Creature Feature: Mexican Free-tailed Bat

Batshit crazy. Batty. Bats in the belfry. Blind as a bat. A cursory examination of these idioms does not exactly paint a pretty picture of the collective opinion of our winged mammalian relatives. Bats, however, have a slew of fascinating behavioral and morphological traits that should elicit awe and recognition. The Mexican free-tailed bat (Tadarida brasiliensis) is a particularly delightful species that Davis residents are fortunate to share a home with (read on for details about where you can spot these critters!). This species, like other insectivorous (i.e., insect-eating) bats, relies on echolocation to capture its prey. T. brasiliensis belongs to the Molossidae family which includes other bat species that similarly lack a membrane connecting their tail to the rest of their body (thus, free-tailed) [1].

The reduced membrane (the uropatagium) connecting the feet and tail of the Mexican free-tailed bat (Tadarida brasiliensis) is a feature shared by bats in the Molossidae family [Source].

The reduced uropatagium (that thin membrane that stretches between the legs of other bats) is thought to contribute to the aerodynamic performance of Mexican free-tailed bats. Researchers have also examined the flight muscles of these bats and found that they have a high concentration of specialized muscle fibers that increase their energetic efficiency [2]. Imagine flying a battery-operated toy helicopter that is powered by a series of double AA Energizer or Duracell batteries rather than those cheaper, knock-off brands sold on Amazon. More efficient batteries will last longer, allowing you to remain airborne for longer. Well, it appears that free-tailed bats are able to stay airborne for much longer than other bat species, and they do so in the most energetically-efficient way possible, thanks to these powerful muscles [2].

Researchers have found that Mexican free-tailed bats have a high concentration of special muscle fibers in their flight muscles (see hatched region in Figure B) [Source].

Beyond this specialized morphological adaptation, Mexican free-tailed bats also engage in a series of behaviors that deserve our attention. This bat species seasonally migrates from parts of Southern and Central America (including states in Mexico and Brazil) to North America (including states such as Texas, New Mexico, and California) [3]. They roost in caves, under bridges, tunnels, dams, buildings, nest boxes, and vegetation. However, several colonies have been returning to the same roosting sites for generations and are of particular conservation importance (see Map 1 below). The largest of these colonies, found in Bracken Cave (Texas, U.S.A.), has been estimated to include about 1-2 million individuals and is one of the largest aggregations of a mammal species on Earth [3],[4]. Individuals will return to their roost after several hours of nighttime foraging for moths, a preferred food item of these bats. In fact, many moth species that are considered agricultural pests are kept at manageable population sizes thanks to the Mexican free-tailed bat [5]. At sundown, a cloud of bats moves seamlessly through the sky as they emerge from their communal roosts as one coordinated beast (see Video 1 below). If you live in Davis, you can admire this creature by watching the daily sunset mass emergence between the months of March and October (see Yolo Basin Foundation’s Bat Walk).

Map 1: The locations of some of the largest colonies of Mexican free-tailed bats, including summer and winter roost sites. Note the two sites in California where you can observe the mass emergence of thousands of bats every night between March and October [Source].

Communication and coordination is an area of particular interest to scientists who study animal behavior. These bats solve coordination problems in a unique way; T. brasiliensis are well-known for singing! Bat biologists believe that singing in T. brasiliensis evolved to coordinate migration and roosting patterns [6]. Surprisingly, females do not sing, and only a small proportion of males within a colony are singers, suggesting that this behavior may attract more bats to a colony [6]. More research must be conducted to determine the exact benefits of this costly behavior (singers spend less time foraging meaning possibly less food intake), and how singing contributes to maintaining social bonds in this species. Advances in animal tracking and acoustic recording technology will hopefully help us answer these questions (see FLIR video). The batshit crazy adaptations in animals such as the Mexican free-tailed bat provide exciting avenues of future research for animal behaviorists.

Video 1: A cloud of bats emerges from a roost at sundown; they will spend several hours foraging for insects such as moths [Source].

Footnote: Although humans benefit greatly from the pest control role that Mexican free-tailed bats play, we have also used them for deadly purposes. Shortly after the bombing at Pearl Harbor, a dentist began to devise a sinister plan to exact revenge and developed a “bat bomb” [7]. The plan was to deploy an aerial container of thousands of sleeping bats which would, upon release, fly to roosts in target cities. The bats were strapped with timer-triggered mini-bombs that could have killed thousands of people. Thankfully, for numerous reasons, the plan was scrapped [7]. Few still remember the minor role the Mexican free-tailed bats played in American weapons testing. Let’s hope we collectively learn from our mistakes.

Main featured image: Source

References:

  1. Vaughan, T. A. (1966). Morphology and flight characteristics of molossid bats. Journal of Mammalogy, 47(2), 249-260. https://www.jstor.org/stable/1378121
  2. Foehring, R. C., & Hermanson, J. W. (1984). Morphology and histochemistry of flight muscles in free-tailed bats, Tadarida brasiliensis. Journal of Mammalogy, 65(3), 388-394. https://www.jstor.org/stable/1381084
  3. Wiederholt, R., López-Hoffman, L., Cline, J., Medellín, R. A., Cryan, P., Russell, A., … & Semmens, D. (2013). Moving across the border: modeling migratory bat populations. Ecosphere, 4(9), 1-16. https://esajournals.onlinelibrary.wiley.com/doi/10.1890/ES13-00023.1
  4. Stepanian, P. M., & Wainwright, C. E. (2018). Ongoing changes in migration phenology and winter residency at Bracken Bat Cave. Global Change Biology, 24(7), 3266-3275. https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.14051
  5. Krauel, J. J., Westbrook, J. K., & McCracken, G. F. (2015). Weather‐driven dynamics in a dual‐migrant system: moths and bats. Journal of Animal Ecology, 84(3), 604-614. https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2656.12327
  6. Smotherman, M., Bohn, K., Davis, K., Rogers, K., & Schwartz, C. P. (2016). Daily and Seasonal Patterns of Singing by the Mexican Free-Tailed Bat, Tadarida brasiliensis. In Sociality in Bats (pp. 197-209). Springer, Cham. https://link.springer.com/chapter/10.1007/978-3-319-38953-0_9
  7. Madrigal, A. C. (2011) Old, Weird Tech: The Bat Bombs of World War II. The Atlantic. Apr 14, 2011. https://www.theatlantic.com/technology/archive/2011/04/old-weird-tech-the-bat-bombs-of-world-war-ii/237267/

Other resources:

National Park Service: All About Bats. URL: https://www.nps.gov/subjects/bats/echolocation.htm

Bats at Yolo Bypass: https://ww2.kqed.org/quest/2011/08/03/science-on-the-spot-bats-beneath-us/

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