Bruce D. n.d. Photograph. Arizona Highways.

This species of bat is an insectivore, meaning they eat the wasps and other bugs you would rather pretend don’t exist.  Their diet varies by region but primarily includes airborne insects due to their method of travel.  However, they will stray from their typical diet if necessary, like in cases of high competition.  Competing for food drives the Mexican free-tailed bat to fly at least 50 miles away from its home (also referred to as a “roost”) in order to find food (Tuttle, 1994).  Since they fly at speeds of up to 60 miles per hour and altitudes over 10,000 feet, the Mexican free-tailed bat can be difficult to track and watch their every move, but it is known that an individual bat can eat its two-thirds of its own body weight (about 12.5 grams) in bugs over the course of one night (Tuttle 1994).  Alone this may seem unimpressive but considering the Bracken Cave colony consists of about 20 million bats, the number of bugs eaten in one night really starts to add up.

Fig. 2. From left to right: Fruit Bat, Bumblebee Bat, Townsend’s Big-Eared Bat, Vampire Bat

            The appetite of a Mexican free-tailed bat colony is worth approximately $741,000 per year due to the consumption of agricultural pests.  The monetary value was calculated by looking at the profits lost due to crop consumption and the cost of alternative forms of pesticide that would be necessary over a 10,000 acre agricultural region (Cleveland et al. 2006).  The most notorious agricultural pest in the studied region is the cotton bollworm, a moth whose larvae can destroy cotton, tomato, and corn crops and whose population is difficult to control.  The bollworm lays its eggs on the leaves or near the fruit of a plant, and the hatched larvae will burrow inside and mine the plant, ruining the crop (“Cotton Insects” 2013).  By eating the adult moths, bats prevent the adults from laying more eggs and continuing to bolster the population of ravenous larvae.  According to an article published in the “Frontiers in Ecology and the Environment,” an average bat’s consumption of 1.5 bollworms in one night prevents five eggs

from being laid (Cleveland et al. 2006).  The Mexican free-tailed bat can provide hundreds of thousands of dollars in pest control services based on the cost of lost crops and the price of pesticides that would be needed without the presence of bats.

            After a long night of eating bugs and protecting crops, what goes in must come out.  The bat guano industry is not commonly thought about, but it’s incredibly bountiful.  The most obvious use for bat droppings is as a natural fertilizer, similar to the use of cow manure. Guano is high in nutrients and contains nitrogen, phosphorus, and potassium, which are nutrients that are useful for improving plant growth and strong stems.  The guano can even help improve the texture of soils that are too loose or too dense due to the presence of beneficial microbes in the droppings that improve the water-holding capacity and presence of air-holes (Koski 2015).   Additionally, guano can be used as a fungicide and to control nematodes.  These agricultural services are possible due to the abundance of bacteria and microorganisms in the droppings that break down the unwanted garden intruders through decomposition and the production of enzymes.  Even though tons of guano are available due to the size of bat colonies, farmers don’t use as much because of its ability to persist in the soil after application (Koski 2015).  Guano is more difficult to wash away than the typical inorganic gardening products, meaning the multitude of benefits are not easily lost during a rainstorm or poisoning the runoff that enters nearby water sources.  Guano is safer than chemical-based agricultural products and provides services that would normally require multiple inorganic products.

Bats should be a farmer’s best friend, but they aren’t treated that way.  Instead, humans have poisoned the bats through the use of pesticides, which has been connected to population decline in bats.  Organochlorine (OC) pesticides, including the infamous and widely discontinued insecticide DDT, have been shown to accumulate in organisms due to their incorporation into the living tissues after consumption, also known as bioaccumulation (Clark 2001).  OC content increases as you go further up the food chain since the predators have to eat more organisms that contain pesticide residue in order to sustain themselves.  This process is also referred to as biomagnification.  This class of pesticides is also very resistant to degradation and evidence of its usage can still be found decades after the last application.  Scientists have found DDT in the fat stores of mammals, such as the Mexican free-tailed bat. 

Even if the initial exposure does not prove to be fatal, breaking down fats during the migration process can lead to toxins entering the nervous system.  It once again accumulates in the living tissue of the brain since it is not able to pass back through the blood-brain barrier, a membrane that selectively protects the brain tissue from solutes in the bloodstream.  This buildup of toxins can lead to potential neurological effects, such as spontaneous neuron firing and spasms (Thies, Thies, and McBee 1995).  The barrier is similar to the wasp traps where the insects can enter but are unable to find the exit due to the structure of the container.  Without a usable outlet, the wasps begin to pile up inside the trap and make it unable to properly function.  Female bats are able to rid themselves of the toxins through lactation, which are then passed on to the offspring, adding to the number of individuals affected (Thies, Thies, and McBee 1995).  Males do not have the luxury of breastfeeding a child and are left with continuously increasing levels of toxins in their bodies.  Man-made mistakes are proving to be costly to the Mexican free-tailed bat, even though their presence has actively been saving the agricultural industry money.

            Even though the Mexican free-tailed bat is the last thing on people’s minds when applying dangerous chemicals, their presence is widely noted when a city can economically benefit from them through ecotourism.  People line the Congress Avenue Bridge and the nearby banks of the Lady Bird Lake in Austin to watch the 1.5 million bats fly out at sunset (Bat Conservation International n.d.).  Boat tours profit from this spectacle, and national parks containing bats make money off ticket sales.   Not to mention, the local economy is supported by the travel expenses incurred by tourists.  Most bat activities occur during the evening, making it more convenient to stay in a hotel if you don’t live nearby.  Of course, there’s always the need for food and transportation or gas if you’re on a long trip.  From personal experience, I know that some cavern-based attractions embody the “ride ends in the gift shop” mentality, which encourages people to spend even more money.  The consumer surplus due all these expenditures brought about by bats was estimated to be about $6.5 million in the southwestern United States (Bagstad and Wiederholt 2013).  Even if a business is not directly related to the ecotourism of the Mexican free-tailed bats, they can still benefit due to the influx of outside spending.

            If you are interested in spotting these beautiful creatures, Mexican free-tailed bats primarily make their roosts in the southwestern region of North America through Central America and partially into northern South America.  Most of these individuals are migratory, but a few will remain in their summer home through the winter in the western United States (Tuttle 1994).  Their ideal roosting locations include caves and hollow trees, as well as buildings, bridges and abandoned mines.  Nursery colonies can contain millions of bats in a single location.  There can be 400 pups per square foot on average, looking more like a swarm of bees than roosting bats (Tuttle 1994). With such dense populations, colonies emerging from caves or structures look like thick plumes of smoke, signaling the locations of even the lesser-known sites to people nearby.  In contrast, the bachelor colonies are much smaller, consisting of a few dozen to a few hundred individuals (Tuttle 1994).  Even though they are seemingly less impressive in scale, watching each bat dive from their lofted home into the open air is just as breathtaking as the flurry of wings of larger colonies. 

It’s no secret that humans have been displacing animals through deforestation and other forms of ecological intervention, and the Mexican free-tailed bats are no exception.  The reduction of natural roosting sites has forced the bats into man-made structures.  Some colonies would roost in abandoned mines, but as these structures are destroyed, killing the bats trapped inside and preventing future colonies from developing there, the bats have moved to urban areas (Bellwood and Waugh 1991). 

Bats tend to favor human-free buildings in low-income neighborhoods, likely due to the lack of visitation and maintenance that would make the building inaccessible to the flying mammals or remove the colony directly.  The stereotype of an abandoned, tall building  being filled with bats is not totally inaccurate.  Structural damage can make the building more accessible for roosting, and the height of the ceilings can make it more difficult to remove the colony (Li and Kenneth 2015).  Resources are also an important factor in choosing a roosting site.  The Mexican free-tailed bats prefer buildings that are close to water and free of obstructing vegetation.  The lights of the city can even help provide a food source as bugs gather around the brighter spaces (Li and Kenneth 2015).  However, these sites are not entirely safe.  With the enactment of more urban renovation projects, many roosts are being removed and potential homes being demolished (Li and Kenneth 2015).  While these projects aren’t necessarily a bad thing, it is important that we create alternative homes for the displaced Mexican free-tailed bats and don’t leave them with a dwindling number of suitable habitats.

As the Mexican free-tailed bats start to intermingle with humans more often, it is reasonable to be concerned about the potential disease transfer that can occur.  Bats are commonly viewed as vectors for devastating viruses such as rabies and various coronaviruses.  The impact of such viruses when transferred to humans can be worsened due to the heightened immune system of their bat hosts, making  it more memorable when bats are  identified as the source of an outbreak.  Their defense systems cause the virus to adapt more quickly, while the host remains asymptomatic or minimally affected (Cottier 2020).  The protein responsible for signalling cells to fortify against an imminent attack, called interferon-alpha, is basically the Paul Revere of the immune system.  It is abundant in the bats’ immune system, leaving viruses to remain in the host without causing major health issues.  However, there are some side effects that typically limit the amount of the protein found in other mammals.  Interferon-alpha causes inflammation as it passes through the body, which can become dangerous in high amounts.  However, the way the bat’s immune system evolved minimizes inflammation, allowing the protein to flood the body and fortify the immune system at a high level (Cottier 2020).  While this is beneficial for the bat host, it causes some serious damage when passed on to a different species.  The human encroachment on bats and their habitat causes the bats to excrete more virus-containing waste, putting people at a higher risk (Cottier 2020).  Humans amplify the problem.  Why should the bats take all the blame?  It’s like poking a sleeping bear and acting surprised when it chases you.

It would be naive to think that habitat destruction is only affecting the human immune system.  White Nose Syndrome is a fungus that affects multiple species of bats and is connected to large-scale population decline.  The Mexican free-tailed bats remain asymptomatic, but they can spread the disease to other species as they share roosts.  Hibernating bats are the most vulnerable(“Bats Affected by WNS” n.d.).  The fungus is associated with individuals waking up during hibernation, severely depleting their energy stores that were supposed to last through the winter, and many infected bats die from starvation or dehydration.  As more roosting sites are destroyed, bat colonies are forced to find homes closer together, increasing the transmission of White Nose Syndrome (Meierhofer et al. 2018).  Important pest-controlling and pollinating bats will be unable to perform their ecological jobs as the fungus spreads further across the United

States into more colonies, causing ecosystems as we know them to change beyond recognition.

            The Mexican free-tailed bats are our friends.  However, it is best to maintain a safe distance from bats, especially if an individual is active during the day and likely diseased.  Animal control services can be called to properly remove an infected bat from the area without putting people nearby in danger.  If you want to attract bats, instead of get rid of them, building a bat box can be an effective way to bring the fascinating creatures to your own backyard.  You can even see a nightly show of the bats swooping through the air, almost too fast to see, and hear their reassuring chirps that tell you that your garden will be protected throughout the night.  If you really want to help the bats and keep yourself safe from potential disease, there is one crucial piece of advice to follow based on historical events.  DON’T. EAT. THE. BATS!


Bagstad, Kenneth  J. and Ruscena Wiederholt 2013. Tourism Values for Mexican Free-Tailed Bat Viewing, Human Dimensions of Wildlife, 18:4, 307-311, DOI: 10.1080/10871209.2013.789573

“Bats Affected by WNS.” n.d. White-Nose Syndrome Response Team. Accessed June 6.

Bellwood, Jacqueline J., and Rachel J. Waugh. 1991. “Bats and Mines: Abandoned Does Not Always Mean Empty.” BATS Magazine Article: Bats and Mines: Abandoned Does Not Always Mean Empty. Bats Conservation International.

Clark, D. R. 2001. “DDT and the Decline of Free-Tailed Bats ( Tadarida Brasiliensis ) at Carlsbad Cavern, New Mexico.” Archives of Environmental Contamination and Toxicology 40 (4): 537–43. doi:10.1007/s002440010207.

Cleveland, Cutler J., Margrit Betke, Paula Federico, Jeff D. Frank, Thomas G. Hallam, Jason Horn, Juan D. López, et al. 2006. “Economic Value of the Pest Control Service Provided by Brazilian Free-Tailed Bats in South-Central Texas.” Frontiers in Ecology and the Environment 4 (5): 238–43. doi:10.1890/1540-9295(2006)004[0238:evotpc];2.

“Congress Avenue Bridge.” 2020. Congress Avenue Bridge. Bat Conservation International. Accessed June 6. summer night, hundreds of,a better place to live.

Cottier, Cody. 2020. “Why Bats Are Breeding Grounds for Deadly Diseases Like Ebola and SARS.” Discover Magazine. Discover Magazine. February 28.

“Cotton Insects.” 2013. Department of Entomology. Kansas State University.

Koski, Michael. 2015. “Bat Poop, Possibly the World’s Best Fertilizer.” Get Bats Out. November 10.

Li, Han, and Kenneth T. Wilkins. 2015. “Selection of Building Roosts by Mexican Free-Tailed Bats (Tadarida Brasiliensis) in an Urban Area.” Acta Chiropterologica 17 (2): 321–30. doi:10.3161/15081109acc2015.17.2.007.

Meierhofer, Melissa B., Hsiao-Hsuan Wang, William E. Grant, John H. Young, Lauren H. Johnston, Lilianna K. Wolf, Jonah W. Evans, Brian L. Pierce, Joseph M. Szewczak, and Michael L. Morrison. 2018. “Use of Box-Beam Bridges as Day Roosts by Mexican Free-Tailed Bats (Tadarida Brasiliensis) in Texas.” Southeastern Naturalist 17 (4): 605. doi:10.1656/058.017.0410.

Thies, M.L., K, Thies and K. McBee. 1995. “Organochlorine Pesticide Accumulation and Genotoxicity in Mexican Free-Tailed Bats from Oklahoma and New Mexico” Archives of Environmental Contamination and Toxicology, 30: 178-187.

Tuttle, Merlin D. 1994. “The Lives of Mexican Free-Tailed Bats.” BATS Magazine Article: THE LIVES OF Mexican Free-Tailed Bats. Bat Conservation International. free-tails typically live,of droppings that they produce.

Leave a Reply

Your email address will not be published. Required fields are marked *