Tag Archives: predation

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.