Of Bats and Men

Can we design urban spaces to work for both bats and us?

A million bats live underneath a bridge in downtown Austin – more bats than there are humans in the whole city. The free-tailed bats started moving in after a reconstruction of the bridge in 1980. While the bats were originally perceived to be pests, the number of curious tourists that began congregating to watch the bats fly out en masse each evening made the city begin to rethink its relationship with these little animals.

The concentration of so many of these little beasts in an urban setting must have an impact on human health, right?

Beginning this story, my hypothesis was two-fold: bats carry and transmit rabies so that’s a strike against human health; but they also eat a bunch of mosquitoes, which spread all sorts of infectious diseases, and reducing those populations would be a positive for people. While there is some truth to both these ideas, these points are far from the limit of how much bats affect people.

Like all mammals, bats can become infected with rabies, a horrifying and nearly always fatal disease of the central nervous system. Fortunately, they rarely pass it to humans, since bats are timid little creatures and humans and bats typically do a great job at avoiding each other. In the past decade, only ten people have been recorded across the United States as contracting the virus from contact with a bat. And though rabies circulates within many bat colonies, no colony becomes “rabid” or “infested” with rabies since fatality from the virus is quick, and the sick animals typically move away from others (this is why rabid animals can be found in some odd places). The good news is you can pretty much prevent rabies from bats by avoiding contact with them:

  • Don’t pick them up off the ground
  • Get your pets vaccinated against rabies
  • If they show up flying around in your house, don’t overreact – just open a window and encourage them to fly away

The second piece of my hypothesis: mosquitoes are hosts for many infectious diseases, including malaria, yellow fever, West Nile, and Zika. Most of these serious infectious conditions are well-controlled in the United States today, and bats can be moderately helpful in that regard. But many bats, such as the free-tailed bats that live under the Congress Avenue bridge, prefer bugs meatier than mosquitoes, such as moths and beetles. For that reason, the role of free-tailed bats in controlling infectious, mosquito-carried diseases is probably not a major factor, though it can be one of several strategies employed to limit mosquitoes in your environment.

So if people rarely acquire rabies from bats, and bats aren’t completely controlling the mosquito population, do they really impact human health?

Free-tailed bat at Austin Bat Refuge

Here’s where things get interesting. I reached out to Bat Conservation International to see if they could point me to a bat expert to speak about the inhabitants of the Congress Avenue bridge. They recommended Dianne Odegard, who operates Austin Bat Refuge to rehabilitate injured and sick bats and to educate schools and community groups in central Texas about the animals. In addition to discussing rabies and diseases spread by mosquitoes, she brought up a few other health considerations like the fungal disease Histoplasmosis and the agricultural impacts of bats consuming pests.

Histoplasmosis is a fungal disease characterized by lung infection, skin abscesses, and varied other symptoms. It grows naturally in soil and in guano from bats and birds, especially in regions of the world with humid climates and the right soil acidity. In North America, it is most common in the broad river valleys of the lower Mississippi, the Ohio, and the Saint Lawrence Seaway. Though most people who become infected have mild symptoms, histoplasmosis can be fatal for people with compromised immune systems. When cleaning up guano, it is best to wear appropriate respiratory protection. And it is wise to avoid bat caves without proper equipment.

A major, and perhaps unexpected, way that bats impact human health is by eating a large number of agricultural pests. This preference boosts crop yields and reduces the amount of pesticides that need to be applied. Free-tailed bats, for example, eat copious quantities of Heliothis moths, which in their larval stage attack corn. In total, bats across the United States have been estimated to increase yields and save farmers pesticides to the tune of several billion dollars annually. In addition, bat guano is a form of manure and is a good source of phosphorus and nitrogen for nearby plants. I could not find any research estimating the human health impact of reduced pesticide use due to bat populations, and this would be a valuable area of future study.

So besides their role in spreading or halting viruses, bats impact human health by indirectly spreading the fungal respiratory disease histoplasmosis and by reducing the need for pesticides in agriculture.

Building the bat bridge

According to Mark Bloschock, a civil engineer who worked on the reconstruction of the Congress Avenue bridge in Austin, the spacing of concrete beams that inadvertently created ideal bat habitat was a “happy accident.” After he saw the way such a minor design detail can impact wildlife in urban areas (and create economic activity), he spent decades working to incorporate bat-friendly designs into bridges across Texas.

The underside of the McNeil bridge of I-35 in Round Rock, Texas, was modeled with the same ~1″ spacing between concrete beams as the Congress Avenue bridge to allow for free-tailed bats to move in. This bridge is currently home to 1.5 million of these bats.

Bloschock mentioned that re-orienting bridge design to make them bat friendly can have a negligible cost if careful planned for, and he believes that consideration of wildlife in bridge design can have multiple benefits for nature and for humans. In west Texas, where long-nosed bats are common, his team combined a bridge project with landscaping to add a significant number of century plants, a type of agave plant used for mezcal production. Long-nosed bats are a principal pollinator of certain agave species, plants that are used to make tequila (a type of mezcal) and natural sweeteners. By creating habitat friendly to these animals, the bridge engineers were able to benefit native species and local agriculture.

However, there are certain circumstances in which he says bridges should not be bat-friendly: where cars park underneath the bridge, or when the structure is too close to schools, for example. But these exceptions still highlight the importance of considering wildlife in designs of structures like bridges.

How can spaces be built to encourage bat habitation?

Dianne Odegard from Austin Bat Refuge is often asked for her expertise on creating spaces that are friendly for bats, especially as urban development greatly changes the environment around the Congress Avenue bridge in downtown Austin. She mentions that the best bat habitat is natural bat habitat, and that means protecting caves from human disturbance and keeping stands of trees available for bats to roost.

Bats and humans each need their own space, and ideally we keep bats from living right where people want to live and work. Secondary to protecting natural habitats is identifying “human only” spaces, such as the inside of buildings (including attics), and keeping buildings well-maintained by sealing or protecting any small holes or crevices that lead indoors. After that, we can create spaces away from people that have small, safe, and protected cracks, such as the underside of bridges or the exterior of taller buildings.

Odegard thinks a lot more can be done to create great urban spaces for bats. More trainings on bat habitat and design for architects, engineers, and developers could help support that effort.

As well, she suggests a lot more research can be done. Specifically, she recommends more studies on how artificial lighting affects different species of bats so that human development doesn’t inadvertently drive a colony away. With more research and education, perhaps humans and bats can coexist peacefully in far more urban settings.