Segment 1: The Question

[MUSIC PLAYING] [MITx Free online courses from MIT] Have you ever wondered how woodpeckers avoid brain injury when they peck? They have to peck pretty hard, because what they’re trying to do is get at little bugs and insects beneath the bark of the tree. One theory is that they have a special foam-like material between their brain and their skull that’s a little like the foam liner in a bicycle helmet. [Lorna Gibson, Professor of Materials Science, MIT] I’m Lorna Gibson. I’m a professor of Material Science and Engineering at MIT and I study the mechanical behavior of foams. I’m also a birdwatcher. And so when I heard that woodpeckers might have some special foam-like material to protect their brains from injury, I had to look into this. A group of neurologists in California who study brain injury in humans also were interested in the woodpeckers and how they avoided brain injury. They got a dead woodpecker and dissected the head. But, unfortunately, they didn’t find anything. There is no foam-like material that protects their brain. At this point, I got even more interested in the question. How do they protect their brains against brain injury when they peck? We’ll look at the mechanics of it in a minute. But first, let’s take a field trip and look at a few woodpeckers. [MUSIC PLAYING] We’re at Harvard University at the Museum of Comparative Zoology. This is one of the great natural history museums of the world. [MUSIC PLAYING] We’re in the Ornithology Department here. And the Ornithology Department has something like 400,000 specimens, representing 80% of the birds of the world. They have everything connected to birds. They have mounted specimens. They have drawers full of specimens. They have skeletons. They have nests. They have eggs. It’s really a truly incredible collection. So we’re going to use the collections here at the Museum of Comparative Zoology to look at woodpeckers and to try and understand how they avoid brain injury when they peck. We’re going to be looking at things like the skulls of the woodpeckers and the orientation of the brain in the skull, and we’ll use this information to understand how the woodpecker avoids brain injury. [MUSIC PLAYING]

Segment 2: Woodpecker 101 

[MUSIC PLAYS] We’re at Harvard University’s Museum of Comparative Zoology to take a closer look at woodpeckers. [Lorna Gibson (Professor of Materials Science, MIT)] So here we have a number of mounted specimens of woodpeckers. This one here is a hairy woodpecker. If you live in New England like I do and you put out a bird feeder in the winter, then you may get a hairy woodpecker come to visit your feeder. This is a red-bellied woodpecker. And you can see, sometimes bird names can be a little deceptive. If you look at the breast, very little red on the belly. Some of them do have a faint red, but this particular one has very little red at all. This next one here is the yellow-bellied sapsucker. And if we look, we can see there’s a little faint yellow on its belly. Most woodpeckers eat insects, but sapsuckers also take sap out of the tree as well. And finally, we have the pileated woodpecker here. This is the largest woodpecker in North America. And when the pileated woodpecker pecks, It’s really very impressive. It sounds like a carpenter hammering but speeded up. It’s very fast when it peaks. So these are different types of woodpeckers, and there’s several more species that live in North America, as well. When woodpeckers peck, they have to apply enough force to chisel out chips of wood to make the holes so they can get at the insects beneath the bark. When they do this, they brace themselves with their tails and their feet. And you can see, these woodpeckers are mounted in such a way that it’s as if they’re about to peck. You can see their tails braced up against the wood as if they were braced up against the wood of the tree. We can see that it’s got tail feathers that have particularly large shafts. We can see the shafts here are really quite large, especially if you compare it, for instance, with the shaft of one of these wing feathers. And those thick shafts are what make the tail feathers stiff, and they help brace the woodpecker against the tree. Another adaptation woodpeckers have is that they have two toes that go forward and two toes that go backward. And that’s different from many other types of birds. The most common arrangement of the toes is that they have three toes forward and one toe backward. And again, this adaptation of two toes forward and two toes backward allows the woodpecker to brace itself against the tree to resist the forces of the pecking. So woodpeckers peck to collect insects and sap to eat, but they also peck for other reasons, too. And so we’ll look at that next. [MUSIC PLAYS] 

Segment 3: Why Peck?

[MUSIC PLAYS] Woodpeckers peck for several reasons. [Foraging] One reason is to forage for insects beneath the bark of the tree or to collect sap. They have another special feature which allows them to project their tongue out quite far to be able to do this. The way this works is their tongues are connected to something called the hyoid apparatus. [Hyoid apparatus] And the hyoid apparatus consists of a bone, as well as some muscles that contract to make the bone move. So we can see a little bit about how this works in these two examples here. We have a downy woodpecker on the left here, a we have a hairy woodpecker on the right. On the downy woodpecker, you can see the hyoid bone here. If I move it around a little bit, you can see how it connects to the tongue down here, and it wraps around the bottom of the skull, comes up the back of the skull, wraps around the eyeball, and then connects in the front. In the example of the hairy woodpecker, you can see that the hyoid and the tongue have been removed, and the hyoid has been stained in this case. You can see the tongue is this little gray piece here, and the hyoid is this red stained bone that wraps around like that. And you can see it curls around kind of in a circle so that it can curl around the back of the skull. So the hyoid is separated from the skull, normally, and when the muscles contract, the hyoid moves closer to the skull and the tongue scoots out. And one of the interesting things is that the hyoid apparatus allows the tongue to move out and collect little insects, and sap, and things like that. So woodpeckers peck to collect insects and sap to eat, but they also peck for other reasons, too. One is that they build cavity nests. [Cavity nests] And here we have an example of a cavity nest that was carved out by an ivory-billed woodpecker. So here’s the mounted example of the ivory-billed woodpecker. Unfortunately, these are now extinct, and so we don’t see any live ones, but this is a mounted specimen of one. And when they build these cavity nests, what they do is they dig out a horizontal tunnel, and then they build a cup below that, and the eggs are laid at the bottom of that cup. And for the ivory-billed woodpecker, we can see just how big that nest is if I take a ruler, and we can actually measure some of the dimensions. So if I put the ruler in horizontally like this, I can see that this nest is about 8 inches deep into the tree. And if I move the ruler this way on, the nest goes right up to the top of this hole here, and it’s about 20 inches deep. So when you think of that, this woodpecker here has carved out this nest that’s about 8 inches in diameter and 20 inches deep to lay its eggs at the bottom. [Drumming] During mating season, woodpeckers peck on a hollow branch to make a loud noise, and they do this to announce their presence and their availability for mating. Sometimes they peck on downspouts on houses, which can be quite annoying for the occupants because it makes quite a loud noise. Acorn woodpeckers peck for yet another reason. So let’s take a look at some specimens of acorn woodpeckers. Here we have a whole drawer of acorn woodpeckers collected at different times– some in California, some in Oregon, some in Arizona. And if we look at a particular specimen, you can see how colorful they are. They have red at the back of the neck here, there’s yellow behind the beak, and yellow goes all around the bottom of their neck there. And they’re sometimes called the clown woodpecker just because they’re so colorful and beautiful. Acorn woodpeckers live in social groups of up to about 16 in California and Oregon, and sometimes in Arizona. And I think of them as the champions of pecking. They really have an amazing behavior related to the pecking. [Food storage] What they do is they store acorns in granaries, and the granaries are decaying tree trunks. They’ll pick little holes in the decaying tree trunk and pop their little acorn in there. And the thing that is the most amazing about this whole behavioral is it’s not just one or two acorns that they store, but it’s tens of thousands. And they can store up to 50,000 or 60,000 acorns in a single granary. Next, we’ll look at the mechanics behind the acorn woodpecker pecking and try to understand the forces involved. [MUSIC PLAYS] 

Segment 4: Impact & Deceleration

[MUSIC PLAYING] Let’s take another look at the acorn woodpecker pecking. [Hairy woodpecker] As it pecks, what happens is it starts with its head back away from the tree, and then its head speeds up and accelerates, and then when it hits the tree, it decelerates. And the impact force depends on how quickly the brain stops when the beak hits the tree. [acceleration? deceleration?] So we need to understand something about acceleration and deceleration to understand the forces on impact. One simple example is to think about what happens if you take an egg and you just drop it and it hits the floor. As you release the egg, right at that very moment, it’s got zero speed. And as it falls, it’s gaining speed. And we say that the egg is accelerating with 1 g. So g is for gravity. And then when the egg hits the floor, there’s some impact. The deceleration will be many more times than 1g. So if we think about brain injury, what sort of decelerations can the brain withstand before there’s some sort of injury? Well, people have studied this quite extensively for humans. [Tolerable Deceleration 100g] And they found that the human brain can tolerate decelerations up to about 100g or 100 times the acceleration of gravity. So one question is, how much acceleration or deceleration could a woodpecker brain withstand without injury? And in the next segment, we’re going to look at how people have measured the deceleration in the woodpecker during pecking. [WOODPECKER SQUAWKING] [MUSIC PLAYING] 

Segment 5: Measuring Deceleration

[MUSIC PLAYING] [SCRIBBLING] [TYPING] How do we study woodpecker pecking? Well, it turns out that a group of neurologists who are interested in human brain injury were also interested in this question of woodpeckers and how they avoided brain injury. [PROPERTY OF DISCOVERY] And they realized that if they took high speed video, they could measure the position of the woodpecker’s head and its beak as it started to peck and as it hit the tree. And with the position at different time points from the video footage, they could then calculate the speed at which the beak was traveling, and also the deceleration when the beak hit the tree. But one difficulty is, how do you actually set this up? [FILM ROLLING] The neurologists were in California. And they had heard about a park ranger in Placerita Canyon State Park who had an acorn woodpecker that was injured and couldn’t fly. And the park ranger had really the key to this whole experiment. The neurologists who did this were doing so in the 1970s. And they actually used film. So they had to be able to start the filming at the moment that the woodpecker started pecking. The park ranger had a typewriter in his office. And when the ranger typed on the typewriter, the acorn woodpecker would peck in response to that. So perhaps the woodpecker thought there was another acorn woodpecker around and it was pecking in response. Who knows what exactly the acorn woodpecker thought. But in any event, it would peck in response to the typewriter keys being struck. So the neurologists set their equipment up. They had an old, dead tree stump. And they could get the woodpecker to peck at a certain spot on the trunk. And then they would also type on the typewriter. And this would get the woodpecker to start pecking. And what they found was truly remarkable. [Deceleration 600g-1500g; Human brain injury 100g] They found that the decelerations on impact were between about 600g and 1500g. And if you recall, we said that human brain injury occurs at about 100g. So the woodpeckers are pecking over and over again at much, much higher decelerations than a human brain could withstand without injury. They also found that the woodpecker hit the tree at a speed of about 15 miles an hour. [CHIRPING] And that’s something like the speed that I ride my bicycle. And if I rode my bicycle into a tree at 15 miles an hour, I would certainly know about it. [Speed 15 mph] The other thing they found was that the duration of the impact was relatively short. [Duration 0.5-1.0 milliseconds] It was between about a half a millisecond and one millisecond. So from these high speed videos, the neurologists were able to get some good data on the woodpecker pecking. And we’re going to use this data to estimate the forces acting on the woodpecker brain. [MUSIC PLAYING] [SCRIBBLING] 

Segment 6: The Explanation: Size Matters

[MUSIC PLAYING] Our Placerita Canyon woodpecker endured a whopping 1,500 g while pecking but we don’t know how much more woodpeckers can endure before injury. Let’s use what we know about human brain injury to figure out the limits of the woodpecker’s tolerance. It turns out it’s all a matter of scale. [force = mass x deceleration] A force is a mass times an acceleration, or in this case, a deceleration. But it turns out it’s not quite that simple. What really matters is the force divided by the area that the force is applied over. [force/area = (mass x deceleration)/area] Imagine you have a rope. And say you apply a tensile force to it and you find that the rope breaks when you apply 1,000 pounds of force. So now imagine if you had two identical ropes. The force to break those would be because you’ve got twice the area. And so what really matters isn’t just the force. It’s the force divided by the area. So if we think of the brain and the skull as the woodpecker’s head hits the tree, both of these things are decelerating as the head decelerates. And we can say that the impact force for the woodpecker is the mass of the woodpecker brain times the deceleration that it undergoes when it hits the tree. [Impact Force: (mass of woodpecker brain x deceleration of woodpecker brain)/area of woodpecker brain] And we’re going to divide that by the area of contact between the brain and the skull. The force per unit area to cause damage is similar in different species. Scaling Law: (mass x deceleration of woodpecker brain)/area of woodpecker brain = (mass of human brain x deceleration of human brain)/area of human brain] So let’s relate the impact force per unit area for the woodpecker to that of the human and solve for deceleration or DW. [mass and area ~ Size of brain] We can simplify this somewhat by saying that both the mass and the area depend on the size of the brain. So the mass depends on how big the brain is, the volume, and that goes as the radius cubed. And the area goes as the radius squared. So in both cases, canceling out the radius times the deceleration that the woodpecker brain can tolerate before injury, is equal to the radius times the deceleration that the human brain can tolerate before injury. [radius of woodpecker brain x deceleration of woodpecker brain = radius of human brain x deceleration of human brain] The deceleration that the brain can withstand without injury depends on its size. This is an example of a scaling law. The size matters. The smaller the brain, the larger the deceleration that it can withstand. But we have to look at the area in a bit more detail. And here we have two skulls. We’ve got one for a human here and one for an acorn woodpecker over here. [Orientation of the Brain] And we want to look at the orientation of the brain because the orientation of the brain affects the contact area that comes into our scaling law. The brain’s roughly a hemisphere. And in the human brain the hemisphere is oriented something like this. And so when the impact occurs in the human brain, the human brain hits the front and hits the back of the skull. And the area of contact is roughly this half area here. If we look at the acorn woodpecker skull, you can see that the back of the skull here that is protecting the brain is actually oriented more in this direction here. So that as the woodpecker pecks and the brain sort of moves forward and backwards in the skull, then it’s the entire area of this hemisphere that is the area of contact. So, relative to the human brain, there’s an increased area by a factor of two for the woodpecker brain. So we have to incorporate that into our scaling law. The factor of two for the woodpecker brain is somewhat approximate. At the back of the brain it’s a reasonably good approximation. At the front, things are more complicated. In the tennis ball analogy, the vertical slice at the front has a plain circular cross section, which does not increase the area by a factor of two. But the surface of the brain is curved so that the area is larger than in our analogy. For simplicity we use a factor of two here. [Deceleration, deceleration of woodpecker brain = 2 (radius of human brain/radius of woodpecker brain) x deceleration of human brain. And we find that there’s this factor of two times the radius of the human brain over the radius of the woodpecker brain times the deceleration in the human. So the next thing we need to know is the ratio of the size of the human and the woodpecker brain. And much to my amazement, I found a paper called “Brain Size in Birds.” This paper, “Brain Size in Birds,” had table after table for many different species of birds. And they had measured the bird-brain mass and from the mass you can estimate the size. The human brain is about eight times the size of the woodpecker brain. The woodpecker brain was about 2 and 1/2 grams and the human brain is about 1,400 grams. [Deceleration, deceleration of woodpecker brain = 16 x deceleration of human brain] So our equation says that the woodpecker brain can tolerate a deceleration 16 times that of the human brain. So this is kind of interesting. We said that the human brain could tolerate 100 g. So our equation so far says that the woodpecker could tolerate something like 1,600 g. And if you recall the measurements that the neurologists made, the woodpecker was pecking and sometimes reached deceleration of 1,500 g. So the other factor that comes into play is the duration of the impact. People have studied human brain injury quite extensively. And they found that if the duration of the impact is shorter, the brain can withstand a larger deceleration. So this graph shows some information that’s been obtained from human brain injury. And on the graph we’ve superimposed the region over which a typical human head impact occurs. So typically for human ahead impacts, things like football player injuries or car crashes, the impact occurs over something like 3 to 15 milliseconds. And we can take this curve here for the humans and we can simply scale it up by our factor of 16. And then we remember that the typical duration of the acorn woodpecker head impact was between half a millisecond and one millisecond. You can see that the tolerable deceleration is something like 4,600 g to 6,000 g. And also on this graph we’ve superimposed the measured decelerations during pecking and you can see they’re substantially below that amount that we’re estimating. So this all kind of makes sense that the tolerable acceleration is really much higher than what you would measure during pecking. [Summary: Small brain size: x8, Orientation of brain: x2, Duration of Impact: x4, Total woodpecker to human impact tolerance ration: x64] So if we summarize what we’ve found here, we can say that there’s really three factors that have allowed the woodpeckers to tolerate these high decelerations. One is their small brain size, that was a factor of eight. One is the orientation of the brain, that gave another factor of two. And one is the duration of the impact and that gives a factor of 4. So that altogether there’s a factor of 64. The woodpecker brain should be able to withstand decelerations about 64 times that that a human brain can tolerate without injury. 

Segment 7: Scaling in Nature

[MUSIC PLAYING] Since we began our journey, we’ve learned some amazing things about woodpeckers. When they peck, they get the tree going at about 15 miles an hour. And they endure decelerations of up to 1,500 G all to eat, to court, to make nests, and to store food for their families. But the answer to how they withstand these decelerations when they peck is really quite simple. It’s a matter of scaling. The size of their brain matters, and their small brains allows them to withstand large deceleration. Scaling phenomena like this are widespread in nature, and many things can be explained by scaling laws. Another example of a scaling law in nature is in the shape of bones. And here we have a variety of bones in front of us. This bone here is the femur of a hummingbird, a very small bird. And these two bones here are femurs from moas, which were very large birds. Hummingbirds are very small. They weigh between about 3 and 8 grams. A penny weighs about 2 and 1/2 grams. So a single penny is the weight of a single, small hummingbird. And if I had three pennies, that’s about the weight of a large hummingbird. So they’re very small, and they’re very delicate. The moa, on the other hand, was a very large bird. The largest of them were about 10 feet tall and weighed about 500 pounds. Moas are now extinct. They were similar to ratites like ostriches or emus. So they walked on their hind legs. They were flightless birds. And you can see one of the striking differences between the hummingbird femur and the moa femur is that the length of the femur relative to the diameter is much larger in the hummingbird femur than in the moa. And this is also the result of a scaling law. [Force per unit area] When you think of the amount of force that the femur or any bone can withstand, what we’re interested in is the force per unit area, a little like we were interested in the woodpecker with the impact on the woodpecker brain. The force is the weight of the bird, which varies as the length cubed. The area is the cross-sectional area of the bone, which carries the weight. The area varies with the diameter of the bone squared. So the force over the area varies as the length of the bird cubed over the diameter of the bone squared. And this can’t exceed the strength of the bone, which is roughly constant for different species. Rearranging, the length has to vary as the diameter raised to the 2/3 power. This scaling law applies to other types of bones too. Here we have the dimensions of the humeri from antelopes ranging from the three kilogram [INAUDIBLE] to a 750 kilogram Cape Buffalo. length link varies with the diameter raised to the 2/3 power again. Scaling laws govern many phenomena in nature. For instance, there’s a scaling relationship between the speed of flight and the weight of a flying object. And this scaling law governs everything from the tiniest little gnat to large birds like swans and even up to the largest planes like Boeing 747s. Scaling laws are universal, and they describe many, many processes in life. 

Segment 8: Hall’s Pond & Bird Conservation

[MUSIC PLAYING] [SCRIBBLING] [INSECTS CHIRPING] We’re at Hall’s Pond in Brookline, Massachusetts. [MUSIC PLAYING] Hall’s Pond is named for Minna Hall, one of the founding mothers of the Massachusetts Audubon Society. [Lorna Gibson (Professor of Materials Science, MIT)] Minna Hall lived near this pond, and she loved it and often came here to go birdwatching. [MUSIC CONTINUES] Minna Hall, with her cousin, Harriet Hemenway, founded Massachusetts Audubon Society in 1896. Harriet had read an article about the hunting of birds, especially birds with very special feathers, for the women’s hat trade. It was very fashionable for high society women at that time to wear feathers in their hats. And she was appalled at the decimation of the birds for this. It’s estimated that in one year alone, something like 5 million birds were killed for the hat trade. So Harriet Hemenway and her cousin Minna Hall decided they were going to do something about this. And what they did was they contacted high society Boston Brahmin women. And they had them over to their houses for tea. And they persuaded them to stop wearing hats with feathers in them. And in the end, they got about 900 women to sign up for the Massachusetts Audubon Society to try to campaign against using birds feathers in women’s hats. They were very successful at this. After 1896, when they founded the Mass. Audubon Society, a number of other state Audubon societies were founded. And these groups acted together to lobby Congress to pass legislation to protect birds. And in 1918, Congress passed the Migratory Bird Treaty Act. And this was the first act to really protect birds. And it was one of the first conservation acts in the United States. Minna Hall lived in this neighborhood. She lived on Ivy Street, just one street over from the pond. [BIRD CALLING] And she used to come here even up to her 90s to enjoy the marsh and the peace and quiet and the birds and the pond. And it’s really lovely that we can still come here today to enjoy the same scene. [MUSIC PLAYING] [SCRIBBLING]

End of transcript

When you hear that distinctive knocking in the woods, you know a woodpecker is nearby. Woodpeckers peck at trees with impressive force. This behavior helps woodpeckers chisel away at tree trunks to unearth tasty insects or sap. Some woodpeckers even use their pecking skills to jam acorns into tree trunks for safekeeping. Rhythmic pecking also helps many woodpeckers keep in touch and coordinate with mates.

But how do they avoid brain injury? Join Professor Lorna Gibson of MIT on her mission to find out. This 8 part series, co-produced by Caitlin Stier, Lorna Gibson and Douglass McLean, is from the Massive Open Online Course (MOOC) MITx 3.054x.

For more on the topic: Gibson, L.J., Woodpecker Pecking: How Woodpeckers Avoid Brain Injury, Journal of Zoology, 2006.