More Things That Fly Around Ponds

The first morning I awoke at the cabin before everyone else, and stepped out onto the deck to look out on the lake. As I did so, I noticed a large, oval leaf floating in the foot-dip bucket*. Being the kind of guy who’s always interested in leaves, I bent down for a closer look. The “leaf” had fur on it.

*Used to get sand off feet before entering cabin.

I first thought the critter dead, but a gentle prod with a twig produced a healthy response. It wasn’t a leaf and it wasn’t dead; it was a Little Brown Bat, Mysotis lucifugus, and it was stuck.

We tend to think of bats as somehow unusual or different, but something like a quarter of all species of mammal in the world are bats (over 1,100 species). If a visitor from another planet landed on Earth tomorrow and asked to see a “typical” mammal, the right thing to show him/her/it wouldn’t be a dog, cat, monkey or mouse; it would be a bat. And if that alien landed in North America, it would be a Little Brown Bat (LBB), the most common, widespread bat in North America.

I’ve blogged a bit about bats before, comparing their wings, lungs and senses to those of birds. But as it turns out, a captive bat, especially one stuck in water, is a fabulous opportunity to take a closer look at its wings. Check out this video:

You never see bat-wings opening and closing that slowly in the air. Nor for that matter do you usually get a chance to see them doing so in one place, in broad daylight.

All About Bat Wings

Back in the Spring, in my post on Swifts, I speculated a bit about birds and bats, why bats were overwhelmingly nocturnal, and why they’d evolved echolocation/sonar* more frequently than birds. In passing, I mentioned differences in flight capability, and basically accepted that birds were superior flyers. But subsequently, as alert reader Doug M. brought to my attention, my off-hand assessment of their flight capabilities probably gave short shrift** to bats.

*The more correct term these days seems to be “echolocation.” But I love the word “sonar”, so that’s what I’m generally going to use in this post.

**What’s a “shrift”, anyway?

Tangent: Doug M. is probably the smartest reader of this blog*. Seriously, whoever you are, Doug M. is almost certainly smarter than you, and he’s for sure way smarter than me. If he wrote this blog, it would totally rock. Doug M. frequently emails me comments on posts, with all sorts of great info and insight on everything from geology to bats to the Eocene to arthropod lungs to echinoderms to the ecology of South Florida. If he ever starts a blog, you should read it.

*Tomodactylus may be another contender, though he comments less frequently. And Christopher. Oh, and Sally, when it comes to plants. And Jube when it comes to rocks. And Ted when it comes to bugs. And KanyonKris and SBJ when it comes to bike parts. OK, so most everyone who reads this blog is smarter than the guy who writes it. But I’m OK with that, because although I am often lacking in smarts and facts, I know that my graphics are totally awesome.

Bat wings do indeed produce less thrust and lift than bird wings. (and as I’ve posted previously, their wing-powering musculature is very different than that of birds, and their/our lung architecture is inferior.) But bat wings enable flight that is in many cases more finely-tuned and more maneuverable than bird flight. Although it’s easy to think of bird and bat wings as basically big arms, that’s a bit of an oversimplification. Check out this graphic (not mine, captions/pointers added by me) of the bone structure of bird and bat wings.

A bird wing is more or less a big arm, with the “hand” as largely a fused-together extension of that arm. You can imagine flapping as a bit like moving your arms*.

*Though the sensations a bird experiences must be radically different. Birds have no deltoid muscles, so raising a wing must feel nothing like raising an arm, but rather stresses a set of muscles we don’t have.

But much of a bat’s wing is effectively its “hand”. Roughly 1/3 of the wing surface area is the membrane between its forefinger and pinkie-finger. So when a bat flaps, it’s sort of a combo arm-flapping and hand-waving.

Extra Detail: And the motion is fundamentally different from bird-flapping. Bat-flapping is more of a “rowing” motion, almost like a sort of breaststroke. As a bat flaps, its elbows lift out to the side with the fingers extended out, and then the arms and fingers pull forward, down, then up again.

Think about your own hand, and its dexterity, sensitivity and fine motor control compared to that of your forearms and upper arms. That’s probably in line with the sensitivity and dexterity a bat feels in its wings, and it enables maneuvers in flight that are hard to replicate in birds.

When you look close up at the membrane of a bat’s wing, you’ll see it’s covered with tiny bumps. These are called Merkel cells, after Angela Merkel, who before she became Chancellor of Germany was a noted bat researcher in the GDR. Haha- just kidding! No seriously, the Merkel cells, each of which has a tiny hair in the center, are ultra-sensitive touch-receptors, through which the bat actually feels the airflow over its “hands” and (subconsciously) dynamically alters its “hand”-position/wing-shape in response. These guys are in tune with the medium through which they’re traveling at a level hard for us to conceive.

Many bats- including the LBB- have another receptor-type in their wings which is sensitized to the stretching of the wing membrane, and its role is to help the bat catch insects in flight.

About ¾ of bats are insectivores. The LBB for example typically consumes about a third of its bodyweight each night in bugs. I always assumed that bats caught bugs like a Loon catches fish: open the mouth and chomp. But it turns out that while bats do consume smaller bugs (gnats, mosquitoes) mouth-first, most larger insect-prey, such as moths, are caught in the wings and/or tail and then “handed” to the mouth.

Extra Detail: Speaking of moths, there are numerous examples of evolutionary “arms races” between moths and bats. Some moths have a special acoustic organ which, when it detects the ultrasonic peeps pf bat-sonar, automatically sends the moth into evasive maneuvers- a twitchy, erratic flight-path. Other moths emit their own ultrasonic signals in response to bat-sonar. These signals appear to be a form of “sonar-jamming”, but it’s also been suggested that in some cases they may serve as a warning to the bat that they’re poisonous or bad-tasting, similar to how other insects warn off predators with distinctive coloration.

Many of the insects preyed upon by LBBs BTW, are insects that- like mosquitoes- develop in an aquatic stage and so are found near bodies of water. As a result LBBs tend to be common near lakes and ponds.

A bat’s wing isn’t just a lame, bare substitute for a feathered bird wing. It’s a completely different appendage, with different structure and different capabilities.

I don’t know how he/she wound up in the bucket. Bats can “see” the surface of the water effectively with their sonar. Many bats routinely skim the surface of a body of water to drink, and I read that the LBB actually sometimes uses its wings to drink (presumably by scooping up a “handful”- I’m not clear.) Certainly the water in the bucket is a much smaller, and proportionally deeper, body of water than say the lake, but small crevices, puddles and pockets full of water occur all over the place in Maine, and I haven’t heard of bats getting regularly stuck.

My one wild guess- and this is pure speculation- is that somehow the plastic of the bucket (a large Tupperware-style container, actually) threw off his/her spatial “sonar judgment.” Bats routinely recognize rock, wood, living things (prey/bugs, other bats). Maybe the sonic “texture” of plastic is just weird/unknown/different enough that the bat misjudged the dimensions and/or structure of the container, and made a bad landing call, akin to when a Loon lands on a highway.

That’s total conjecture, of course, but it does get you thinking about bat sonar, and how completely amazing it is. We all know the basics of course: bats emit ultrasonic peeps, and then hear those peeps bounced back at them, which lets them know where various things are and enables them to fly around in the dark while not hitting walls and catching bugs. Just like sonar on a submarine, right?

All About Bat-Sonar

Well, not really. Submarine sonar (and accompanying hydrophones) is reasonably good at detecting things like the ocean floor, ships, other submarines, and torpedoes. Bat sonar detects things like moths and gnats, distinguishes them from dozens- or hundreds of other flying things- bugs, other bats, what bat researchers call “clutter”- and enables the bat to make repeated split-second decisions, flight-corrections, attacks and evasive maneuvers. Bat sonar provides bats with a real mental image of the world around them; they “see” the world through sound.

Still, the concept sounds simple enough, but when you scratch your head and think about it, it’s amazing that bats make sonar work. Let’s take a simple example. For bat sonar to work, the emitted peeps need to be LOUD. This makes intuitive sense when you think about it, though chances are you probably never did think too much about it, since bat-sonar-peeps are ultrasonic, too high-pitched for us to hear.

So how loud are bats? Pretty freaking loud. When measured from a distance of 10cm, the sonar-peeps of insect-hunting bats are up to 125 -130 decibels. That’s about the volume of a jackhammer when you’re standing next to it. So here’s this little critter, with these teensy-weensy, ultra-advanced, hypersensitive little ears, which are being blasted by a jackhammer between 10 and 200 times a second. How on Earth does a bat not blow out its own eardrums?

Peep Frequency vs. Sonic Frequency

Before we answer, 2 things worth covering here. “Frequency” is a loaded term with bat-sonar. There’s the sonic frequency of the pitch of the peeps, which generally ranges from ~20kHz to ~60kHz (with extremes from 11kHz to 212 kHz), and then there’s the frequency of the emission the peeps, which usually ranges from ~10Hz to 200Hz. Insect-hunting bats- as I mentioned last year in the post about Vampire Bats- use a lower peep-frequency scanning sonar when cruising along, but switch to a higher peep-frequency “targeting” sonar when closing in on a bug. The higher peep-frequency sonar enables more accurate tracking of the bug, and quicker flight corrections, etc.

Special Side Note for the Scientifically Impaired/Illiterate, Generally Spacy and/or my Mother: 1Hz or “Hertz” is once per second. 1 kHz or “Kilohertz” is 1,000 times per second.

The bat doesn’t use the high-peep-frequency all of the time because a) it presumably requires a lot more energy and b) there’s some evidence that the lower peep frequency may provide a better distance/big picture view of the world. (This may particularly be true for bats that use “chirp sonar”, where each peep starts at a high sonic frequency and then drops. The actual frequency returned may provide additional distance information, and these reflected trailing chirps may be clearer if not packed too closely together.)

The sonic frequencies of bat-sonar are of course ultrasonic to human ears. Our hearing ranges to an alleged maximum of about 20kHz. Given how loud bat sonar is, we should probably be grateful that we can’t hear it, but courtesy to human beings probably wasn’t the evolutionary driver in determining sonar frequencies.

Side Note: I say “alleged” based on my own recent testing. Our own hearing detects higher frequencies best when we’re children, and then deteriorates progressively as we age. While we were back East we visited the (very excellent) Boston Museum of Science. One of the interactive displays on hearing and acoustics was a set of headphones with a user-controlled sound frequency. I (age 46) was able to detect a tone up to about 14.2kHz. Bird Whisperer (age 11) was able to hear up to nearly 17.5kHz.

The reason bat sonar is high frequency is that so many of the things that bats need to “see”- like bugs- are small. Lower frequency sounds have longer wavelengths, and when the wavelength of a sound wave gets longer than say the wingspan of an insect, it becomes hard to “see” that insect. An analogy with our own vision would be if our eyesight was tuned to the electromagnetic frequencies of radio or TV waves, which range in wavelength from roughly 1 to 10 meters; it would be very hard for us to see much of anything smaller than a piece of furniture.

Tangent: Something occurred to me in doing this bit on frequency. 20kHz – 60kHz is out of range for us humans, but a large portion of that range is easily audible to dogs, whose hearing regularly extends way up over 40kHz. What do dogs hear in the evenings when bats come out? If it’s really that loud, how come they aren’t freaking out? The same is true for most other mammals, BTW. Cat hearing ranges even higher than dogs (up over 60 kHz). Rats and mice hear way higher frequencies than either of them, and even horses and cows can hear pitches up over 30kHz.

Yeah, yeah, frequency-schmequency. So what about the blowing out the eardrums thing? Turns out that bats have 2 ways of dealing with this problem. The first is that many species have an organ in their heads that partially mutes their hearing in the exact instance they’re peeping. So if that bat’s peeping 100 times/ second, it’s muting its hearing 100 times/second.

But the second workaround is even more interesting. Many bats peep at slightly lower frequencies than that which their hearing is optimized for. But because the bat is flying forward, the return echoes are received at the slightly higher frequency for which its ears are tuned. This is an example of the Doppler Effect, which is why the siren of an ambulance or the horn of a train sounds slightly higher when it’s coming towards you than when it’s going away from you.

Sounds cool, but how can this possibly work? A bat flies at all kinds of different speeds, and- as anyone who’s watched a flying bat knows- they change speed and direction all the time? Apparently the bat’s brain is wired to- automatically and subconsciously- modulate the pitch of its own peeps to compensate for changes in flight speed.

A seemingly even thornier problem is how to do all this stuff- peeping, flying, water-scooping, bug-catching, Doppler-adjusting- in the company of dozens or hundreds of other bats. How does the bat keep straight its own echoes from those of other bats, whether of the same or different species?

For a long time these questions were unanswered, and much is still not understood, but research in the last decade has started to make things clearer. First, bats can distinguish peeps of their own species from those of other species. This isn’t surprising. Most bats, including the LBB, are highly social, roost in large groups, and it would be problematic if they routinely confused groups/roost of other species with their own. But the more surprising finding has been that bats can distinguish between specific different/other species, so when they hear an “alien” peep, they don’t just think, “Oh, this is some other kind of bat…” but rather, “This is a type X bat.” This is potentially useful info for a bat. Some species may be more aggressive competitors, other species may favor similar feeding or roosting areas.

Secondly, it seems that bats can recognize “voices” amongst their own kind. For many years researchers have known that if you want to mess up a bat’s ultrasonic “vision”, you can’t do so by just playing recordings of bat-peeps. But you can mess it up by playing back its own recorded, individual peeps. It wasn’t clear why this was, but now it seems that bats recognize their own voices, and that instant recognition is what allows them to sort out the echoes they need to accept to construct a sonic “view”.

Bats not only recognize their own voices, but also those of common associates. So the distinction in a bat’s mind when hearing a voice isn’t just, Me/Other Bat Of My Type, but Me/Bat I Know/Other (Unknown) Bat Of My Type. Interestingly, bats in this research learned to “recognize” specific “new” bats over 2 to 3 weeks of regular exposure/interaction.

Tangent: There are 2 interesting corollaries here. The first is that when bats are out hunting, they must recognize the calls of their regular associates. (“Oh, that’s Roy closing on a target off to my right…”) The second is that bats regularly and comfortably do something we’re sort of awkward with: recognizing our own voices. When I’m listening to a recording or a video of family, friends, coworkers, etc., I’m always slightly jarred when hearing my own voice. “That’s what I sound like?” I think. Somehow my own voice, pitch* and even inflection** all seem a bit wrong, like there’s this presumptuous stranger in the midst of my friends/family/team. Why is that? Why can’t I “hear” myself?”

*Always higher than I think.

**Oddly credible, but sometimes slightly affected, like I went to a snooty school or something. Which I did, so I guess that makes sense, but again, I don’t “sound” that way to me when I speak…

This effect is especially unsettling when an “echo” is present in conference calls. (Who is that guy with the irritating voice, and why does he keep repeating everything I just said??)

I’ve touched on just 2 examples, but the point is that there’s far more complexity and sophistication in bat sonar than just shouting at walls; the brains of bats are optimized in all sorts of ways to process acoustic information and translate that data into a working model of the world, much in the way significant portions of our own brain are adapted to processing visual information in creating our model of the world.

It’s interesting to think about how sonar came about. Like vision, we know that it’s evolved multiple times- no one thinks bats and porpoises shared a common sonar-enabled ancestor! But how many times has it evolved in bats? The answer is at least twice, and maybe more.

Bats are generally divided up into 2 types- microbats and megabats. Most microbats, like the LBB, are insectivorous and use sonar, but many- particularly the larger ones- hunt frogs, lizard, fish, scorpions and even birds*. (Vampire Bats BTW are microbats.) Megabats are overwhelmingly fruit and nectar-eaters, have excellent senses of smell, and do not- with one important exception- use sonar. Many megabats are important pollinators and/or seed-dispersal agents.

*Thanks, Doug M.!

Extra Detail: For a long time it was assumed that bats were monophyletic (descended from the same common ancestor species) and that early on they diverged into the micros and megas. But in the late 1980s some researchers suggested an alternate hypothesis: that bats had evolved not once, but twice, and that the similarities between micros and megas were the result of convergent evolution. This would mean that flight- true powered flight- had evolved not just once, but twice independently among mammals. The supporting evidence for this hypothesis included a number of anatomical differences between the 2 groups, but most tantalizingly, the researchers pointed to apparent structural similarities between the brains of the megas and the brains of primates. In other words, maybe the megas were more closely-related to us than they were the micros!

It was one of the coolest, most incredible convergent evolution hypotheses ever. Unfortunately, it was almost certainly wrong. Subsequent DNA research established the monophyly of bats. But that same research did disrupt the traditionally accepted family tree. Bats don’t divide cleanly into megas and micros; instead the megas are just one branch within the (now paraphyletic) micros.

And that branch is overwhelmingly non-echolocating. Whether or not their ancestors were sonar-capable is unclear*; so the megas could be descended from bats that never evolved sonar, or from echolocating bats that subsequently lost the ability.

*Fossil evidence seemed in some cases to indicate bats evolved flight before echolocation, but more recent genetic evidence suggests that the common bat ancestor may have been echolocating. Fossil evidence for bats isn’t all that great BTW, they don’t fossilize that well.

That one megabat exception- the species that does use sonar- is Rousettus aegypticus, the Egyptian Fruit Bat (EFB) (pic right, not mine). But its sonar is completely different from microbat-sonar. Echolocating microbats produce sound from the larynx, like us. In effect, an LBB spends its evenings flying around shouting. But the EFB produces its signals by clicking its tongue, something no other bat does.

It used to be thought that the EFB’s sonar was a sort of lame substitute for “real” bat sonar, but more recently it’s been re-assessed as more capable and more comparable to microbat sonar. So clearly sonar has evolved at least twice in bats. More recently, DNA research suggests that traditional, “mainstream”, laryngeal sonar may have evolved twice within microbats (for a minimum of 3 times overall). The question is still unresolved, but bats have a complex and fascinating evolutionary history.

When I come a across an easily or already-captured critter, I often try to show it to the Trifecta, if I think I can do so without excessive harm or trauma to the critter in question. In this instance I was especially motivated to do so, as I wasn’t clear as to when, if ever, they might see another LBB close-up.

As a boy in the 1970s I remember sitting on the beach after dusk, watching dozens of bats flit through the air. We saw bats this trip, but only onesy-twosy. Within the last 5 years, numbers of bats- especially LBBs- have been dying in huge numbers. The dead and dying bats exhibit a white fungus, Geomyces destructans*, growing around their muzzles and wings. Geomyces species do well in cold, dank environments, such as permafrost soils, and, presumably, bat caves. G. destructans seems related to other Geomyces species, though it has an unusual spore morphology.

*Totally bad-ass Latin name.

Frustratingly, it’s not known with the fungus is the killer, or an opportunistic infection after the bat has been weakened by some other killing agent. Infected bats are typically emaciated, dehydrated and unable to hibernate. LBBs are social and sleep, mate and hibernate huddled closely together in huge roosts- ideal conditions for communicable pathogens. In less than 5 years since its discovery in upstate New York, it’s spread from Quebec to Oklahoma.

Extra Detail: The fungus is present in Europe, where- presuming it is in fact the killing agent- it doesn’t seem to cause bat die-offs, leading researchers to wonder whether European species are resistant. One of the leading suspects for its spread to North America is humans, and perhaps specifically cavers.

The day we left Boston to drive up to Maine, the New York Times* ran a piece on White-Nose Syndrome. Researchers from Boston University have determined that the probability of LBBs being wiped out completely in the Northeastern US within 20 years is over 99%.

*My parents subscribe to the New York Times, despite living 200 miles from that city. I used to poke fun at them for doing so, but have since desisted, as after several days’ exposure to the Times, my return home to the Salt Lake Tribune makes for a bit of a rough landing.

My LBB’s muzzle looked clean. I gently removed him from the bucket and placed him under another container on the deck.

Side Note: Yes, I was careful to use tongs, and wash everything/hands afterward. You hear a lot about bats and rabies, and it’s true that rabies regularly afflicts many bat species, including the LBB. So how likely is it that a given bat is infected? Typical infection rates are less than 0.5%. But- and this is a big “but”- symptoms of rabies include loss of coordination, which in bats often means loss of flight. So while there’s a small chance that any given bat is rabid, infected bats are probably statistically over-represented amongst those you encounter on the ground in broad daylight.

Last month I made a little extra effort to swim at a Gulf Coast beach, with the thought in the back of my mind that perhaps one day I might not be able to do so. In the same spirit I waited patiently for the Trifecta to awake and check out our visitor.

Maybe someday they’ll tell their kids about it.

Note About Sources: There are tons and tons of great info about bats and echolocation online. My best source was Bat echolocation calls: adaptation and covergent evolution, Gareth Jones and Marc Holdereid. Additional evolutionary and phylogenetic info came from Integrated fossil and molecular data reconstruct bat echolocation, Springer et al. Info on voice recognition in bats came from The Voice of Bats: How Greater Mouse-eared Bats Recognize Individuals Based on Their Echolocation Calls, Yosi Yovel et al. The link to the referenced New York Times article is here. The hearing ranges used in the animal-frequency graphic came from this site. My introduction to bat-sonar some years back came from the excellent description in Richard Dawkins’ The Blind Watchmaker, which though somewhat dated now, is still a wonderful read.