December 4, 2013
In 1917, British artist Norman Wilkinson experienced a eureka moment while serving in the Royal Naval Volunteer Reserve. Throughout the month of April, German U-boats had been mercilessly torpedoing British ships, sending around eight of those vessels per day into the watery abyss. Hiding a ship traveling on the open ocean from plain sight was impossible, Wilkinson knew, but a bit of artistic trickery may be able to muddle the Germans’ ability to accurately judge the exact location of that ship, he realized.
From that idea, Wilkinson devised a type of camouflage called “razzle dazzle” (its slightly more serious name is dazzle camouflage). The technique consists of squashing together contrasting geometric patterns, shapes and colors to create a pattern of optics that would confuse enemies by distorting the object’s dimensions and boundaries. All in all, more than 2,000 ships received such a makeover, although the scheme’s effectiveness seemed to produce mixed results.
By World War II, razzle dazzle had largely fallen out of favor, but as it turns out, this technique lives on in the natural world. High contrast patterns–nature’s equivalent to dazzle camouflage–are used by animals ranging from snakes to zebra to fish. Like those hidden World War I ships, many creatures seem to use dazzle patterns to conceal themselves from predators. Until now, however, researchers hadn’t considered the flipside of this relationship: could predators use razzle dazzle to sneak up on prey as they mounted an attack?
To investigate this possibility, biologist Roger Santer of Aberystwyth University in the U.K. turned to locusts. These insects are particularly well suited for vision studies due to something called a single lobula giant movement detector neuron, a unique cell that specializes in detecting looming objects (think of a car speeding towards you, or a hand reaching for your face). Researchers think this neuron works by measuring the shape and movement of patterns of light and dark across the eye. Whatever the mechanism, as looming objects approach a locust, its detector neuron fires away, alerting the insect to imminent potential danger and triggering it to flee.
To see how the locusts responded to dazzle camouflage, Santer created an array of visual patterns using a graphics software. He situated the locusts just in front of the computer monitor, and then projected a simulated approach of those objects from about 10 meters away to about 0.07 meters from the cowering insects. The objects varied in contrast: black, grey or white on a grey background. Around 20 locusts took part in the experiment, and Santer measured their cellular reactions to the various shapes through copper wires inserted into the locusts’ neck.
The locusts’ neurological responses to the looming objects depended on which patterns they saw, Santer reports in Biology Letters. Squares with both a darker-than-background top and bottom half elicited the strongest panic response, followed by squares with a dark upper half, but a bottom half that was the same color as the background. Squares that had an upper half that was dark but a bottom half that was bright (in other words, the razzle dazzle ones) produced a significantly weaker panic response, as did squares that were brighter than the background. Finally, squares that were the same color as the background produced no response at all.
These results are interesting in that they correlate with similar dazzle tests performed on humans, who also had trouble quickly registering dazzle patterns. However, at this point, whether or not locust predators actually use dazzle to catch their unsuspecting insect prey remains a matter of speculation. Though lab tests confirm this strategy might work, Santer did not investigate whether or not a dazzle dance of death is carried out in the real world.
Hypothetically speaking, dazzle camouflage, Santer concludes, would help a predator but would not be the most effective way of snagging a locust lunch. Instead, classic camouflage–blending in with the background rather than creating an optical illusion–seems to be the most effective means of tricking would-be prey. However, in the case that other selection pressures favor high-contrast patterns (such as if females of a predator species prefer bold stripes in males), Santer thinks that predators could indeed evolve to give ‘em the old razzle dazzle.
November 27, 2013
When we think of ants as builders, we normally imagine them digging intricate tunnel networks as part of underground colonies.
But David Hu, Nathan Mlot and a team of other researchers at Georgia Tech are studying a very different type of building behavior specific to one ant species: The ability of Solenopsis invicta to construct bridges, rafts and even temporary shelters using their own bodies as building material.
“Fire ants are capable of building what we call ‘self-aggregations,’” Hu says. “They can build little boats, little houses called bivouacs and even bridges to cross streams by being the building material, linking their bodies together and forming strong networks.”
The ants are now considered an invasive species in 25 states, Asia and Australia, but their unusual behavior is a survival strategy shaped by their native environment: a particular area of wetlands in western Brazil that are flooded frequently. “The ants live underground, but when it begins to flood, they have to gather the colony members, pull them out of the ground and build a floating raft,” Hu says.
When this raft hits land, the ants keep building. To cross small streams during their subsequent migration, they make living bridges that allow the entire colony to scramble to safety. Afterward, using their bodies, they construct a temporary aboveground encampment to provide shelter for the few days it takes to re-dig underground tunnels. All the while, the ants that form the temporary shelter are continuously moving, but still preserving the structure. “It’s really living architecture—it has well-constructed, organized tunnels, brooding rooms,” Hu says. At least for the ants in the inside, this provides protection against hostile weather or predators.
Hu, an engineer, is primarily interested in studying the swarming ants as a novel material with unprecedented characteristics. As part of his group’s recent research, presented yesterday at an annual meeting of the American Physical Society, he and colleagues considered the ants within the context of other “active materials”—substances that can respond to changing conditions, such as self-healing cements that can use the energy in sunlight to expand and fill their own fractures.
“We wanted to characterize what kind of material it is—is it a fluid, or is it a solid, and how does it respond to stress?” he says. “In nature, for instance, these rafts might float down a river and bump into rocks, or raindrops might hit them.”
To test these self-aggregations, Hu’s team used a few techniques, comparing live ant structures to clumped dead ants as a control. Using a rheometer—a device that can precisely measure the stress response and flow of a fluid, and is often applied in industrial situations (such as the development of a new shampoo)—they found that the ants continuously reorganize their structure to maintain stability.
Many materials behave like a solid when stressed by forces moving at certain speeds, and a fluid when stressed by slowed ones. Water, for instance, behaves like a fluid when you stick your hand in it, but a solid when hit by a human body jumping off a diving board—the reason that a belly flop hurts so much.
But the ant structures are a combination of solid and fluid when stressed by forces at all speeds, the researchers found. They actively deform their structure to accommodate a stress (like a fluid) but then bounce back into place afterward (like a solid). Check out what happens when one of their structures is compressed by a petri dish, for instance:
“This makes sense, based on their natural environment,” Hu says. “If they’re floating in a raft down a river, they have no control over where it floats, so if there’s something in the way—say, a twig—you see respond and flow around the twig, kind of like an amoeba.”
The ants’ sheer resiliency and buoyancy is also remarkable. When the researchers tried to push the floating rafts below the water’s surface, they found they could resist a significant amount of force and float back up:
This is enabled, in part, by the ants’ exoskeletons, which are naturally hydrophobic (i.e. they chemically repel water). When many ants clump together to form a structure, water doesn’t penetrate into the gaps between then, so when they’re forced underwater, the air that remains in these cavities helps them float.
Perhaps the biggest mystery of these ants’ remarkable living structures is how the creatures communicate to build them. Most ant communication is based on trails of pheromones left on the ground, but in such an interconnected form, that type of communication seems unlikely. Microscopic examination reveals that the ants grasp each other using both their jaws and little claws on the end of their legs. Noting this, Hu adds, ”We think they’re communicating through touch, but we really don’t understand it yet.”
November 26, 2013
Seahorses belong to the genus Hippocampus, which gets its name from the Greek words for “horse” and “sea monster.” With their extreme snouts, weirdly coiled bodies and sluggish movements produced by to two puny little fins, these oddly shaped fish seem like an example of evolution gone terribly awry. And yet, new research published today in Nature Communications shows that it is precisely the seahorse’s uncanny looks and slow motions that allow it to act as one of the most stealthy predators under the sea.
Seahorses, like their close relatives the pipefish and sea dragons, sustain themselves by feasting on elusive, spastic little crustaceans called copepods. To do this, they use a method called pivot feeding: they sneak up on a copepod and then rapidly strike before the animal can escape, much like a person wielding a bug swatter tries to do to take out an irritating but otherwise impossible-to-catch fly. But like that lumbering human, the seahorse will only be successful if it is able to get near enough to its prey to strike at very close range. In the water, however, this is an even greater feat than on land because creatures like copepods are extremely sensitive to any slight hydrodynamic change in the currents around them.
So how do those ungainly little guys manage to feed themselves? As it turns out, the seahorse is a more sophisticated predator than appearance might suggest. In fact, it is precisely its looks that make it an ace in the stealth department. To arrive at this surprising conclusion, researchers from the University of Texas at Austin and the University of Minnesota used holographic and particle image velocimetry–fancy ways of visualizing 3D movements and water flow, respectively–to monitor the hunting patterns of dwarf seahorses in the lab.
In dozens of trials, they found that 84 percent of the seahorses’ approaches successfully managed not to sound the copepod’s retreat alarms. The closer the seahorse could get to its unsuspecting prey and the faster it struck, the greater its odds of success, they observed. Once in range of the copepod, seahorses managed to capture those crustaceans 94 percent of the time. Here, you can see that method of attack, in which the seahorse’s giant head looks like a floating bit of marine sludge drifting toward the blissfully ignorant copepod:
The way the seahorse’s movements and morphology–especially its head–interact with the water particles, the researchers found, likely take the credit for its exceptional hunting skill. The animal’s arched neck acts like a spring for generating an explosive strike, they describe, while the shape of its snout–a thin tube with the mouth positioned at the very end–allows it to drift through the water while creating minimal disturbance.
To emphasize this pinnacle of engineering, the team compared water disruptions caused by seahorses with those of sticklebacks, a relative of the seahorse but with a more traditional fishy look. Thanks to the shape and contours of the seahorse’s head, that predator produced significantly less fluid deformation in the surrounding water than the stickleback. The poor stickleback possesses neither the morphology nor posture to generate “a hydrodynamically quiet zone where strikes occur,” the authors describe. In other words, while the seahorse may appear a bit odd so far as fishes go, evolution was obviously looking out for that funny but deadly animal’s best interests.
November 21, 2013
Over the course of 1700 miles, they sampled the water for small pieces of plastic more than 100 times. Every single time, they found a high concentration of tiny plastic particles. “It doesn’t look like a garbage dump. It looks like beautiful ocean,” Miriam Goldstein, the chief scientist of the vessel sent by Scripps Institution of Oceanography, said afterward. “But then when you put the nets in the water, you see all the little pieces.”
In the years since, a lot of public attention has been justifiably paid to the physical effects of this debris on animals’ bodies. Nearly all of the dead albatrosses sampled on Midway island, for instance, were found to have stomachs filled with plastic objects that likely killed them.
But surprisingly little attention has been paid to the more insidious chemical consequences of this plastic on food webs—including our own. “We’d look over the bow of the boat and try to count how many visible pieces of plastic were there, but eventually, we got to the point that there were so many pieces that we simply couldn’t count them,” says Chelsea Rochman, who was aboard the expedition’s Scripps vessel and is now a PhD student at San Diego State University. “And one time, I was standing there and thinking about how they’re small enough that many organisms can eat them, and the toxins in them, and at that point I suddenly got goosebumps and had to sit down.”
“This problem is completely different from how it’s portrayed,” she remembers thinking. “And, from my perspective, potentially much worse.”
In the years since, Rochman has shown how plastics can absorb dangerous water-borne toxins, such as industrial byproducts like PCB (a coolant) and PBDE (a flame retardant). Consequently, even plastics that contain no toxic substances themselves, such as polyethylene—the most widely used plastic, found in packaging and tons of other products—can serve as a medium for poisons to coalesce from the marine environment.
But what happens to these toxin-saturated plastics when they’re eaten by small fish? In a study published today in Scientific Reports, Rochman and colleagues fill in the picture, showing that the toxins readily transfer to small fish through plastics they ingest and cause liver stress.This is an unsettling development, given that we already know such pollutants concentrate further the more you move up the food chain, from these fish to the larger predatory fish that we eat on a regular basis.
In the study, researchers soaked small pellets of polyethylene in the waters of San Diego Bay for three months, then tested them and discovered that they’d absorbed toxins leached into the water from nearby industrial and military activities. Next, they put the pollution-soaked pellets in tanks (at concentrations lower than those found in the Great Pacific garbage patch) with a small, roughly one-inch-long species called Japanese rice fish. As a control, they also exposed some of the fish to virgin plastic pellets that hadn’t marinated in the Bay, and a third group of fish got no plastic in their tanks at all.
Researchers still aren’t sure why, but many small fish species will eat these sort of small plastic particles—perhaps because, when covered in bacteria, they resemble food, or perhaps because the fish simply aren’t very selective about what they put in their mouths. In either case, over the course of two months, the fish in the experiment consumed many plastic particles, and their health suffered as a result.
“We saw significantly greater concentrations of many toxic chemicals in the fish that were fed the plastic that had been in the ocean, compared to the fish that got either clean plastic or no plastic at all,” Rochman says. “So, is plastic a vector for these chemicals to transfer to fish or to our food chain? We’re now fairly confident that the answer is yes.”
These chemicals, of course, directly affected the fishes’ health. When the researchers examined the tiny creatures’ livers (which filter out toxins in the blood) they found that the animals exposed to the San Diego Bay-soaked plastic had significantly more indications of physiological stress: 74 percent showed severe depletion of glycogen, an energy store (compared to 46 percent of fish who’d eaten virgin plastic and zero percent of those not exposed to plastic), and 11 percent exhibited widespread death of individual liver cells. By contrast, the fish in the other treatments showed no widespread death of liver cells. One particular plastic-fed fish had even developed a liver tumor during the experimental period.
All this is bad news for the entire food webs that rest upon these small fish, which include us. “If these small fish are eating the plastic directly and getting exposed to these chemicals, and then a bigger fish comes up and eat five of them, they’re getting five times the dose, and then the next fish—say, a tuna—eats five of those and they have twenty-five times the dose,” Rochman explains. “This is called biomagnification, and it’s very well-known and well-understood.”
This is the same reason why the EPA advises people to limit their consumption of large predatory fish like tuna. Plastic pollution, whether found in high concentrations in the Great Pacific garbage patch or in the waters surrounding any coastal city, appears to be central to the problem, serving as a vehicle that carries toxins into the food chain in the first place.
November 20, 2013
In 1834, Charles Darwin discovered a strange animal during his exploration of Chile’s southern coast. The creature, a small frog, was shaped like a leaf with a pointed nose, but appeared puffed up as if had been blown full of air, like a balloon. As it turned out, those fat male frogs hadn’t been gorging themselves on too many mosquitoes, but instead were enacting duties that earn them distinction as one of nature’s best dads. They were incubating several of their squirming babies in their vocal sac.
These peculiar animals, known as Darwin’s frogs, are today divided into two species, one that occurs in northern Chile, and another that lives in southern Chile and Argentina. When a female Darwin’s frogs lay her eggs, her mate keep a careful watch until the tadpoles hatch. The eager dad then swallows his young, allowing the babies to safely grow within his vocal sac until they turn into frogs and are ready to strike out on their own. Here, you can see a dutiful papa frog seemingly vomit up his living young:
Northerly Darwin’s frogs, however, have not been spotted in the wild since 1980. Researchers are nearly certain the species is extinct. Meanwhile, their southerly cousins are in steep decline and seem to be heading down extinction’s death row as well. For once, it seems that humans are not entirely to blame for these biodiversity disasters (unlike the western black rhino, which bit the dust a couple years ago after enduring decades of poaching for its valuable but medicinally worthless horn, used as an ingredient in traditional Chinese medicine). Instead, the deadly amphibian chytrid fungus, researchers report today in PLoS One, is likely to blame.
The chytrid fungus has popped up in amphibians in North and South America, Europe and Australia. The fungus infects the animals’ skin, preventing them from absorbing water and other nutrients. The fungus can rapidly decimate amphibian populations it comes into contact with, and has been called (pdf) “the worst infectious disease ever recorded among vertebrates in terms of the number of species impacted, and its propensity to drive them to extinction” by the International Union for Conservation of Nature.
To identify chytrid as the likely culprit behind the Darwin’s frogs disappearance and decline, researchers from Chile, the UK and Germany conducted a bit of historical sleuthing. They dug up hundreds of archived specimens of Darwin’s frogs and closely related species dating from 1835 until 1989, and then tested them all for fungal spores (the problematic form of chytrid fungus was first recorded in the 1930s and reached epidemic-status around 1993, but researchers aren’t certain of when it first emerged). They also took around 800 skin swabs between 2008 and 2012 from 26 populations of still-living southern Darwin’s frogs and other similar frog species that live nearby.
Six of the old museum specimens, all collected between 1970 and 1978–just before the northern Darwin’s frog’s disappearance–tested positive for the disease. More than 12 percent of the living frogs tested positive for the fungal spores. In places where the Darwin’s frog has gone extinct or is experiencing drastic declines, however, rates of infection jumped to 30 percent in other amphibian species. Although these events don’t prove that the fungus killed the northern Darwin’s frogs and are now wiping out the southern species, the researchers strongly suspect that is the case.
Despite evidence that the disease has spread throughout the Darwin’s frog’s range, the researchers are not giving up on hope to save one of the world’s greatest dads from extinction. “We may have already lost one species, the Northern Darwin’s frog, but we cannot risk losing the other one,” Claudio Soto-Azat, the study’s lead author, said in a statement. ”There is still time to protect this incredible species.”