The science of bats

Oct 31, 2012

Just in time for Halloween, the world's first artificial bat cave is expecting the arrival of its first winged visitors. The nearly 80-foot-long concrete chamber was built to protect bats against white nose syndrome, a disease named for a white fungus that infects the skin of the muzzle, ears and wings of hibernating bats.

But how much do we really know about bats? Bat expert Seth Horowitz and Post science reporter Stephen Ornes answered your questions about bats, current research about them, and about the artificial bat cave.

Hi, I'm Seth Horowitz, an auditory scientist who has done research in hearing and balance with a wide variety of animals (including humans), and the author of "The Universal Sense: How Hearing Shapes the Mind."  There is a short interview with me on the National Science Foundation website http://www.nsf.gov under the “Discoveries” tab.  Today I'm happy to ask your questions about bats.

We have had bats in our eaves, not the attic, for 20+ yrs. How do we get rid of them so we can sell our house? We are pretty sure a potential buyer will balk at having our approximately 50 mosquito eaters around.

Bats on the outside of a house usually don't do too much damage unless you are seeing huge piles of guano around, and they are, as you pointed out, good for controlling pest insects.  if you really feel you need to get rid of them, do NOT try and relocate them yourselves.  You can try calling a local university with a bat lab - they may be happy to take them off your hands safely.  Otherwise call your local pest control company.

I live in Central NY State and have always seen Little Brown Bats in my yard and pool area. This year the numbers seemed to be four fold. Are small colonies of Little Brown bats in the northeast doing better than large colonies of bats as a whole?

This has been a rather warm and wet year in the northeast, so the little brown bats' (Myotis lucifugus) food supply of small bugs is probably way up, helping support a bugger population.  The other major species in your area would be the big brown bat (Eptesicus fuscus) who probably see the same benefit.  

What critters are bats afraid of? (What animals eat or annoy bats?)

Bats (depending on the species) are often preyed upon by evening-foraging birds such as hawks and owls.  In central america there is a species of frog that sits and lurks at the entrance to caves and will occasionally grab a bat on the way in.  In some areas like India, there is a species of bat that preys upon on bats.  

Hi, I'm Stephen Ornes. I'm a science writer in Nashville. I recently visited the artificial bat cave, and the timing for it is remarkable. White Nose Syndrome was just identified this year near the cave.

How large of bandwidth is the bat echolocation signal? Is it fairly constant or does it change from pulse to pulse based on information from previous pulses? What resolution can a bat achieve with its signal? Is it able to discriminate between a model of an insect and a real insect?

Good questions - bats use a wide variety of acoustic signals but they largely break down into FM (frequency modulated) bats who make "chirps" that sweep across frequencies from high to low and CF (constant frequency) bats that make a steady signal (sometimes with a little FM tail on the call).  Species have different bandwidths; the FM big brown bat's call sweeps downward from about 90 to 20 kHz and rely on tiny changes in the echo structure compared to the original signal to reconstruct distance and 3D shape; CF bats will have different center frequencies and rely on doppler shitfing of the signal to alert them to an insects wings beating.  

 

The signals are basically consistent but recent work by the Simmons lab at Brown showed that bats will actually shift frequencies at the low end to avoid jamming signals from other bats.

The resolution a bat can achieve depends on the frequencies it uses in its calls - bats with higher frequency emissions such as the little brown bat can resolve smaller prey like mosquitos, whereas bats with slightly low ranges such as the big brown bat focus more on large prey like moths and june bugs.

And bats are very good at figuring out if they're being tricked; scientists have used everything from necco wafers on strings to artificial echoes to simulate bugs.  Even if they get tricked once, they usually don't make the same mistake again.

How close is anyone to finding a cure or vaccine for white nose? How decimated is the population of the little brown bat in Indiana? We had (in Indiana) a lot of bigger bats visit in October-is this normal? If a vaccine can be created, would placing a vaporizer in a cave to distribute a mist be a feasible way of administering it?

No cure or vaccine have been developed yet, as far as I know. Some mycologists (fungus scientists) do think a vaccine could effectively treat the disease, but Ann Froschauer at the US Fish and Wildlife Service told me there are not currently any vaccines for fungal infections in mammals. I know researchers are looking at both treatments and ways to distribute those treatments, from in-cave misters to artificial caves like the one in Tennessee. I can't speak to the increase in bats in Indiana this year. 

How do bats hear in three dimensions with their ears?

All animals hear in 3 dimensions, but bats are very good at creating 3D models of their world the same way we do with their eyes.  It's still not completely understood, but the short (and to be continued) version is that for bats like the Big Brown Bat, they emit a "chirp" sound which sweeps down in frequency from about 90 kHz to about 20 kHz.  Their brains have a template of this sound.  When the echo comes back, their auditory system compares the echo with the original sound and calculates where the differences are.  The time of delay (period fro mwhen the sound is emitted to when it returns) tells the range or distance to the target and changes in the echo at each frequency gives information about the relative size of the target due to the fact that every frequency has a specific wavelength - the higher the frequency, the shorter the wavelength.  Even though the bat's brain is about the size of a peanut, they are carrying out massively complicated computations on this auditory information, allowing them to build a 3D world from the echo/signal differences. 

Hi Seth -- We had two bats in our house (or maybe it was the same bat) this summer. We had the house inspected and there was no signs of a colony. We have since located a likely entry point and sealed it up. Was it likely just a curious/lost bat checking out our house?

There's a good chance you just had solitary or scouting bats looking for a good place to live and if you blocked their entrance while they were out hunting they just looked for another place.  Maternity colonies where you get large numbers of bats are relatively rare and usually grow over multiple season of not being bothered.  So you probably put the "not welcome" mat out in time. 

I know the brown bat population in the NE U.S. has been devastated by white-nose syndrome over the past few years. Are there any signs that bat populations are recovering?

There are some early studies that suggest little brown bats can recover from infection if they're given supportive care. Those lab studies suggest recovery is not impossible. Most studies to date are observational, as scientists try to understand how it spreads and the behavior of the disease. 

Do insect eating bats mainly use active echolocation or the natural sounds of the flying insect to find dinner?

Great question - I worked with echolocating bats who are active hunters, but there are quite a few bats who hunt by passive listening (also called gleaning).  You can often tell these type of bats (like the Pallid Bat) because of their oversized ears.  They often prey on insects on the ground by listening for and targeting the sounds they make as they crawl around.  (Owls do somethign similar with ground-based rodents).

Is white nose syndrome getting worse or better recently? Will any bats be left in 10 years?

Worse. All hibernating bats are believed to be vulnerable to the disease, and it spreads farther westward every year. European bats appear to be resistant to WNS - they may carry the fungus without dying. 

What is it like to be a bat?

I wish I could answer that  - one of the holy grails of animal behavior is trynig to figure out "what is it like to be X?" and then replace X with your favorite species.  The closest I've come to figuring out part of it was an animation project I did where I used a 3D modeling and animation program and substituted propagation qualities of sound for those of light (basically tricking the program to show light reflections substituting for echoes).  What it showed was what I called "glassworld" with glints of lights reflecting off structures.  What was interesting using this model was that if things were still it looked like a static holiday light display, but when the shape of a bug flapped it's wings or moved relative to the trees, it was really easy to figure out what you were looking at.  While this is a human-centric model, it's likely that bats, being relatively close relatives of humans, use similar perceptual organization tools. 

Stephen - How does an artificial bat cave work? Did you get to go inside it?

I did get to go inside. From the outside, the human entrance kind of looks like the entrance to some ancient Egyptian tomb built in a hillside. The bat entrance sticks up out of the ground at the top like a chimney. The inside looks like a giant, rectangular concrete tunnel. The echoes from any noise are so strong that you can hear people whispering from far away - but because the sounds all overlap, it's hard to make out words. (It was hard to do interviews in there, too!) I can only imagine how noisy it will be once the bats move in...  It's designed to trap cold air - colder than the ambient temperature of the surrounding earth - which is attractive to bats. There are all kinds of textured add-ons where the bats can roost, as well. 

I have never understood why so many people are scared of bats. Is it lore, or a fear that actually has some basis in fact?

Bats (at least night time echolocating bats) are usually seen as fast moving shadowy things, moving silently (unless you have very weird ears).  Since bats are evolutionarily quite old, we've seen them since we were humans, usually moving faster than we can track at a time of day when we are quite vulnerable (also it's hard to accurately gauge the size of something flying around in near darkness) so those factors probably kicked in early in our cultural history.   Also if you see them close up, very few of them are what most people think of as cute and cuddly - some of them have very ornate and weird faces and ears.  (Also if you see an ecolocating bat close up in daytime it's sick and likely to bite).  In regions where  large fruit eating bats are common, you can have this vaguely human shape about the size of a child flying around, covering your trees and eating your crops while staring at you with big eyes.

Bats pose little risk to humans (at least if you keep your distance) except for vampire bats and even they prefer livestock to humans.  So short answer, it's probably a night time, barely seen, vaguely human and hence creepy cultural thing.  

I love bats. I watch keenly the first spring twilight when their acrobatics fill my homestead sky. They have continued to emerge each year from their wood edge home. Are the small populations that do not reside in caves more insulated from disease because of their relative isolation or is it pretty inevitable that the fungus will spread out to them. They are little brown bats. I am so sorrowful at the drop of their numbers.

Species that hibernate in caves, where the fungus may live in the soil, are most vulnerable. The disease can also spread from bat to bat, so social contact can also increase risk. I'm not completely sure, but I would guess that it's too soon to know to what extent the spread is inevitable. Researchers are still watching the fungus and the disease to understand more about it. 

We live in Anne Arundel county and have two small bats, maybe brown bats, that have been in a bedroom window for a few months now, they shimmy in between the storm window and a screen. It was just one bat there it came around mid summer and from what I read, I expected it to move on once it got cold. Then, 2-3 weeks ago, another bat joined our first one and they've both been there ever since. They don't sit close to one another, they usually try to be as far away as the small window allows. Any chance that this is turning into a bat family? Should we be calling someone to remove them? I wasn't upset when I found the bat and quite honestly, my family finds it very interesting to watch it come and go, but I'm starting to think we should be concerned. We've cleaned out the guano, but it's getting harder now that the bats seem to be taking their time leaving at night. There is no chance that they could get into the house from the window and we're really not scared of them at all, but I'm worried about letting them live here for too long. Thanks.

It's been a pretty warm fall so far, so while they may still move on, they may have decided that your window is not a bad place to hang out.  I doubt they're forming a family - most east coast bats in the USA breed in early summer, so I think you have two solos who like your place.  The problem is that if they are in an accessible window could kids or pets get to them?  That's not great for them or the bats.  Also while sterilize bat guano is great fertilizer you don't really want it building up and you shouldn't be handling it.  If the bats are still leaving to hunt, I would tighten down the storm window so they can't get back in and they will find somewhere else to live.  If they seem to be hunkering down, I'd call a local university with a bat lab and see if they will take them, or a local pest control company (but make sure they're just going to remove and release them, and then tape up the storm window).

I love bats. I watch keenly the first spring twilight when their acrobatics fill my homestead sky. They have continued to emerge each year from their wood edge home. Are the small populations that do not reside in caves more insulated from disease because of their relative isolation or is it pretty inevitable that the fungus will spread out to them. I am so sorrowful at the drop of their numbers

The white nose fungus is definitely hitting cave-dwelling bats much harder than othe bats.  House- and open-dwelling populations seem not to be as affected so far.

Hi! Thanks for taking my question. I recently installed a bat house, but have no idea if any have taken up residence. Any ideas to encourage them?

At the bat cave in Tennessee, they're going to be using recordings of bat echolocation sounds to try to bring in the bats. I'm not sure that's a good solution for a home bat house, though. Cory Holliday at the Nature Conservancy also told me that once a few bats move in and poop, other bats will come streaming in, so the key is to get that first bat to move in. Again, I'm not sure that's helpful at home. 

Are there any captive breeding programs being considered to safeguard the population due to the danger of infection? Would it even be feasible?

There are several bat breeding programs (such as that run by the Lubee bat Conservency) but breeding bats is not like breeding mice.  Echolocating bats typically only have 1-2 babies per year and only breed once a year, so they invest a lot in their offspring.  They also need a lot of space to fly and if you captive maintain them you have to hand feed them, which means they then have to learn to hunt if you're planning on releasing them (which is one reason why zoos usually just keep fruit bats - you just put out strings of fruit rather than have to wrangle mealworms and moths with tweezers).  So while some places do captive breeding, it's probably not practical for population restoration in the wild.

(On the other hand, as long as you keep them supplied with fresh blood, vampire bats breed REALLY well in captivity - this can be a problem).

I want to encourage bats to the yard (not attic necessarily) of our new home, which has a terrible mosquito problem in summer. We have three tall, slender oaks standing fairly close together in the backyard--are there things we can do to make the area bat-friendly, to encourage them to come hang out and eat some mosquitoes?

I recently spoke with a naturalist who suggested building bat houses to attract bats. In the spring, female bats are looking for a place to nest, so I believe that's the best time to encourage them to take up residence. 

How does the artificial bat cave help prevent the spread of disease? I visited Carlsbad Caverns in the May 2011 and watched the bats come out at dusk. The Park Ranger told the audience that unfortunately, the bats do NOT eat mosquitoes (which were biting us) but eat a lot of moths instead.

The disease is caused by a soil fungus that loves to live in caves. Observational studies have shown that the bats start to show symptoms - like the telltale white fuzz - about 3 years after the fungus is found in a cave. An artificial cave can be disinfected every year, which means that every time the bats come to hibernate, they're not going to get the disease from their environs. The conservationists say they're trying to reset the clock every year, and they hope that reduces the spread of disease. Bats can still get sick from other bats, however. 

Any thoughts on why bats mainly use auditory information to find prey while birds are mostly visual? Is the mammalian brain maybe more efficient at processing auditory information?

Mammals definitely have the most bandwidth for auditory processing, with ranges from infrasonic sub 1 hz in elephants to 160 kHz + in some dolphins.  Even birds who are very auditory-oriented  usually only hear up to 5-7 kHz  although owls and some other specialized birds can hear up to 15 kHz).  But mammals also have excellent vision.  Echolocating bats evolved from insectivore like animals that naturally had good high frequency hearing and over time took advantage the evening niche where vision is of limited use and predatation from daytime predators was probably lower.  Some recent evidence indicates that bat echolocation may have emerged from an evolutionary shift in development that let them retain gap junctions in their auditory system.  Gap junctions are common in the developing mammalian brain and are a type of neural synpase that uses direct ion flow from neuron to neuron rather than the more flexible chemical synapses found in adults.  A study colleagues and I did in 2007 found that the cochlear nucleus, the first place in the big brown bat's central auditory system to receive input from the auditory nerve, was loaded with the protein that makes up gap junctions, so this could lead to figuring out the evolutionary/developmental shift that let bats echolocate with such stunning temporal resolution. 

It hasn't happened in a while, but what do I do if bat does get in the house? I don't like to harm living things, but I'm not sure I could deal with a bat in the house. I'm not sure the dogs and cats would tolerate it either. Assuming this would happen at night and it would be difficult to call someone to help, what should be done?

There's a quick trick to this and I've used it for years.  Since bats rely on high frequency sound to echolocate, if you open a window in the room and then move away from it and shake a set of keys, the bats will try and move away frmo the noise from the keys.  Don't walk right up to it and shake the keys over it - that will make them freeze.  Just start shaking them and move around so that they only quiet area is the one by the open window.  The bat will fly towards the quieter zone.  You're basically herding the bat with sound.  Works great.

I was told was visiting Austin that the people who designed the bridge where bats took residence never considered that their bridge attract them. It turned out to be great for Austin tourist. I am wondering, what exactly is it about the bridge shape that turned out to be perfect for bats?

Great question - that's an amazing sight. I don't know for certain, but I would guess that the bridge offers all kinds of perfect nooks and crannies for the bats, and since they're social creatures they followed their fellow bats in... 

I've heard there are insects who can fool echolocating bats, how do they do that?

Bat echolocation is considered a classical example of a natural "arms race."  Bats develop echolocation, their prey develop defenses.  Some moths like the tiger moth can actually generate ultraosnic signals that jam the bat's echolocation signal.  Others detect the echolocation signal and preferentially drop one wing or the other using a direct connection from their "ear drums" and their wing muscles and hence drop from their predicted flight path and hopefully  out of the bats range.

Last year, I evicted a bat from my bedroom. I managed to put a bucket over the bat and scoop a piece of cardboard under the bucket. I took the bucket outside about 30 feet from the house. When I removed the lid, I swear that the bat started chasing me. It was flying just behind me as I ran back inside. Does this sound like something a bat would do on purpose or was it just disoriented?

It's not going to chase you - probably just disoriented especially if you did this in daytime.

I live in Seychelles, and although we have loads of fruit bats, insectivorous bats are here only in small numbers. I would love to do whatever I can to encourage populations to increase, but have no idea what I can do to help. There's certainly no shortage of insects and no predation I know of the little guys. Any suggestions?

I don't know about your local bat populations but you should probably go to websites like Bat Conservation International and see if there are any programs in your area.  They do great work.

Thanks for your questions. As of the last time I checked in, no bats were living in the artificial cave. But as the researchers told me when I visited, it only takes a few to come in and poop to attract other bats inside. Here's to hoping it works to slow the spread of WNS! 

Okay thank you for all your questions and my apologies for those I couldn't get to.  Take care and remember that bats are really some of the most amazing and generally helpful animals around.  

In This Chat
Seth Horowitz
Seth S. Horowitz, Ph.D. is a neuroscientist who has researched and published extensively on sensory perception by bats in the dark, human hearing, balance and sleep. His new book, The Universal Sense: How Hearing Shapes the Mind, (Bloomsbury: 2012) explains how bats see in 3-D with their ears

Dr. Horowitz's research has been funded by grants from the National Science Foundation, the National Institutes of Health, The Deafness Foundation, and NASA. He lectured in undergraduate and graduate level classes in animal behavior, neuroethology, brain development, the biology of hearing, and the musical mind. As chief neuroscientist at NeuroPop, Inc., he has applied his basic research skills to real world applications ranging from health and wellness to educational science outreach.
Stephen Ornes
Stephen Ornes is a science writer for The Washington Post.
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