Pelicans, like many sea birds, are aerial divers. They spot their prey from high above, bank, and dive into the water to catch the fish. Although they hit the water at high speeds, pelican diving techniques differ somewhat from plunge divers like gannets or boobies. Pelicans are only aiming for a shallow dive, so they have features – like their expandable neck pouch – that help them decelerate quickly instead of taking a full-body plunge. The goal is to increase drag after the head enters, slowing everything down. That can add more stress to the bird’s neck – the rest of the body is still moving quickly even after the head begins to slow. To counter this compression, the birds must have strong neck muscles to stabilize their spines during the impact process. (Video and image credit: Deep Look)
One of my favorite things to do while we were sailing along the Galapagos was watching the blue-footed boobies hunt. Like the gannets shown above, boobies are plunge divers. They circle overhead until they spot their prey, then they fold their wings and dive headfirst into the water, impacting at speeds of more than 20 m/s (~45 mph). It’s absolutely incredible to watch. The physics involved are impressive, too, especially considering how badly a human would be injured diving at their speeds!
Fluid dynamically speaking, there are three important phases to the birds’ entry. The first is the impact phase, which lasts from initial contact until the bird’s head is underwater. In the second phase, an air cavity forms behind the head and around the neck as it enters the water. Finally, when the chest – the widest point of the bird – hits the water, the bird reaches the submerged phase.
Mechanically, the most interesting part is the air cavity phase. During this time, the bird’s head is slowing down due to high hydrodynamic drag from the water, but the rest of the bird is still moving fast. That means the bird’s slender neck experiences strong compressive forces, which would tend to make it buckle. Researchers at Virginia Techexamined this very problem and found that the birds’ sizing – its head shape, neck length, and so forth – combined with their typical diving speeds kept these birds well away from the conditions that would cause their necks to buckle. With the added stabilization from the birds’ neck muscles, they estimated that gannets and other plunge divers might be able to safely dive at speeds twice what would kill a human! Check out the BBC video below to see high-speed footage of gannets diving. (Image credits: G. Lecoeur; B. Chang et al.; research credits: B. Chang et al., pdf; video credit: BBC)
Tomorrow will be the final day of Galapagos Week. Catch up on previous posts here.
One of the most unique inhabitants of the Galapagos Islands is the marine iguana. These reptiles live in colonies of thousands and subsist entirely on marine algae. Smaller iguanas are intertidal feeders, grazing on green and red algae when it is exposed near low tide. But the largest iguanas feed near midday by swimming out and diving to feed on richer pastures.
The iguanas are surprisingly good swimmers, even though marine iguanas exhibit little extra specialization for it compared to other iguana species. They swim both at the surface and underwater with an undulatory motion driven by their tails. The iguana also streamlines its body somewhat by tucking its legs along its sides. Although the marine iguana is a much slower and less efficient swimmer than a bony fish of equal size, swimming is still a good choice for getting around. The marine iguana expends only 75% as much energy per distance swimming as it does walking. The big challenge is staying warm in the cold Galapagos waters. Small iguanas are both less efficient swimmers and lose body heat faster. This is why you’ll only see the biggest iguanas feeding underwater. (Image credits: N. Sharp; research credit: K. Trillmich and F. Trillmich; J. Videler and B. Nolet; G. Bartholomew)
This is the first post of Galapagos Week here on FYFD. Check back every day for new Galapagos-themed posts!
Water striders are masters of life at the interface of water and air. Their spindly legs are skinnier than the capillary length of water, meaning that, at their size, surface tension is strong enough to overcome gravitational effects. Thus, their feet leave dimples on the interface, but the water itself holds them up. To keep from getting accidentally drenched (and thus weighed down), the striders are covered in tiny hairs that trap a layer of air that makes them hydrophobic or water-repellent. To get around, these masters of the interface use their middle legs in a manner similar to oars. They push against the dimple around their legs, which generates vortices under the surface and helps propel them. Even more impressive, the water strider can jump off the surface, a feat that requires remarkable adaptation in order to maximize the jump without breaking surface tension. (Video credit: Deep Look)
Earlier this summer, I looked up on a bright, sunny day and saw a quartet of black and white figures soaring overhead. Initially, I thought it might be a formation of kites or unmanned aerial vehicles (UAVs) because I saw no flapping as the group wheeled about. With the help of the Cornell Lab of Ornithology’s awesome Merlin app, I was able to identify the soarers as American white pelicans – not a species I’d expected to find flying along the Front Range of the Rocky Mountains! (Turns out, they breed on lakes around here.)
The reason I saw so little flapping is that the birds were riding thermals. As the sun heats the ground, air near the surface warms up and begins to rise due to its buoyancy. Pelicans interested in flying between breeding and foraging grounds will start testing the thermals early in the day, as soon as they begin to form. As the heating continues, the intensity of thermals strengthens and they extend higher into the atmosphere. This is where the birds can really excel at using atmospheric energy for their flight. Pelicans will circle within a thermal until they reach roughly the middle of its height. Then they will glide, gradually losing altitude until they reach another thermal where they can climb without expending their own energy. With a 2.7 meter wingspan and a relatively low drag coefficient, the pelicans can glide and soar remarkably well. Researchers have even suggested using them as a sort of biological UAV for studying atmospheric dynamics! (Image credits: D. Henise, M. Stratmoen; research credit: H. Shannon et al., pdfs – 1, 2)
Humans are hairy on the inside. Not in the way that we are on the outside, but in the sense that many interior surfaces of our bodies are covered in small, flexible, hair-like protrusions like the papillae on our tongues or the cilia in our intestines. Many of these fibers are immersed in fluids, raising the question of how the flow and the hairs interact. An elastic fiber immersed in a flow will bend in the direction of the flow (bottom); this helps reduce the drag and widens the channel flow goes through compared to a stiff, upright fiber.
But what happens when the fibers are all mounted at an angle? In this case, researchers found an asymmetric response. If flow moves in the direction of the fibers’ bend, the hairs don’t impend the flow at all. If flow moves against that direction, however, the hairs start to stand upright, blocking the flow channel and increasing the drag. The researchers suggest this sort of mechanism could be use in micro-hydraulic devices in the same way as a diode in a circuit – allowing flow in only one direction. For another biological example of flow control, check out how a shark’s denticles can prevent flow separation. (Image credits: hairy surface – J. Alvarado et al., flow around a hair – J. Wexler et al.; research credit: J. Alvarado et al.)
Salps are small, jellyfish-like marine invertebrates that swim by ejecting a pulsatile jet. They are unusual creatures whose lives have two major stages: one in which salps swim individually and one in which they link together and swim in large chains. In the chain, salps don’t synchronize their jetting; each salp jets with its own phase and frequency. A new study suggests that, in spite of this lack of synchronicity, the salp chain’s swimming reduces the animals’ drag. There are several factors that contribute to this result. One is that drag is generally lower on a body moving at constant speed compared to one moving in bursts. When linked together and firing randomly, all the individual jets tend to average out into one continuous swimming speed. There’s even a benefit to being out of sync: previous work showed that synchronized jets lose some of their thrust when they are too close together. Salps avoid that loss by keeping to their own beat. (Image and research credit: K. Sutherland and D. Weihs, source; via Gizmodo)
Substances don’t have to be a liquid or a gas to behave like a fluid. Swarms of fire ants display viscoelastic properties, meaning they can act like both a liquid and a solid. Like a spring, a ball of fire ants is elastic, bouncing back after being squished (top image). But the group can also act like a viscous liquid. A ball of ants can flow and diffuse outward (middle image). The ants are excellent at linking with one another, which allows them to survive floods by forming rafts and to escape containers by building towers.
Researchers found the key characteristic is that ants will only maintain links with nearby ants as long as they themselves experience no more than 3 times their own weight in load. In practice, the ants can easily withstand 100 times that load without injury, but that lower threshold describes the transition point between ants as a solid and ants as a fluid. If an ant in a structure is loaded with more force, she’ll let go of her neighbors and start moving around.
When they’re linked, the fire ants are close enough together to be water-repellent. Even if an ant raft gets submerged (bottom image), the space between ants is small enough that water can’t get in and the air around them can’t get out. This coats the submerged ants in their own little bubble, which the ants use to breathe while they float out a flood. For more, check out the video below and the full (fun and readable!) research paper linked in the credits. (Video and image credits: Vox/Georgia Tech; research credit: S. Phonekeo et al., pdf; submitted by Joyce S., Rebecca S., and possibly others)
ETA: Updated after senoritafish rightfully pointed out that worker ants are females, not males.
The mantis shrimp is a tiny, clown-colored juggernaut of underwater physics. Some species have modified claws that serve as clubs for punching their prey, and the mantis shrimp swings that club fast – its acceleration is comparable to a bullet’s! Moving that quickly in water causes a drastic drop in local pressure, low enough to form a cavitation bubble. Such low-pressure bubbles themselves are not particularly dangerous, but their collapse is incredibly violent, especially near a solid surface, like the shell of the shrimp’s prey. Collapsing cavitation bubbles can send out shock waves, shatter glass, and even generate light. In the case of the mantis shrimp, it’s more than enough to stun, if not outright kill, its prey. (Video credit: Physics Girl)
Many fungi use coalescing water droplets to launch and spread their spores. The process is recreated in the laboratory in the animation above. Initially, there is a small spherical drop and a second, flattened drop stuck to the backside of the spore. In the animation, the large object on the right is actually both spore and droplet. The spore is spherical on one side and flattened on the other and starts out tipped up on its edge. When the spherical drop gets large enough to reach the flattened drop, they merge. This reduces the total surface area of the drop and thus releases some surface energy. It’s that surface energy that drives the spore’s jump. Even launching just a centimeter away from the host fungus is enough for a breeze to carry the spore further, allowing the fungus to reproduce. (Image and research credit: F. Liu et al., source; submitted by Kam-Yung Soh)
