During the summer months, as the sun embarks on its dusky descent, the evening sky comes alive as bats look for their next meal. There are probably few urban neighborhoods that lack representation from the Chiropteran family. But the activities of bats undergo a distinct change with the turning of the seasons. Let’s look at how this plays out.
Wait...which bats?
So let’s be clear which bats we’re talking about. After all, there are over 1200 species of bats across the globe. We will focus on the bats of Iowa, and more specifically, the two most common bat species in Iowa. If you live elsewhere and landed on this article just looking for an enjoyable read, then stay on board! But if you need info on a species outside of Iowa, then you’ll need to dig around online to find which live in your area. Let me help!
If you live in the United States, look up which bats live in your state using the USGS Bat Population Database. To our worldwide friends, check out this bat list on Wikipedia. You might also be interested in groups actively conducting or highlighting bat research, such as the North American Society for Bat Research and Bat Conservation International.
Here in Iowa we have thirteen species of bats. The two most populous species are the little brown bat (Myotis lucifugus) and big brown bat (Eptiscus fuscus). Both species are found in every part of the state. While they do not form families together, they are perfectly comfortable cohabitating. I’ve even seen them cozying up to each other to keep warm in the late fall. To understand what happens to these bats in the winter months, let’s cover their seasonal habits.
Some quick bat biology
Our local bats are strictly insectivores. They do not feed on berries and fruits, and certainly don’t consume blood. We humans benefit from this nightly onslaught on the local insect population. But this limits which times of year that bats can feed. No bugs, no food.
The majority of the bats in Iowa will have emerged by late April. This fluctuates from year to year, but is a good rule of thumb. Bats chomp on a variety of flying insects, including gnats, crane flies, mosquitoes, aquatic flies, and beetles. These feedings continue until the insect populations freeze out or go dormant in the late fall.
Bats are hardwired to know that they will have no access to any calories during the winter months. They bulk up their fat reservoirs in the fall to provide enough stored energy to last through this period. But this fat energy would be grossly insufficient if they maintained normal activities throughout the winter months. They resolve this shortage by not only avoiding flight, but also by lowering their rates of respiration and circulation—from a normal of over 200 beats per minute to around ten. This low metabolic state is called a torpor state, and it allows bats to survive without food for months at a time.
Should I stay or should I go?
A bat’s state of torpor is in part induced by low temperatures. As seasonal temps drop, each bat must choose whether to look for a local place to overwinter, or migrate to a communal area—usually large caves. I’m unaware if scientists have unraveled how individual bats decide what to do. Is it generational knowledge, passed on by its relatives? Is there an innate drive to stay or to migrate? Do individual bats make the same choice each year to stay or go? Perhaps the answer rests in one dominant factor, or perhaps its a combination.
It appears that the big brown bat generally favors migration, while the little brown bat prefers to stay near its summer home. Pruszko and Bowles published a survey of several known cave hibernacula to take a count of which and how many species had migrated. They found
This study is helpful, but we must exercise caution in what conclusions we draw. For example:
How many of the bats counted were already there prior to the fall migration?
What are the estimated total species populations across the state, so we can better gauge these reported counts as a percent of the total?
What other hibernacula (caves or otherwise) are used as migration destinations, which haven’t been accounted for in this study?
These questions certainly are not to criticize the study, but point out the limits of generalizing our conclusions regarding the question of bats staying in place versus migrating elsewhere.
Those that stay
A good number of bats, both of the little brown and big brown species, choose to overwinter at their summer locations. These locations must be suitable to their survival. It must offer concealment, protection, and an appropriate temperature range. The attics of residential and commercial buildings are often well suited. Since an unfinished attic is not heated, the temperatures will be low enough to induce the torpor state. However, an attic sits on top of the heated living space, so the temperatures do not drop low enough to allow a bat to freeze solid.
As winter temperatures fluctuate, these bats will potentially move around to select a spot better suited to maintaining torpor. One study showed that big brown bats in the winter months had skin temperatures that fluctuated between 39-degrees and 98-degrees Fahrenheit, the upper temperatures corresponding to having come out of their torpor states. The study also suggested that these bats probably require dwellings that are above freezing. This supports the idea that, in residential and commercial building attics, bats will move around based upon outdoor temperatures.
Did we answer the question?
The question posed in this article is “Do Iowa bats stick around for the snow?” We considered the two most common Iowa bat species, the little brown bat and the big brown bat. For both species, some opt to stay in their summer locations. Attic spaces provide suitable locations for overwintering, as bats can move in order to find suitable temperature ranges.
There is one species of bat that hibernates in the snow! Check it out!
A portion of the populations of both species leave their summer homes and congregate in large hibernacula—mainly caves. Once the spring thaw takes place, these bats will return to their summer homes. These bats will often sleep and roost in the exact same locations as in previous years, and will resume their same flight routes to capture their food. All of this happening with most of us being totally unaware!
Have you had a close encounter with a bat? Share your story in the comments below!
Highlighted Resources: USGS Bat Population Database
Pruskzo, R and Bowles, J (1986). Survey of some Eastern Iowa caves for wintering bats. Proceedings of the Iowa Academny of Science, 93(2): Article 4.
Halsall, A, Boyles, J, and Whitaker, J (2012). Body temperature patterns of big brown bats during winter in a building hibernaculum. Journal of Mammalogy, 93(2): 497-503.
Joe Taylor, M.S. Science Education, B.S. Biology, is a full time nuisance wildlife control operator in Eastern Iowa, and owner of Paw Control, LLC. Follow him on Twitter: @pawcontroller at LinkedIn, or the Paw Control blog: https://www.pawcontrol.com/blog