Tag Archives: predation

Staying airborne: How bird wings are built for aerodynamic and efficient flight

Flight is a concept that has, until relatively recently in history, eluded humanity. However, birds have been successfully flying for approximately 130 million years, proving themselves to be a physical marvel of the natural world. And while our means of flight have historically been crude in design and performance, nature provides an elegant, efficient solution to get creatures off of the ground. Rüppell’s griffon vultures have been recorded flying as high as 37,000 ft, while some species of shorebirds have been recorded flying as far as from Alaska to New Zealand over eight days without stopping. But how exactly do birds seem to effortlessly overcome gravity so effectively? And perhaps more importantly, how might we apply these answers to improve manmade aircraft?

Morphology

Obviously, the exact aerodynamics and physical characteristics of birds will vary from species to species, but there are still underlying similarities that enable birds to fly. A bird’s wing consists of a shoulder, elbow, and wrist joint which establish the wing’s basic shape and allow a range of motion. Covering the wing are structures called primary, secondary, and coverts, which are all groups of feathers that provide lift and stabilize flight. Feathers consist of flexible fibers attached to a center shaft, called the rachis. Overtime, the rachis will become damaged from fatigue and large instances of stress. As a result, birds will molt and regrow their feathers on a regular basis. 

A diagram of the structure of a bird wing
Picture by marcosbseguren on Wikimedia Commons

Generally, a bird’s body will be adapted to either gliding flight, in which the wings flap very infrequently, or active flight, in which the wings flap nearly constantly. For gliding birds, such as the ocean dwelling albatross, the wings will extend far away from the body, and prioritize both wing and feather surface area over flexibility. Additionally, these wings will have a thick leading edge, and will be much straighter. However for fast, agile birds, such as falcons, the opposite is true. Consequently, agility is sacrificed for energy efficiency. In both cases, the rachis will change shape and rigidity, becoming larger and stiffer for gliding flight and smaller and more flexible for agile flight. 

Aerodynamics

One of the most unique aerodynamic characteristics of birds is that nearly all of their lift and thrust is exclusively generated by their wings, as opposed to aircraft that implement both wings and engines. This provides, among other things, near instantaneous control of both flight direction and speed. In other words, this gives birds an advantage when hunting, escaping from predators, and maneuvering through a landscape. 

To aid in the generation of thrust and lift during flight, birds will change their wing shape through a process called active morphing. During flight, the wing will be bent inwards and twisted up during the upstroke, and extended and straightened during the downstroke. As a result, this minimizes drag while maximizing thrust and, consequently, energy efficiency. This can aid in anything from traveling farther distances to hunting prey.

An osprey folding its wings in while catching a fish
Photo by Paul VanDerWerf on Wikimedia Commons

Applications

Initially, these principles may seem difficult to realistically utilize in aircraft. After all, we are limited by the materials available and the size that aircraft must reach. However, small steps could be taken to improve the energy efficiency and responsiveness of aircraft. For example, wing shape, material flexibility, surface finish, and moving joints could all be explored. In fact, research at MIT is currently being conducted on flexible wings made of scale-like modular structures. If experiments like this are successful, it could show that aircraft designs inspired by nature may be the future of the world of aeronautics.

Do Hammer-Shaped Heads Help Sharks Swim?

With their sandpaper skin, cartilage skeleton, electroreceptive sensors, and rows of dangerous teeth, sharks fascinate many people. However, even within this distinctive group the hammerhead sharks that make up the Sphyrnidae family have attracted a special attention due to the unusual shapes of their namesake heads, called cephalofoils. Several evolutionary benefits of the cephalofoil have been proposed by researchers. The wide hammer-shaped head may allow the shark to house more sensory receptors in its snout, to bludgeon prey, and to move and maneuver through the water more easily. Here we will address the question posed by the third theory: Does the cephalofoil found on hammerhead sharks provide an advantage in moving and maneuvering underwater?

Great Hammerhead Shark
Image of “Great Hammerhead Shark” by Wendell Reed showing a close-up of the cephalofoil. https://search.creativecommons.org/photos/3fa18a9b-9085-4867-93e1-15a88b01389b

Many advancements in the aviation and nautical industries have been developed from the study of sea creatures. There is certainly some potential that research into the mobility of sharks could someday be used as inspiration to advance locomotion technologies. In addition, a deeper understanding of the physiology and behavior of hammerhead sharks could help us to better preserve their habitat and species from endangerment – a crisis which some of them are already facing.

The theory that the cephalofoil provides advantages in forward swimming to hammerhead sharks relies on it supplying some hydrodynamic lift similar to the wing of an aircraft. Aircraft wings provide lift partly by creating a pressure difference between the top and bottom wing surfaces. An area of higher pressure on the bottom surface of a wing will generate upward lift. One study by Matthew Gaylord, Eric Blades, and Glenn Parsons applied a derivation of the Navier-Stokes equations – a set of partial differential equations for analyzing fluid flow – to water flow around digitized models of the heads of the eight most common species of hammerhead. It was found that in level, forward swimming there was some pressure differential that developed between the dorsal (top) and ventral (bottom) surfaces of the cephalofoil, but for each species it was very small and often in the direction to produce negative lift. The drag coefficients of the cephalofoil of each hammerhead species were then calculated and shown to increase as the size of the cephalofoil increased. The drag created by a cephalofoil was always much greater than the drags caused by the heads of a control group of non-hammerhead Carcharhinidae sharks.

Images of the pressure contours at zero angle of attack on the dorsal (top image) and ventral (bottom image) sides of the cephalofoil. The eight leftmost sharks are the hammerheads. Taken from Gaylord, Blades, Parsons.

However, the same study showed that the pressure difference between either surface of the cephalofoil did significantly increase in some species if the shark raised or lowered its head. This extra hydrodynamic force caused by the pressure differential at nonzero angles of attack would help the shark to turn its head up or down very quickly. This more explosive maneuverability was particularly present in the hammerheads that commonly feed on fish and less present in the species that feed predominantly on slower bottom dwellers. Another study by Stephen Kajiura, Jesica Forni, and Adam Summers theorized that the unbalanced, front-heavy cephalofoil may provide extra stability during tight turns by preventing banking. The unbalanced head would create a moment – or torque – to counteract the force from the tail that causes most sharks to roll into their turns. Not banking around turns could be important to some hammerhead sharks that often swim so close to the seafloor that banking into a turn could cause their head or fins to bump into the floor. It is likely that hammerhead sharks evolved the cephalofoil at least in part to provide more explosive and stable maneuvering.

Featured image “Hammerhead Shark” by bocagrandelasvegas.

Swimming Fast and Slow: What We Know About the Sailfish’s Iconic Fin

Indo-Pacific Sailfish
Indo-Pacific Sailfish. Credit: D. Corson/Shostal Associates

Sailfish, or Istiophorus platypterus, are one of the most recognizable fishes in the ocean due to their large sail-like dorsal fin. But, did you know that they are also iconic because they are one of the fastest swimmers in the ocean? Sailfish top speeds have been recorded to be up to 30 m/s, which roughly translates to 67 miles per hour. When researchers examined the sailfish swimming out in the ocean, they discovered that they have the unique ability to retract and deploy their sail and other fins. Furthermore, they saw that when swimming at top speeds, swordfish retract their sail, and when the fish are hunting prey, they deploy it. How does retracting the sail help them swim fast, and how does deploying the sail help them hunt? 

Sailfish swimming with both the sail retracted (0:24) and extended (0:40). Disregard the silliness of the video. Credit: Pew

Speeding Up

When it comes to swimming fast, the answer may have to do with drag. Due to their hydrodynamic characteristics, sailfish have a very low drag coefficient, which is the quantity used to describe the resistance of an object moving through fluid. Sailfish have a drag coefficient of 0.0075, which is similar to smaller fish such as pike, dogfish, and small trout. Additionally, due to their size, sailfish are able a generate much more force with each swimming motion than their smaller peers. The combination of these two factors allows them to move at such high speeds. The thing that truly sets sailfish apart, though, is this ability to retract their sail and pectoral fins. Studies show that when the sail and other fins were retracted, sailfish are able to reduce their drag by about 18%. With less drag to worry about, the fish can be more efficient in generating thrust from its swimming motion, which allows it to speed ahead of the competition. To get a sense of just how fast these fish move, watch the end of the video above, where they only get up to 40 mph!

Slowing Down

Sailfish usually feed on smaller fish that swim in schools, like sardines. These small fish “exhibit higher performance than large fish in unsteady swimming,” which is to say making quick lateral movements. So, if a sardine senses a sailfish coming at it at full speed, it will likely have no problems avoiding it. With the sail dorsal-fin raised, the increase in drag slows the fish down dramatically and steadies its swimming path.

In the figure below, sub-figures C and D show how much the tail oscillates (in grey) and the direction in which the fish is moving (in black) when the sail is retracted and deployed, respectively. Zero on the Y-axis represents the tail being directly in line with the rest of the body and the fish is moving straight forward. While the sail is up, the tail moves far less aggressively and the fish more consistently moves in a straight line.

Side by Side comparison of Sailfish swimming with and without sail
Affect of Sail on stability while swimming. Credit: Stefano Marras

This allows the sailfish to become more controlled in its movements and help pursue a sardine performing evasive maneuvers. When it gets within striking distance, the sail helps counterbalance the fish’s swift lunge at its prey, which provides much-needed accuracy. Biologists have also theorized that the large size of the sail helps herd the school closer together, but no official research has been done on this.

Take-Aways

The sailfish’s ability to retract and deploy its sail dorsal-fin gives it unique advantages in both long-distance swimming and hunting. Minimizing drag is one of the most important concepts for travel through any fluid (air or water), so understanding how the sailfish is able to reduce drag could provide a new perspective on things. The sail’s ability to provide stability at lower speeds has potential use in any sort of water travel, whether it be with cargo ships (like the Evergreen in the Suez), boats, or submarines.  

If you are interested in another unique feature of the Sailfish, its bill has been a popular topic as of late, especially with its feeding habits. The bill is often used like a sword, either spearing or stunning prey, and also has been thought to have helpful drag-reducing qualities. Click here for more information.

Sources

Woong Sagong, “Hydrodynamic Characteristics of the Sailfish (Istiophorus platypterus) and Swordfish (Xiphias gladius) in Gliding Postures at Their Cruise Speeds,” Plos One, https://doi.org/10.1371/journal.pone.0081323

Stefano Marras, “Not So Fast: Swimming Behavior of Sailfish during Predator–Prey Interactions using High-Speed Video and Accelerometry,” Integrative and Comparative Biology,” https://doi.org/10.1093/icb/icv017

P. Domenici, “How sailfish use their bills to capture schooling prey,” The Royal Society Publishing, https://doi.org/10.1098/rspb.2014.0444


Down to the Bear Bones: How Polar Bears evolved from Grizzlies to hunt in the Arctic

Katmai National Park in Alaska holds an annual “Fat Bear Week”, in which Twitter followers are asked to vote for the fattest bear in the park. This year’s winner was Holly, somewhere in the range of 500 to 700 lbs. That’s a big bear. However, in 1960, a male polar bear in Kotzebue Sound, Alaska, weighed in at 2,209 lbs. In fact, on average, polar bears weight up to 60% more than Grizzly bears, their closest animal relative. 

A very fat grizzly bear standing on rocks.
Holly, aka Bear 435, the 2019 winner of the Fat Bear Contest. From Katmai National Park via Twitter.

So just how did Polar Bears get so big? Well, as anyone in the Midwest knows, a harsh winter requires a good winter coat. The advantage of thick skin and fur, as well as a higher capacity to put on weight made heavier polar bears more adept to survive. However, bigger bears that could survive the cold were more likely to fall through the ice, so these adaptations required better foot mechanics.

Consequently, polar bears developed a distinctive gait. A rotary gait is a “double suspension” gait, meaning the animal bounces both off the hind limbs and then the fore limbs . This is contrasted from the grizzly bear’s transverse gallop, which involves only one “bounce,” — this loads each limb for a longer time and more vertically. The rotary gait improves stability, giving the polar bear the ability to travel quickly and smoothly on icy surfaces. 

A series of drawings depicting the gait of a galloping polar bear.
A series of drawings depicting the gait of a polar bear. Modified from S. Renous, J.P. Gasc, and A. Abourachid, Netherlands Journal of Zoology (1998).

Another significant difference between the species are their skulls, which, while similar in size, vary greatly in bite force and bone strength. The polar bear has a stronger bite, but a weaker skull. Polar bears are one of the most rapid instances of evolution in surviving species of animals, having evolved from the grizzly bear within the last five hundred thousand years. So why are their skulls weaker if their bite is stronger? 

Simply put: seals are easy to chew. Grizzlies are omnivores, as most bear species. Their diet subsists of salmon, elk, and small game, but includes a hefty amount of vegetation. Polar Bears, in the ice and cold, were forced to eat seals (as well as penguins, fish, even belugas). Seals are largely blubber, providing the caloric intake necessary to sustain these large beasts, but offering little resistance in the chewing process. 

Two line drawings of skulls, one of a polar bear and a grizzly bear
Skulls of the polar (left) and grizzly bear (right). Modified from P. Christiansen, Journal of Zoology (2006).

The polar bear’s skull morphed quickly, elongating to allow it to hunt for seals and fish through small holes in the ice. This weakened and lowered the density of the skull; however, because the seal-heavy diet required less effort to chew than vegetation, there was no selective advantage to a skull reinforcing. So, with a more efficient gait and a stronger bite, the polar bear developed into a killing machine in the icy north.

Interested in more of the polar bear’s hunt? Learn about how they can swim for hundreds of miles, or to see these arctic advantages in action, check out this video of a polar bear hunting a seal.

Fish in Flight: The Science Behind Great White Breach Attacks on Cape Fur Seals

Great white shark employs vertical attack on prey decoy
Great white shark employs vertical attack on prey decoy – from Sharkcrew via Wikipedia Commons

If you’ve ever turned on Discovery channel during Shark Week, then you’ve probably seen the iconic footage of a 2.5-ton great white shark leaping out of the water to catch its next meal.  If you’re weird like me and you’ve ever tried to mimic one of these epic breaches in a backyard pool, then you realize just how difficult it is to generate enough momentum to jump even partway out of the water and therefore have a real appreciation for what it takes to pull off this incredible feat.

Great white breaks the ocean surface
Great white breaks the ocean surface – from Alex Steyn via Unsplash

So if a breach attack is so difficult to pull off, how are great white sharks able do it, and why do they do it?  As per usual, some basic physics can help us answer both these questions.

 

Great white shark mid-breach
Great white shark mid-breach – from Alex Steyn via Unsplash

According to a 2011 paper by Martin and Hammerschlag, who spent 13 years studying great white predation in South Africa, breach attacks allow great whites to play to their strengths and maximize stealth.

Millennia of evolution have left great whites with long bodies great for straight-line speed (can reach speeds  >11m/s) but not so great for agility. Additionally, roughly 95% of a great white’s muscle is white muscle, which allows for rapid contraction (e.g. speed bursts) but also results in poor endurance.  Considering these aspects of their physiological makeup, it’s in a great white’s best interest to attack swiftly, avoiding prolonged chases.  Martin and Hammerschlag report that the majority of great white attacks on seals are over within 2 minutes and that the longer an attack drags on, the less likely it is to be successful.

Great white shark chases decoy prey from behind
Great white shark chases decoy prey from behind – from Sharkcrew via Wikipedia Commons

As great whites are less agile than seals, maximizing stealth and minimizing the time seals have to react is imperative.  Having evolved to have a dark grey dorsal (top) surface, great whites are hard to distinguish from the coral on the ocean floor when viewed from above (seal’s perspective).  Additionally, since very little of the light entering the water is reflected back towards the surface, it is estimated that under even the best lighting conditions, a seal could only reliably distinguish a shark a maximum distance of roughly 5m below it, which explains why great whites attack from below rather than behind. Great whites need about 4m to reach top speed, so due to this acceleration distance and seal vision, Martin and Hammerschlag report that great white attacks generally start between 7m and 31m below the ocean surface, with the majority staring closer to 30m.  Looking at data for great white breach attacks ranging from vertical to 45 degree ascents, Martin and Hammerschlag estimate that it typically takes a shark between 2 and 2.5 seconds to go from initial acceleration to surface breach, and that when considering shark speed and average visibility conditions, a seal generally has only about 0.1 seconds to react if it spots the shark before contact is made.  Ultimately, due to the advantages it gives them, great whites are successful in over half their breach attacks when lighting conditions are ideal.

Schematic of geometry and optics of great white shark attacks on cape fur seals from Martin and Hammerschlag - not to scale
Schematic of geometry and optics of great white shark attacks on cape fur seals from Martin and Hammerschlag – not to scale

 

Sources & Further Reading:

Fallows, Chris & Aidan Martin, R & Hammerschlag, Neil. (2012). Predator-Prey Interactions between White Sharks (Carcharodon carcharias) and Cape Fur Seals (Arctocephalus pusillus pusillus) at Seal Island, South Africa and Comparisons with Patterns Observed at Other Sites

Martin, R. Aidan, and Neil Hammerschlag. “Marine Biology Research.” Marine Biology Research, vol. 8, no. 1, 30 Nov. 2011, pp. 90–94., doi:10.1080/17451000.2011.614255.

Egdall, Mark. “New Research Reveals Physics Behind Great White Shark Attacks.” Decoded Science, Decoded Science, 10 Dec. 2011, www.decodedscience.org/new-research-reveals-physics-behind-great-white-shark-attacks/7497.

Sloat, Sarah. “Shark Week: Here Is the Wild Physics of a Great White Leap.” Inverse, Inverse, 25 July 2018, www.inverse.com/article/47437-shark-week-great-white-jumps.

Madrigal, Alexis C. “The Physics of Great White Sharks Leaping Out of the Water to Catch Seals.” The Atlantic, The Atlantic Monthly Group, 9 Dec. 2011, www.theatlantic.com/technology/archive/2011/12/the-physics-of-great-white-sharks-leaping-out-of-the-water-to-catch-seals/249799/.

 

Archerfish: Nature’s Master Marksmen

Picture of an archerfish swimming
Photo by Chrumps on Wikipedia

The name archerfish refers to seven species of freshwater fish that are all members of the Toxotes genus. These fish derive their name from their ability to hunt land-based creatures, ranging from insects to small lizards, using jets of water shot from their mouth with remarkable accuracy. They only grow to a maximum of a foot long, but they’ve been recorded in the wild propelling their water jets distances of up to two meters. A recent study in the Journal of Experimental Biology was conducted by Stephan Schuster to investigate the mechanics behind their unorthodox hunting technique.

Archerfish Learning to Aim

As it turns out, the archerfish has many circumstances working against it. Not only must they account for gravity when hunting, but because their eyes remain underwater during the attack, they must also adjust for the effects of the light refracting through the water. Refraction could cause the fish’s perceived angle of the prey to be off by up to 25˚ from the actual angle. They compensate for these factors by adjusting their spitting angle prior to releasing the jet. The ability to compensate for various environmental factors seems to be a skill developed through experimentation and practice. Archerfish will indiscriminately fire off shots at objects that are new to their habitat. They then judge whether they should continue to attack those objects in the future by remembering if a successful hit rewarded them with food. This was tested in the lab and shown that when the fish wasn’t rewarded, it would ignore the target in the future.

A diagram showing archerfish potential targets. Once the archerfish was only getting consistently rewarded for a single kind of target, insects, that's the only target it would shoot at
Modified from Schuster, Journal of Experimental Biology

In the past, studies were conducted that underestimated the accuracy of archerfish because of the variation in test specimens. Studies now show that a well-practiced archerfish can achieve a 100% hit rate on targets within distance of 65 cm. Accuracy will begin to drop off with increased height and distances beyond 65 cm.

Hunting Mechanics

While most animals who utilize their ability to spit depend on the spit’s glue-like properties or venom, archerfish hunting relies on the force at which the jet contacts the target. Depending on the size of their prey, archerfish can modulate their jets between the range of 40-500 mN. To exert strong enough forces to dislodge targets from leaves and branches, the fish expelling the jet acts like a vessel under pressure. The water starts out slow and increases in velocity as the fish’s mouth stays open, like turning a valve. This means that the jet’s tail catches up to its tip as the water travels along its trajectory, causing a more focused impact. As the water travels, the stream will break into individual blobs of water due to Plateau-Rayleigh instability, creating multiple jets. Since water ejected later is moving faster, these jets will join over time and create a single, larger jet which will impact the target.

Race to the Food

While utilizing their ability to create forceful jets of water can be an effective method of hunting, it does come with one disadvantage. Archerfish have evolved to react to plummeting objects much quicker than other fish, regardless of what caused the object to fall. While this means that an archerfish having its prey stolen by another type of fish is very rare, its prey will often be stolen by other archerfish which happen to be closer to where the prey impacts the water.

Additional information on archerfish hunting can be found at wired.com and in the video from below. There are also some beautiful photos of archerfish in action at stevegettle.com.

 

How the Mantis Shrimp Packs its Punch

The mantis shrimp, a six inch long crustacean residing in the warm waters of the Pacific and Indian oceans, may look harmless with its rainbow shell, but it is able punch its prey with the same acceleration as a 0.22 caliber bullet, providing around 1500 newtons of force with each blow. The mantis shrimp can shatter the glass of aquariums, catch and kill their prey with minimal effort, and punches so fast that cavitation bubbles form behind their hammer-like clubs. Cavitation bubbles are pockets of low pressure air that form when a liquid is moved faster than it can react, and collapse with tremendous heat and force—enough to crack the shells of other crustaceans or even a glass bottle.

Rainbow colored mantis shrimp on ocean floor, with two white clubs visible.
Mantis shrimp on the ocean floor, with its two white dactyl clubs visible. (Source: Wikimedia Commons)

The mantis shrimp gets the power for its punches from elastic energy storage—that is, it stores energy in its muscles as they are compressed when cocking its dactyl club back into the locked position. A four bar mechanism within the club and body of the shrimp is used to hold the club back in place until it is ready to punch and a latch is released, transferring the stored energy into rapid motion of the club.

Diagram of how the mantis shrimp locks his club back, storing energy, then releases it and swings it forward to attack prey.
Mechanics of the mantis shrimp’s club swing. (Source: Maryam Tadayon/Nanyang Technological University)

The material composition of the mantis shrimp’s shell enables it to hit so hard without damaging itself. The two layers of the shell on the club allow it to withstand large stresses in both tension and compression, which is uncommon of most shells since they are ceramic materials, which are very brittle. Both layers are made out of hydroxyapatite (HA), a highly crystalline and hard calcium phosphate that is found in human bone, and chitin, a hard biopolymer fiber found in outer shells of most crustaceans and insects. Differing structures and amounts of each material provide different functions in the layers of shell. The outer layer is arranged in a wave-like pattern of HA and chitin that effectively redistributes stresses evenly over the whole surface, greatly decreasing the chance of cracking. The inner layer is less stiff than the outer layer, and is composed of HA and chitin in helical patterns, which allows for small cracks to form in a spiral shape instead of cracking straight through the shell. This greatly increases the lifetime of the club’s shell.

Ceramics are typically not used in robotics due to their brittle nature, but, if modeled after the shell of the mantis shrimp, this could be overcome and provide a stiffer and lighter material to be used in the robotics industry. Additionally, the material composition of their clubs could be used to develop resilient protective body armor for the military and police forces, or helmets for bike riders or football players.

Additional information on the mantis shrimp can be found at Wired.com and theoatmeal.com, and in the following video.