The Underground

Cool, dark, peaceful and calm, the subterranean world is a many a bat’s perfect escape from the din and clatter of the daytime. Whether alone or in massive colonies, bats are uniquely suited to take advantage of the many benefits caves confer. For hibernating, resting during the day, at night between meals, for mating, or for social meet-ups, underground spaces in their myriad forms are important places for many species of North American bats.

Roughly half of all of North American bat species rely on subterranean habitat for at least some part of their life cycle. Though not all bats live in caves, and not all caves have bats, they hold outsized importance for the chiropterans that do rely on them. Because many individuals tend to congregate in relatively few roosting sites during breeding or hibernation, disturbance of even one site can impact a large proportion of a given species.

Do cave bats only live in caves?

No! Although some bats have evolved to live in caves, they can, and do, take advantage of any cave-like structure with the right environmental conditions. This include the obvious, like sinkholes and abandoned mines, but also places people don’t tend to think of as often: bunkers, burial crypts, basements, derelict sewer systems, and crevices in cliffs. Some bats will only hibernate in these structures, while others migrate and occupy them seasonally.

Why did bats go underground in the first place?

Science is still working out exactly why bats headed under the hills at some point during their evolution. One reason may be due to their wings: all that exposed skin makes them susceptible to dehydration, and caves provide habitat with relatively high humidity. They’re also safer from predators.

Caves also provide stable habitat for very long stretches of time, even as climate conditions above may be changing. Access to a variety of underground roosting spaces is key for bats to able to exploit shifting conditions, or to expand into new areas.

Does every cave or mine have bats living in it?

No. In fact, only a small fraction of available subterranean habitat is suitable for the bats that need it. In temperate North America, many underground features are simply too cold for bats to occupy them long-term; they must have the right kind of airflow, humidity levels, ceiling texture and light levels for bats to choose to settle down there. But when a site does check all the right boxes, it’s one bats will return to again and again.

How do we know which subterranean places are most important to bats?

Through careful, systematic and sustained observation and investigation of sites suspected or reported to be frequented by bats. BCI’s SubT program team is equipped with a specialized set of skills to be able to venture into these hard-to-reach places and confirm which sites are used by bats, as well as gather biological and environmental data about the bats and their roosting sites.

The team also uses this data to advise land management partners in the public and private sectors regarding proper closure of mines and other potentially hazardous subterranean features; for instance, helping coordinate installation of gates at known bat habitats to keep people out while still allowing bats access to a roosting or hibernation site.

What are the greatest threats to these kinds of habitats?

In general, human disturbance poses the greatest threats to natural and manmade bat habitat. While cave exploration is a popular pastime for many people, careless visitors can prompt bats can abandon a roost after even short periods of disturbance. Humans can also inadvertently carry in pathogens on their clothes or shoes that can be harmful to the bats living there. And in expanding urban and suburban areas, development around caves or old mine reduces or eliminates foraging habitat and puts humans in closer proximity to the roosts.