The Physics of Diving Gannets, Bird Navigation, and Speedy Tiger Beetles

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

Jen-Luc Piquant was at the APS March Meeting in San Antonio, Texas this week, a longtime favorite conference, and often touted as the largest physics conference of the year, covering a diverse range of topics: biophysics, fluid mechanics, materials (exotic or otherwise), complex systems, quantum mechanics -- it's a treasure trove of cool cutting-edge physics. There was so much going on, we are only now getting around to writing about some of our favorite talks at the meeting. First up: a tantalizing trio of tidbits providing insight into the physics behind how the seabirds known as gannets survive high-speed deep dives; how the "internal compass" migratory birds use to navigate their journeys might work, and a proportional control law that could explain how the tiger beetle moves to hunt its prey.

Deep Diving. Virginia Tech's Sunny Jung spilled the beans on how he and his colleagues studied the physics of gannets diving into the sea to capture prey. Some may recognize Jung's name from prior work his lab has done on the physics of how dogs and cats drink water. A diving bird, from a physics standpoint, is essentially an elastic body impacting the water's surface, but the two mediums -- water and air -- are vastly different mediums in terms of density. And that means the birds must be able to tolerate very strong forces upon impact. "If you're moving from one medium to another but one is much denser, you're going to feel a huge force when you move from air to water or vice versa. There's a huge stress on the animal body," says Jung. This is especially true for gannets, who hit the water moving as fast as 55 MPH in search of underwater prey, often up to 60 times. They're like "torpedoes hitting the water," Jung said. Check it out:

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Professional divers who participate in the Red Bull high dive competition typically make dives from heights of around 50 meters, and Jung estimates the impact force of such dives at around 5000 Newtons. It's quite dangerous; even a professional diver can fracture a leg or a couple of thoracic vertebrae if they don't hit the water in perfect position. So how do the gannets manage to make high-speed dive after dive after dive without injury? Jung and his students collaborated with researchers from the Smithsonian's Natural History Museum to find out.

Jung et al. managed to snag a deceased gannet ("This gannet has ceased to be! It is an ex-gannet!" Jen-Luc chortles, always eager to sneak in a random Monty Python reference) for the first stage of their study, but the body was just too, shall we say, "floppy" to drop into tanks of water from a high platform. So they carefully arranged the gannet corpse in peak diving position and froze it -- manmade rigor mortis -- then dropped the frozen bird repeatedly into the tanks. Cameras captured the impact and mechanics of the bird's movement through the water.

This revealed three distinct phases of the dive. Phase 1 is the initial impact force on the head as the bird first hits the water's surface; this produces a slight compressive force on the animal's long neck as downward gravity pushes against the neck in one direction, and the upward impact force pushes in the other direction.

Phase 2 is when the head is fully immersed in the water but the body is still in air. At this point, the downward gravity pull becomes a drag force caused by the water, thereby producing a very strong compressive force on the neck. (An air cavity also forms around the bird's neck.)

Phase 3 is when the animal's entire body is underwater with steady drag on both the head and body, so once again there is very little compressive force. The danger zone from a diving standpoint is Phase 2.

Jung's team then made 3D printed models of gannets based on CT scans of the dead bird (courtesy of a local veterinary hospital) and repeated the experiment. They also made 3D printed models of what can only be described as "spherical gannets": reducing the biological reality to an elastic beam (neck) with a cone (head and beak) on the front end and a sphere (body) on the back end, and tried the experiment again, varying the length of the elastic beam, as well as impact speed, and the angle of impact with the water. Physicists are nothing if not thorough.

Either the model birds' necks stayed straight, or they buckled up upon impact, and those model birds with longer necks buckled more easily. Gannets have very long necks, amounting to more than half their body length, with a framework of 25 elongated slender bones. (In contrast, the human neck has seven disk-shaped bones.) Jung et al. found that the trick is to dive slowly enough to dive safely; there's a sweet spot for safety. If the birds dive faster than a certain speed, their necks buckle up.

Added protection comes from the shape of the bird's beak and how it is attached to the skull. "If the bird has a very pointy beak, it can reduce the force on the body," said Jung. Furthermore, the shape of the skull must transition smoothly into the beak for the bird to experience less force when diving for prey, rather than flattening out. Jung and his team are continuing to investigate the physics of diving gannets, with an eye towards using what they've learned to better understand the diving mechanics of human beings.

Animal Magnetism. The arctic tern will travel more than 40,000 miles to migrate from pole to pole and back again each year, according to Oxford University's Peter Hore. But how do they -- and other migratory birds who might make shorter though still impressive journeys -- navigate such tremendous differences when they can't just Google the directions like the rest of us?

One idea involves iron-infused nanoparticles, but Hore presented evidence for another hypothesis: birds and possibly other animals are equipped with a sensitive internal compass based on the quantum mechanical properties of electron spins contained in biological molecules known as cryptochromes -- a magnetically sensitive protein known to mediate circadian rhythms in plants and animals.

"We are interested in the possibility that migratory birds use a magnetically sensitive chemical reaction to detect the direction of the Earth's magnetic field," he said. That chemical reaction can be induced in cryptochrome by shining blue or green light onto the molecule. This serves as a trigger for the protein to produce pairs of radicals. And those radicals have electron spins that are highly sensitive to magnetic fields -- even very weak ones like the Earth's. "As we vary the strength of the magnetic field, we can alter the progress of these photochemical reactions inside the protein," said Hore.

This internal compass is so sensitive that even very subtle disruptions in the magnetic field can make it difficult, if not impossible, for the birds to navigate, as Hore discovered when he collaborated with Henrik Mouritsen on a behavioral experiment at the University of Oldenburg in Germany, placing the birds in funnel-shaped wooden huts surrounded by magnetic fields to test their navigational abilities with no visual cues available (eg the sun's position in the sky). But for some reason, in this urban setting, the birds couldn't navigate well.

Hore concluded that there was interference emanating from AM radio signals and electronic equipment in operation on campus. It was only when the researchers covered the huts in aluminum sheeting and electrically grounded them to block all the EM noise in the problematic frequency range other than the earth's static magnetic field, that the birds regained their navigational ability. "We would like to know how such extraordinarily weak radiofrequency fields could disrupt the function of an entire sensory system in a higher vertebrae," he said. "Our feeling is that this is likely to provide key insights into the mechanism, either of the magnetic compass sense or of some important process that interferes with the bird's orientation behavior."

Hore suggested that the EM noise is affecting the ability of cryptochrome to perform its function at a quantum level. When the radicals first form, they are entangled, so radical pairs in cryptochrome could have the unusual ability to preserve their quantum coherence for much longer than previously believed possible. "Physicists are excited by the idea that quantum coherence could not just occur in a living cell, but could also have been optimized by evolution," he said. "There's a possibility that lessons could be learned about how to preserve coherence for long periods of time." That's a big deal for quantum computing in particular, where decoherence is perennial challenge.

Beetle Mania. Finally, let us consider the question of how species with very small brains nonetheless are able to perform fairly complex tasks -- something that could help scientists design more effective simple machines capable of performing similar complex tasks. Robert Noest, a graduate student in Z. Jane Wang's lab at Cornell University, reported on his work investigating how the tiger beetle manages to accurately assess distance from its prey even when both predator and prey are scuttling about.

Wang's lab is known for investigating how insects fly; one intriguing question is, why they fly in certain directions. But it's a difficult thing to study with flight movement in 3D, so they opted to study tiger beetles scuttling along a 2D plane -- a standard "chasing problem."

The tiger beetle is a fascinating creature, an aggressive hunter that can run 5 MPH at top speed -- okay, maybe that doesn't sound too impressive, but think about how tiny the beetle is. This translates into being able to cover 120 times its body length per second, according to Cornell entomologist Cole Gilbert, who has collaborated with Wang's lab on this research. (A cheetah, in contrast, covers 13 times its body length per second.)

It's so fast that it blurs the beetle's vision -- think of trying to photograph any fast-moving object using a camera with woefully slow shutter speed -- giving the creature a bizarre herky-jerky gait when it hunts. "Their behavior is really interesting in that they run with little jerky, stop-and-go movements, and that's not the way most creatures move," Gilbert told the Cornell Sun last year. He teamed up with Wang's lab to figure out the physics behind this odd behavior.

Noest and his cohorts in Wang's lab examined high-speed digital video footage of a tiger beetle chasing after "prey" -- in this case, a small black bead dangled on a string serving as a dummy victim. Map out all those trajectories and you get what appears to be a tangled mess. Wang et al. found an underlying method to the seeming haphazard madness.

Specifically, the beetle first runs toward its prey head-on, before stopping to adjust its movement when the prey starts to "flee." According to Noest, the creature is constantly re-assessing the angle between its current trajectory and where it saw the prey about half a stride before and translating that sensory information into a sense of rotational velocity -- i.e., determining just how much and in what direction to turn to find its prey. "Even a small insect is able to tell distance," said Noest. It's a kind of optimal reorientation dance.

"The idea [of our research] is to find laws that animals use to intercept their prey," Wang told the Cornell Chronicle last year. "We do it, too [interception] -- when trying to catch a baseball, or when chasing someone. But since insects have a smaller number of neurons, their behaviors are more likely hardwired, which makes it possible for us to find and understand the rules they follow." For the tiger beetle, it seems to be the result of a proportional control law: "the angular position of prey, relative to the beetle's body axis, drives the beetle's angular velocity with a delay of 28 milliseconds" -- about half a beetle-sized stride.

One theory as to how the creature manages the feat is motion parallax, a common technique in used in astronomy, among other areas. The beetle's head is constantly swaying back and forth as it scuttles, which might enable the creature to focus on the prey from two different viewpoints and then determine the difference. "But stars don't move and the prey is running," Noest pointed out, so somehow the beetle would also have to account for that motion. The other, more likely theory, he said, relies on elevation angle in the field of vision. Picture a row of chairs; the further one will appear to be the highest in your visual field. There is some evidence that the tiger beetle shows a preference for prey at higher elevations.

These are just three examples of the kinds of lessons physicists can learn from studying biological systems. Mother Nature, after all, is pretty talented engineer, with millions of years of evolutionary trial-and-error to her credit.

References:

Engels, S. et al. (2014) "Anthropogenic electromagnetic noise disrupts magnetic compass orientation in a migratory bird," Nature 509: 353-356.