In my last post, I wrote about how I can’t walk in salt marsh mud without getting stuck. Great blue herons, on the other hand, walk on salt marsh mud all the time without getting stuck.
Part of the reason great blue herons can walk in salt marshes and I can’t is because I may weigh around 40 times more than the average adult great blue heron. But part of the reason is also because their goofy looking feet are much better than my goofy looking feet for walking in mud.
It all has to do with pressure. “Pressure” is the force created whenever one thing presses on another thing. Any animal that is not currently swimming or flying or hanging out on a plant or sitting on your fence or living in the walls of your house (or whatever other exception you can think of) is putting pressure on the ground. Part of what is creating the pressure is the force of gravity pulling the weight of the animal down. Pressure can also be created by the force of their muscles. For example, if you stomp on the ground, you are putting more pressure on the ground than if you just stand there like a person pretending to be the Statue of Liberty in Times Square.
Pressure is not just created by force, though. It is also created by the amount of space the force is spread across (the name for that amount of space is “surface area”). If the person pretending to be the Statue of Liberty is standing on both of her feet, half the force of her weight is going into one foot and the other half is going into the other foot. If the person pretending to be the Statue of Liberty stands on one foot, the force of her body weight is all going through that one foot. She creates a lot more pressure standing on one foot than when she stands on two feet, because the force of gravity pulling down her body weight is going through half the surface area.
If you put enough pressure on something, like salt marsh mud, you can actually cut through it or squeeze it into a smaller space. The best way to cut through something is to apply force to it with an object that has very little surface area. There is a reason we don’t cut cheese with the flat side of a Frisbee (actually there may be more than one reason). We use a knife because the edge of it is very thin, meaning it has very little surface area. When you press down on a knife, the pressure from your weight and muscular activity is all going into one thin edge. That knife’s edge has so little surface area that you only have to put a little force on it to create enough pressure to cut the cheese. If you tried to cut the cheese by putting the same amount of force onto the flat end of a Frisbee, you might flatten the cheese a little, but you would not cut it. By spreading out the force over the larger surface area of the Frisbee, you reduce the pressure a lot, and there is no way you can cut the cheese with that little pressure.
So what does this all have to do with great blue heron feet? Great blue heron feet are adapted to apply very little pressure on anything the heron stands on.
Great blue herons have really long toes. They also have a small amount of webbing between some of their toes (“webbing” like a gull’s webbed feet and not like 1960s’ Spider-Man’s armpits).
Both the long toes and the webbing on the great blue heron’s feet do not really do anything except create more surface area, but this is an important adaptation for great blue herons because it makes their feet like full-time snowshoes. We wear snowshoes to expand the surface area of our feet when we walk on snow. More surface area reduces the amount of pressure we are putting on the snow. Less pressure means we are less likely to step on a snow bank and have our entire leg disappear into the snow bank and our boots fill with wet snow. Because a great blue heron's feet always have lots of surface area, they are able to put very little pressure on the ground as they walk across salt marsh mud.
So one thing you may be wondering is: “Kevin, don’t your giant feet have way more surface area than a great blue heron’s skinny feet?” Well, yes, but my feet are still not big enough to be adaptations for walking on salt marsh mud. Despite having more surface area, my feet create a lot more pressure than a great blue heron’s feet, because the force of gravity pulling my much more massive weight down is a lot higher than the force of a great blue heron’s 6-pound body. The combination of the force of my weight and the surface area of my feet easily produce enough pressure to cut through and squeeze down the mud and cause me to sink and sink until I’m stuck.
Great blue heron feet, on other hand, are adapted for walking on mud. They allow these birds to survive in environments like salt marshes, because if great blue heron couldn’t stand on mud without sinking, there is no way they could walk into the water to catch the fish they need to eat.
To learn more about great blue herons and salt marshes, read my book A Day in the Salt Marsh.
To learn more about pressure (well, about water pressure and it’s effect on deep sea animals), read my book A Day in the Deep.
PS. When I was stuck in salt marsh mud, one of the things that got me out was increased surface area. Before I was dragged across the mud by marine biologists pulling me with a rope, the marine biologists threw me a bucket. They then instructed me to put the bottom of the bucket on the mud and push down on the top of the bucket with my hands. Because the bottom of the bucket had more surface area than my hands did, I was able to push on the bucket without creating enough pressure to cause it to sink. I could then use the bucket as something to hold onto to pull myself out of the mud. Once I was no longer stuck, I laid down on the mud, because spreading my weight across the surface area of my entire body did not create enough pressure to cause me to sink back into the mud. Then the scientists threw me a rope and I flopped around like a fish out of water until I was able to grab hold of it. They pulled me into the water and I swam to the boat. This all happened on my first day working for the marine biology research center, so my first impression on my brand new coworkers and bosses was to show them what I look like when I'm stuck in mud.
Online Resources and References
BBC, Bitesize, "Pressure."
Cornell Lab of Ornithology, All About Birds, "Great Blue Heron."
Encyclopedia of Life, "Ardea herodias, Great Blue Heron."
National Geographic, "Adaptation."
NOAA National Ocean Service, "What is a salt marsh?"