Field of Science


Vampire Spiders Pounce on Victims with the Best Costumes

A version of this post first appeared on June 7, 2012. Happy Halloween!

It's reasonable for a hungry predator to hesitate when its prey appears to be two halves of an animal glued together and reanimated. Jumping vampire spiders that were faced with this decision took it slowly. But they eventually chose tasty mosquitos stuffed full of fresh human blood. Before pouncing, these discriminating diners considered their victims' headwear as well as their belly size.

New Zealand biologist Ximena Nelson led the investigation of how Evarcha culicivora spiders choose which prey to attack. The East African jumping spiders are very picky eaters, dining solely on mosquitos that have recently filled up on blood. (Since only female mosquitos drink blood, this means males are mostly off the menu.)

A blood meal doesn't just satisfy a rumbly abdomen; it also makes the spiders more attractive to the opposite sex. So it's worth a spider's effort to make sure that it's hunting the right prey. Stalking and killing what it thinks is a jelly-filled, only to discover it's only a glazed, would be disappointing.

Earlier research had shown that vampire spiders can find their prey by sight alone—they don't need the smell of a blood-engorged insect abdomen to direct them. So Nelson wondered what visual cues help E. culicivora home in on its prey. The spiders have great vision and eyes that point in all directions. But will they simply attack any mosquito with a belly full of blood, or do they use other signals to find females? If they only glimpse part of a mosquito, what hints will give them enough confidence to pounce?

To find out, Nelson's team tempted vampire spiders with various mosquito "lures." They used dead mosquitos rather than living ones, since these are less prone to flying away in terror when a spider comes after them. Dead mosquitos are also more amenable to being cut up and Frankensteined together with each other, which is exactly what the researchers did.

The scientists used three kinds of mosquitos: males, females fed only on sugar water, and females recently fed on mammal blood. After being dispatched, each insect was cut in half. Then the front and back halves were shuffled and recombined. This created female heads with male abdomens, male heads with blood-filled or regular female abdomens, and control insects that were normal (except for the seam across the middle).

Those female mosquitos fed on mammal blood, by the way, had eaten well. I asked Ximena Nelson if the researchers fed their mosquitos using their own arms, an unappetizing practice I'd heard of. "Yes, unfortunately that is the case," she said. "Not a pleasant experience for the victim."

After the monstrous mosquito lures were "mounted in a lifelike posture," the vampire spider subjects went through a series of tests. In some cases, the researchers made the lures jerk up and down to seem a little more lively. Sometimes one end of the lure was hidden behind a wall, so that only the mosquito's head or only its abdomen was visible.

Whenever females with blood-engorged abdomens were on the menu, the spiders went for them. But when given the choice between a normal-looking female mosquito and one with a male head, the spiders preferred their meals with the correct heads on them. And when the mosquitos' back ends were hidden behind a wall, spiders went after the ones with female front ends. A test using virtual mosquitos on a screen (which the frustrated spiders jumped at to no avail) confirmed how the spiders were choosing their victims: In addition to blood-stuffed stomachs, they looked for female antennae.

Male mosquitos have plush, feathery antennae, while females' are simple and unadorned, like car antennas. In the photo below, a male is on the left and a female is on the right (with her head pointing toward the bottom corner).

Although E. culicivora spiders do choose victims for their blood-filled bellies, they use antenna shape as an extra clue to make sure they're choosing well. Plain female antennae tell a spider that its intended victim isn't just a male mosquito that overdid it on the sugar water.

It seems to be "a combination of cues from the head and from the abdomen that truly 'flicks the switch,'" Nelson says. Vampire spiders are cautious, deliberate hunters that look closely before they leap. Once they take their pick, that well-fed female mosquito may be left regretting her own lunch choice.

Nelson, X., & Jackson, R. (2012). The discerning predator: decision rules underlying prey classification by a mosquito-eating jumping spider Journal of Experimental Biology, 215 (13), 2255-2261 DOI: 10.1242/jeb.069609

Images by Robert Jackson.

The Shambulance: 5 Reasons Not to "Cleanse" Your Colon

The Shambulance is an occasional series in which I try to find the truth about bogus or overhyped health products. Physiologist Steven Swoap is with me at the helm.

If you've been tempted by promotions for "colon hydrotherapy"—that is, sessions in which you would pay someone to put a tube up your rectum and wash out your large intestine with water, like a dirty garage being hosed down in summer—then you've already overcome some impressive mental hurdles. Maybe you're almost ready to enjoy the relaxation, renewed energy, and improved health that the procedure promises. Before you take the plunge, as it were, here are a few points to consider.

It's not the 19th century.
People who offer colon hydrotherapy (also called a colonic) tell you the large intestine is full of "toxic waste and toxins." It does, of course, carry waste out of your body. But is it a two-way street?

"Intestinal autointoxication," the idea that poisons from your feces can move backward from your colon into the rest of your body, is an old one. Old as in ancient Egyptian. The Greeks were into it too, including Hippocrates and Galen.

In the 19th century, doctors prescribed laxative pills and enemas to cure all manner of illnesses. One man created and promoted a popular device called the Cascade. As alternative medicine researcher Edzard Ernst describes it, this was a rubber bottle with a nozzle for a person to insert into his or her rectum. When the person then sat on the bottle, it squirted 5 liters of fluid into up into the colon.

By the 1920s, though, some actual scientific study had been done on the subject. Unlike the Cascade, the theory of intestinal autointoxication did not hold water.

A toilet is not a gym.
"Having colonics is like taking your colon to the gym," declares the website for one colon hydrotherapy center. Filling the colon up with water and emptying it again, the theory goes, "exercises" the intestinal muscle so it can do its job better in the future.

"Injecting water into the colon will cause the colon to swell, and cause so-called 'stretch-activated' contractions of the smooth muscle surrounding the intestine," says Williams College physiologist Steven Swoap. These contractions are called peristalsis. "But the colon does this naturally as food stuffs pass through," he adds. "There is definitely no need to help this along for peristalsis to occur."

Colorectal surgeon Francis Seow-Choen points out in a review paper that since the colon is lined with smooth muscle (a type we can't voluntarily control), it cannot be toned like the muscles you work at the gym. Toned muscles are ones that we've consciously flexed so often, our brains learn to flex them automatically. Sit-ups work; water up the rectum doesn't.

It's rude to firehose your friends.
In addition to waste, your colon houses a large portion of your body's friendly bacteria. These gut microbes manufacture several vitamins we need, and seem to be involved in defending us from dangerous microorganisms and generally keeping us healthy.

One study found that cleaning the colon to prepare patients for a colonoscopy—in this case with a straightforward laxative, not with large volumes of pumped-in fluid as in a colonic—immediately altered the types of bacteria in patients' intestines. Another study found that cleaning out the colon both knocked down the bacterial population there and seemed to make it easier for new, potentially unfriendly bacterial strains to move in.

It might kill you.
"My biggest worry would be perforations caused by the water," Swoap says. "If abrasions or tears in the colon occur, you have the possibility of a dangerous bacterial infection." Ironically, one way to make the material in your colon as dangerous as colonic practitioners claim is to blast it with water. Breaking up the feces and creating tiny tears in the colon can turn a one-way street into a two-way hazard.

According to a paper in the Journal of Family Practice, reported complications from colon cleansing include cramping, abdominal pain, vomiting, rectal perforation, blood poisoning, kidney failure, fatal amoebic infection, and fatal accumulation of gas in the veins. Even if such a consequence is rare, Swoap points out, "it is sure not to happen if I don't let some technician put a hose in my rear."

Everybody poops.
Colons have been doing their job without outside intervention for hundreds of millions of years. "Your colon does not need help in a non-disease state," Swoap says. "Your colon is a pro!" If you want to thank it, step away from the hose and have some broccoli.

You Have to Hear This Beluga Mimicking a Person

At first, they couldn't tell where the sound was coming from. Researchers at the National Marine Mammal Foundation kept hearing what sounded like a muffled conversation, as if two people were talking just around a corner. It was only when a diver climbed out of the enclosure holding a male beluga whale and said, "Who told me to get out?" that they realized the whale himself was making the speech-like noises.

It was 1984, and the beluga, called NOC, had been living at the research center for seven years. Belugas are known for being noisy. But they usually stick with a standard whale repertoire of squeals, whistles and clicks. As for NOC, it seemed that after all his years of hearing human speech, both from trainers and researchers above the water and from divers talking on underwater equipment, he had developed a decent impression:

I like to imagine the whale is griping to one of its whale friends. "So I was working with the trainer today, and he was all like DURP DE DOO, BLER BLER BLER, BLOOOOOR DE DUM DUM! Pfft."

By inserting tubes into their beluga mimic's nasal cavity (or, more accurately, convincing the beluga to allow them to insert tubes into his nasal cavity), the researchers learned exactly how he was manipulating pressure in his nasal tract to produce the strange sound. They describe their results in Current Biology. But even though they discovered how the whale was performing his signature trick, they could never know exactly why.

As NOC matured, he retired his human impression. Five years ago, the beluga died. His legacy will be the lasting knowledge that to whales, humans sound just like the Swedish Chef.

Sam Ridgway, Donald Carder, Michelle Jeffries, & Mark Todd (2012). Spontaneous human speech mimicry by a cetacean Current Biology, 22 (20) : 10.1016/j.cub.2012.08.044

Image: Jason Pier (not the same beluga whale). Audio: Current Biology, Ridgway et al.

Dolphins Pull Endless All-Nighters by Resting Half of Brain at Once

Prepare to be jealous, perpetual to-do-listers of the world: because it shuts down only one side of its brain at a time, the dolphin never needs to sleep. Dolphins don't even seem to slow down. When researchers tried their hardest to wear out a dolphin by it making it do the same echolocation task nonstop, the experiment came to an end—15 days later—before the dolphin's attention flagged.

Although dolphins (as far as we know) don't have checklists, they do have a couple very good reasons not to doze off. One is sharks. By staying attentive at all times, dolphin pods can prevent predators from gaining the element of surprise.

Another reason is breathing. Like other mammals, dolphins are stuck with the task of periodically inhaling air. Constantly coming to the surface to breathe doesn't leave much room for rest.

Their solution is to sleep, literally, with one eye open. Along with other marine mammals and migrating birds, dolphin conk out with one side of their brain—and close one eye—while the other side stays wide awake. In an earlier study, researchers showed that dolphins could pay attention to a task for five straight days without seeming to get tired. Now, they set out to push the animals even farther and see what happened.

Brian Branstetter of the National Marine Mammal Foundation in San Diego and other scientists conducted their experiments on two dolphins, a 26-year-old male and a 30-year-old female. Both dolphins had participated in at least one study on this subject before, so they knew the drill.

For each experimental trial, a dolphin stayed within a 20-by-20-foot floating pen, enclosed by nets, in San Diego Bay. Around the pen's perimeter were eight underwater microphones. The dolphin's task was to constantly patrol the enclosure, sending out echolocating clicks at each station. If it heard its sonar signals echo back (something that would normally indicate another animal was out there swimming around, but in this case was merely a computer-generated phantom), the dolphin had to press a paddle. For performing the task accurately, the dolphins got fish snacks.

When tested for five straight days, both dolphins continued to patrol the pen and press the paddle accurately the whole time. (They may have wondered, though, why trainers were willing to stick around and keep tossing them fish.) Both animals lasted through three separate five-day trials. They began to perform a little worse by the fifth day, although this effect had shrunk by the third trial. So although the dolphins may have gotten tired or bored with the experiment, they seemed to adjust to the schedule as they went on.

Out of the two dolphins, the female was the star performer at this task. She was both more accurate and, according to the researchers, more eager to participate, making "victory squeals" when she got the right answers. So she alone moved on to the next round of the study.

This was originally planned as a 30-day marathon of clicks, paddle pushing, and fish tossing. But a winter storm rolled into the bay after 15 days and forced the team to cut it short. Throughout the 15 days, the female dolphin continued to perform her task with near-perfect accuracy.

The experiments were set up so that the number of computer-generated targets varied somewhat depending on the dolphins' performance. In this case, the female dolphin was challenged with almost 80 imaginary sonar targets every day, or more than 3 an hour on average. That doesn't even leave room for a cat nap. But even at the end of the 15 days she showed no signs of slowing down.

"How much longer she could have performed the task," the authors write, "is unknown." Dolphin mothers with newborns have been observed swimming without rest for two months or more. To keep their babies fed and protected from predators, mothers remain alert and vigilant for as long as it takes.

It seems like a cruel joke that the ability to stay sharp for weeks, or months, without resting belongs to an animal that doesn't know what a deadline is. They can work or play for as long as they feel like, a skill we humans only have in our dreams.

Brian K. Branstetter, James J. Finneran, Elizabeth A. Fletcher, Brian C. Weisman, & Sam H. Ridgway (2012). Dolphins Can Maintain Vigilant Behavior through Echolocation for 15 Days without Interruption or Cognitive Impairment PLOS ONE : 10.1371/journal.pone.0047478

Image: Jeff Kraus

The Hazards of Being an Athletic Ape

This post first appeared at the Scientific American Guest Blog and is republished with permission.

With a single bad step as he ran untouched across a field this September, one of the best cornerbacks in the National Football League removed himself from the game for a whole season. New York Jets fans who saw Darrelle Revis’s left knee buckle under him that day may have pled with their televisions: not the ACL. But it was too late for Revis and his anterior cruciate ligament, which will undergo surgery this week.

Football fans are all too familiar with the ways in which a knee or ankle can fail a person. But athletes, like other humans, are simply doing the best that an ape running around on two legs can.

Before we lived and walked on the ground, our ancestors inhabited the tree branches. They didn’t look quite like chimpanzees or any other modern animal, but they were large apes built for climbing. They had big, grasping toes and extremely flexible feet and ankles. “These things were just brilliantly adapted for living in the trees,” says Boston University anthropologist Jeremy DeSilva. He studies the evolution of ape and human locomotion by looking at both ancient fossils and modern-day animals in motion.

When our ancestors descended from the trees and began walking upright, they faced some major mechanical challenges. “Being on two limbs is just a real problem,” DeSilva says. “If you were taking shop class and your assignment was to build a chair, and you built a chair with two legs, you’d fail the class because it would fall over all the time.” Simply balancing an animal upright is a feat of evolutionary engineering—and that’s before the animal starts moving around.

To walk on two limbs, our ancestors had to make several modifications to the feet they’d inherited from tree-climbing apes. Flexible, grasping appendages with 26 individual bones had to become stable surfaces that we could push off of with each step. “We’ve stiffened things up by patching these bones together with a bunch of ligaments that make up the arch,” DeSilva says. And muscles that were once used for grasping branches now support the foot’s arch. “But boy,” he says, “these are just a bunch of band-aids.”

Though these new two-legged bodies worked well enough to keep our lineage alive, bipedalism may not be the best idea evolution has ever had. “If you look across the animal world,” DeSilva says, “good ways of moving evolved multiple times.” Flight, for example, has evolved many times. So has a streamlined body in swimming animals. But striding on two legs evolved just once in mammals.

The only other animals that walk like we do are birds. And with a couple hundred million years to work on the problem, rather than the mere 5 million or so that we’ve had, birds have come up with what DeSilva thinks is a tidy solution: they’ve fused several bones together to create rigid, immobile feet.

In humans, DeSilva says, “I find the foot to be incredibly problematic.” He thinks a lifetime of walking and running on feet held together by evolutionary band-aids is bound to lead to the kinds of problems people frequently experience: plantar fasciitis, collapsed arches, shin splints, Achilles pain.

What’s more, DeSilva says, “We have evidence that these things are not just modern problems.” In the ancient hominins whose fossils he studies, there are many who suffered from the same injuries that plague us. There are broken ankles in individuals 1.9 and 3.4 million years old (both healed). There’s osteoarthritis in a creature that may have been Homo habilis. An Australopithecus has what looks like a compression fracture in its heel. Another individual sustained, and healed from, a severe high ankle sprain 1.8 million years ago.

Modern-day humans know a thing or two about twisted ankles. The most commonly sprained ligament in the whole human body is a tiny one in the ankle called the anterior talofibular ligament. What’s notable about this ligament, DeSilva says, is that almost none of our living ape relatives has it.

DeSilva’s opinion is that humans evolved this ligament to keep the ankle stable. An upright human is like a balanced stack of blocks, he says. Our ankle bones have flattened surfaces that sit on top of each other, unlike the curved and snugly fitted ankle bones of a chimp. When a human steps on an unexpected rock, this extra ligament in the ankle might be necessary to keep the whole stack of blocks from slipping off its foundation. We don’t dislocate a foot entirely when we trip on a curb—but we might be benched for a couple of months.

Like our ankles, our knees have wide, flattened surfaces that spread out the weight we’re carrying on two limbs instead of four. And they’re large, compared to our body size. “The whole bed-of-nails idea is at work here,” DeSilva says. “Human joints tend to be very puffy.” Structurally, though, our knees are similar to those of our climbing relatives; they have all the same components that a modern chimp’s knee does.

But chimps don’t ever land funny after a layup shot, or change direction too sharply while cutting upfield. That kind of sudden sideways motion is the knee’s downfall, and can rip or snap the ligaments that stabilize the joint.

The infamous ACL sits inside the front of the knee joint, holding the thigh bone in place on top of the shin bone. Its counterpart at the back of the knee is the posterior cruciate ligament. The MCL and LCL, or medial and lateral collateral ligaments, cradle the knee joint on either side and are especially vulnerable to sideways jarring. Too much twisting in the knee can tear the menisci, pads of cartilage tucked inside the knee socket.

Our knees have no problem with the normal folding and straightening of our legs. “When you go too far out of range in the other directions, that’s when you get in trouble,” says Irene Davis.

Davis is a physical therapist and biomechanics researcher at the Spaulding National Running Center at Harvard University Medical School. Despite how often we suffer injuries, Davis says, “I think we’re designed really well for both walking and running.”

Davis cites the theory, promoted by Harvard anthropologist Daniel Lieberman and others, that early humans evolved as so-called persistence hunters. Before they developed effective spears, the theory goes, our ancestors obtained meat by separating an animal from its herd and simply chasing it on foot until it couldn’t run any farther. Researchers point to various skeletal features and cooling mechanisms—and the fact that some people seem to enjoy it so much—as evidence that our species is built for long-distance running.

Of course, early humans would have done it without Reeboks on. In the clinic, Davis advocates what she calls a more natural style of running. She teaches people to land gently on the front of their foot with each step, as barefoot runners do, rather than hard on their heels as people with cushioned running shoes tend to.

Davis believes that wearing structured, arch-supporting shoes makes feet weak and lazy, and that this weakness leads to common foot injuries such as plantar fasciitis. Yet feet are largely ignored until they give us trouble. “You don’t see people at the gym strengthening their feet,” she says, but you should. “Strong feet are healthy feet.”

Despite what DeSilva sees as evolutionary patchwork, Davis thinks the human foot is “just a fantastic structure.” Each time the foot hits the ground, it must be both flexible enough to absorb shock and adjust to uneven terrain and rigid enough to push off of again. Davis thinks the problems come when we don’t use our feet and legs as evolution intended.

When treating patients with overuse injuries, Davis teaches them to run with better mechanics so they avoid getting the same injury in the future. Runners receive feedback on their motion from tools such as accelerometers or mirrors, then practice carrying their bodies in better alignment.

Davis says people can also be taught to prevent future acute injuries such as ACL tears. Most ACL injuries are non-contact; as Darrelle Revis knows, one awkward step is all it takes. So there are programs that teach athletes to land their jumps more gently, or aim to strengthen stabilizing muscles around the knee to protect its ligaments. Though some people will still choose to put themselves in the paths of linebackers, they can at least learn ways to run and jump that put less strain on their vulnerable ligaments to start with.

Having recovered from recent injuries of his own, Jeremy DeSilva will be lacing up his minimalist Nike Free sneakers to run a marathon this weekend. Influenced by the research on barefoot running, he’s left cushioned sneakers behind and is now propelling himself more like his Australopithecus subjects did. “I guess I take my work home with me,” he says.

Davis runs completely barefoot, though in the winter or when she needs more protection for her feet she’ll wear a minimal covering such as water shoes. She also rollerblades.

One sport Davis doesn’t enjoy is football. “I don’t like watching the injuries,” she says. “I see a big pile of people with someone underneath it and it just drives me crazy.”

Image credit: Cpl. Michelle M. Dickson

Urinating Through Your Mouth Is Great. Ask This Turtle.

Even if you did ask the Chinese soft-shelled turtle what's so great about excreting bodily waste through one's mouth, you would probably just get gurgling in reply. The animal spends a lot of time with its face underwater. But its unusual strategy may be what allowed it to move into its favorite swamps and ponds in the first place.

Pelodiscus sinensis lives in China and other parts of Asia, as well as more remote spots, such as Hawaii, where it was introduced by turtle-soup-eating immigrants. It spends time both out of water and underwater, coming to the surface to breathe air into its lungs. When it has to stay submerged for long stretches, though, the turtle can breathe almost like a fish. It fills its mouth with water and empties it out again while rhythmically pulsing its throat, a maneuver that lets the turtle get all the oxygen it needs via specialized surfaces inside its mouth.

Scientists in Singapore observed that a soft-shelled turtle on land will occasionally dunk its head into a puddle for no clear reason. It may stay this way for an hour and a half or more. While in this position, the turtle makes the same throat-pulsing motion, which led the scientists to wonder whether the turtle's specialized underwater-breathing equipment had some other role as well.

Yuen Kwong Ip at the National University of Singapore, along with other scientists, investigated organs at the back of P. sinensis's mouth called the buccopharyngeal villiform processes (BVPs, if you prefer). These look sort of like gills and are on the roof and bottom of the mouth, near the throat.

To find out exactly what materials were entering and exiting the soft-shelled turtle, the researchers performed a series of tests. They restrained turtles for several days on a dry platform, with a box of water in front of their heads for drinking and whatever else it was they were doing. A second container waited under the turtles' tail ends to collect urine that came out the traditional way.

Urinating is a way for animals to dispose of the waste products their cells constantly generate. Nitrogen, one of these waste products, is filtered out of the blood and sent on its way—in P. sinensis and in many other animals, including mammals—as a molecule called urea. It's usually collected by the kidneys and dumped in the urine.

But when the researchers observed their soft-shelled turtles' waste production, they found that although some wastes showed up in the urine, most urea came out through the mouth.

Ip writes in the Journal of Experimental Biology that turtles on dry land dunked their heads in water for 20 to 100 minutes at a time. While submerged, they repeatedly "rinsed" their mouths with water while rhythmically pulsing their throats. He discovered that this motion simultaneously pulled oxygen out of the water, so the turtles could keep breathing, and expelled urea into it.

Looking at the turtle's DNA, the researchers found what looked like a gene for a urea transporter, a protein that carries urea molecules across membranes. The gene was active in the turtle's mouth and the gill-like BVPs, but not—as it would be in humans or almost any other vertebrate animal—in the kidneys.

Ip thinks the Chinese soft-shelled turtle's strategy is not, unlike most cases of misplaced urine, an accident. He notes that P. sinensis and other soft-shelled turtles often live in salty marshes and swamps, or even in the sea. If they excreted urea in the usual way, they would need to continuously drink the water around them to make urine. But like human castaways in the ocean, the turtles would be ill-advised to drink this water; their kidneys can't handle that much salt. So instead, P. sinensis—perhaps along with the other soft-shelled turtles—sends urea back toward its mouth after filtering it from its blood. To dispose of the urea, the turtle only has to rinse its mouth with water, not drink it.

Chinese soft-shelled turtles aren't the only animals that know the taste of urea. Cows and other ruminant animals excrete some urea into their saliva. Their reasons are quite different: they use nitrogen in urea to feed the friendly bacteria that live in their guts and help them digest plant matter. By swallowing their urea-carrying saliva, cows send it to their stomach and keep their microbes alive.

You may find this trick unappealing, but to Ip it's inspirational. Hypothetically, he says, doctors could one day treat patients who have kidney failure by turning on genes for urea excretion in their mouths, just as these genes are turned on in turtles. "Urea excretion can still occur through rinsing the mouth with water, just like the soft-shelled turtle," he says, "without having to go through blood dialysis." Then we'll be able to ask them just how great it is.

Yuen K. Ip, Ai M. Loong, Serene M. L. Lee, Jasmine L. Y. Ong, Wai P. Wong, & Shit F. Chew (2012). The Chinese soft-shelled turtle, Pelodiscus sinensis, excretes urea mainly through the mouth instead of the kidney Journal of Experimental Biology DOI: 10.1242/jeb.068916

Image: Chinese soft-shelled turtle by muzina_shanghai (Flickr)

Music Is an Acquired Taste (for Mice)

Though mice are skittish and naturally seek out quiet, undisturbed spaces, you can turn them into music fans—if you get them while they're young. Scientists have found that mice who hear music during a narrow window of their development will enjoy it when they've grown up. They'll even behave less anxiously when there's music playing.

The brains of mice, like the brains of humans, go through phases while they're growing. During certain windows of time, we're especially ready to learn particular skills (such as seeing with two eyes, keeping our balance, and maybe speaking a language) or develop particular preferences (like a bird imprinting on its mother). Outside of those windows, it may be difficult or impossible to learn the same things.

Takao Hensch and his colleagues at Harvard University used music made by humans to study these developmental windows, called "critical periods," in mice.

They started with classic mouse-testing setup: a square "arena," 45 centimeters on each side, with shelters holding nesting material in two opposite corners. When left in the arena for a few hours, mice will explore the available shelters, scurrying back and forth between them, and ultimately choose one to settle down and build a nest in. In Hensch's setup, one shelter also held a speaker that was playing music. (The mice heard one of three pieces of music that included the first movements of Beethoven's first and ninth symphonies and a bossa nova tune called "Agua de Beber," the latter of which is now stuck in my head so badly that I wonder if I was trapped in a lab and overexposed to it as a child).

Normal adult mice, when dropped into this setup, usually chose to snuggle up in the silent shelter. But Hensch was able to teach some mice to prefer the musical shelter instead. He housed 15-day-old mice in a chamber where one of these pieces of music was playing continuously, for 10 days straight (talk about an earworm). Then he let the mice grow up normally. At 2 months old, he tested the mice in the square arena; one shelter was silent and the other played the music each mouse had heard in its youth. These mice chose the musical shelter at a significantly higher rate than normally raised mice.

When mice were exposed to the 10 days of music as adults, though, they still preferred silence afterward. And an earlier study had found that mice exposed to music within their first 10 days of life also choose silence as adults. So, as Hensch reports this week in PNAS, the time around the third week of a mouse's life seems to be a critical window for learning to love music—or, at least, learning not to hate it.

But the flexibility, or "plasticity," of a young brain can be recreated in adults. Hensch writes that various "molecular brakes" put a stop to the brain's flexibility in adulthood. These changes prevent neurons from making new connections, for example, or tuck away certain genes in hard-to-reach pockets of a coiled DNA strand. Hensch studied two groups of mice who were missing certain molecular brakes: one thanks to a genetic mutation, and another because of a chemical injection he'd given them.

Hensch used these mice, and their young-acting adult brains, to further explore how the animals developed musical preferences. Both types of mice could learn to like music in adulthood, just as other mice did when exposed at a young age. And out of the three pieces of music used in the study, the mice preferred the one they'd been taught over the other two.

But it had to be music, not just sounds. Although the mice learned to like Beethoven or Jobim, they could not be taught to appreciate the sound of one note repeated over and over. "It was fascinating to see that music would shape mouse behavior, whereas pure tones would not," Hensch says. "This suggested something special about music was capturing the animal's interest."

Of course, mice don't usually grow up hearing human music. But Hensch says his group is also researching "song-like" ultrasonic noises that mice might hear from each other. "Importantly, the same critical period seems to hold there as well," he says.

In one final twist to the story, the researchers saw that mice who'd been taught to tolerate music spent more time scurrying across the center of the experimental arena, between the two shelters. This happens to be a classic measurement of anxiety: mice who are more anxious will avoid the dangerous, exposed area. Mice who liked music acted less anxious than others during the experiment, suggesting that the music they heard in the shelter was calming.

"This has obvious relevance to music therapy," Hensch says, for both humans and animals. It also provides new clues about the windows of opportunity that exist in our own brains. Events during small spaces of our childhood may determine what we like, what we find familiar, and what tunes will never, ever leave our heads.

Eun-Jin Yang, Eric W. Lin, & Takao K. Hensch (2012). Critical period for acoustic preference in mice. PNAS : 10.1073/pnas.1200705109

Image: Roger McLassus (Wikimedia Commons)

How Rice Plants Kick Out Party Crashers

To really sympathize with rice—and to understand why it's developed tricks for bossing insects around—I need you to imagine you're a plant throwing a party.

Have you got it? Let's say it's a sushi-making party, since, you know, you're a rice plant and you already have one ingredient.

So you're Oryza sativa and you're growing in a field somewhere in Asia, and you're enjoying your party with the other rice plants. But then a notorious moocher shows up: the brown planthopper, Nilaparavata lugens. And it brings a whole crowd of its friends.

Next thing you know, the moochers are stealing all your sushi and eating it. Which is to say, they're causing severe crop damage. Brown planthoppers are one of the world's biggest rice pests. They feed on sap from the stems, carry viruses that infect rice plants, and lay new generations of eggs on the leaves.

Since that kind of behavior can ruin a party, you (the rice plant) want to drive the freeloading insects out. You can't physically remove them, so instead you change the tone of your party to something that's not at all their taste. Let's say you switch off the classical music and crank up some heavy metal.

Really, the rice plant emits a chemical called S-linalool into the air when it senses the familiar chewing. Brown planthoppers don't care for the molecule. But if any of them stick around, they'll be sorry. That's because the loud music simultaneously attracts the rice plant's real friends, who love both heavy metal and laying their eggs inside the eggs of brown planthoppers.

These metal fans are the parasitic wasps Anagrus nilaparvatae. Their larvae grow inside the planthopper eggs where they're laid, consuming the unhatched planthoppers from the inside out. And the same chemical signal that hustles the mooching planthoppers out of the party summons the wasps to punish any that stay behind.

Yonggen Lou at the Zhejiang University in China, along with other researchers, found out what the rice plants were up to by spying on them both in the lab and in the field. They knew already that rice plants emit S-linalool when they're chewed on by planthoppers. In fact, all kinds of plants are known to release certain compounds in apparent self-defense. But to understand the crashed house party one step at a time, the scientists created genetically altered rice plants that couldn't make S-linalool at all.

Starting with regular, non-altered rice plants, the researchers showed that S-linalool (the heavy metal music) was turned on whenever brown planthoppers fed on the plants—or when humans repeatedly stabbed the rice stems with tiny needles, imitating planthoppers in search of sap.

Next the researchers released groups of brown planthoppers into cages holding the rice plants. They saw that females preferred to hang out and lay their eggs on the genetically altered plants that couldn't make S-linalool. This means regular, unaltered plants, which could crank up the music when they wanted to, ended up with fewer pest eggs.

To explore the tastes of the parasitic wasps, researchers didn't let them see the plants or the planthoppers at all. Instead, they put the wasps in tubes and gave them whiffs of chemicals previously released by  rice plants. The wasps followed the smell of regular plants that had been chewed by planthoppers—and released S-linalool—but weren't interested in the smell of genetically altered, non-heavy-metal-playing plants.

When the same genetically altered plants were growing in a field, scientists found more than twice as many female planthoppers on them (along with their eggs) than on the regular, S-linalool-producing plants. And the insects on the altered plants had significantly fewer parasitic wasps attacking them. There were also fewer predatory spiders on those plants. The rice plant, despite being stuck in one place and seeming pretty passive, is dictating in detail who's invited to its party. 

Not all compounds emitted by plants are for deterring pests, though. The researchers also studied a second chemical that comes from the rice plant called (E)-β-caryophyllene. Presumably it's helpful to the plants, because they make it all the time. But brown planthoppers are attracted to it—as are their parasitic wasps. At the sushi party, let's call it the beer.

Yonggen Lou thinks farmers might be able to take advantage of the compounds rice plants naturally emit. For example, they could grow rice around the outside of a field that's genetically engineered to produce only (E)-β-caryophyllene (the beer) but not S-linalool (the loud music). These plants would attract both brown planthoppers and their pests. The rest of the rice plants in the field would be engineered in the opposite way, cranking up the heavy metal without providing any beer. Since pests would gather at the edges of the field, where the more attractive molecules were in the air, farmers could reduce their pesticide use and protect their crop. They're the ones, after all, who would really like to be in charge of the guest list.

Xiao Y, Wang Q, Erb M, Turlings TC, Ge L, Hu L, Li J, Han X, Zhang T, Lu J, Zhang G, Lou Y, & Penuelas J (2012). Specific herbivore-induced volatiles defend plants and determine insect community composition in the field. Ecology letters, 15 (10), 1130-9 PMID: 22804824

Image: Brown planthoppers by IRRI Images (Flickr)

Enslaved Ants Get Even by Killing Captors' Babies

In the world of ants, a "slave rebellion" isn't a sudden uprising: it's the slow and stealthy murder of the next generation of captors while they're too young to fight back. Ants that have been seized and put to work tending another species' young instead tear some of those young to shreds. Now scientists have found that these rebellions aren’t random acts of retaliation, but an ongoing strategy in the war between these species.

The battles take place in the leaf litter. In the summer "slavemaker" ants send tiny scouts out from their nests to search for targets, explains Susanne Foitzik, a professor at Johannes Gutenberg University of Mainz in Germany. Foitzik studies a North American species of slavemaker ant called Protomognathus americanus, but there are others too.

When a scout finds a target colony, she sneaks inside to see how big it is and what species live there. In the case of P. americanus, she’s looking for one of three ant species in the genus Temnothorax. Then the scout returns home to recruit help. An army of P. americanus workers returns to attack the target site. The invaders release a chemical weapon from their bodies that sends the other ants into confusion; the besieged ants may start attacking each other while the invaders cut off their legs and antennae.

Once all the adults have fled the colony, the slavemakers either move in themselves or pick up the colony's abandoned young and carry them home. These Temnothorax babies will grow up to become workers for the slavemaker colony. They’ll go on raids and defend the home nest alongside their P. americanus captors. They’ll even tend their captors' own young—or pretend to.

Foitzik and her colleagues first observed the revenge of Temnothorax a few years ago. Spying on slaveholding P. americanus colonies in the lab, they saw that the captive ants tasked with taking care off the young sometimes do just the opposite. "Slaves carry healthy pupae out of the nest, where they die from neglect," Foitzik says. At other times, they saw slave workers turn on young, soft-bodied pupae and—as in the photo above—tear them apart and eat them.

In a new study, Foitzik and other researchers sampled ant colonies from New York, West Virginia, Ohio and Michigan to find out whether the rebellions they’d seen were a fluke or a common occurrence. They collected a total of 352 colonies, both P. americanus and Temnothorax longispinosus, and brought them back to the lab for observation. In the untouched Temnothorax nests and in slavemaker nests tended by captive Temnothorax ants, they monitored how many young survived to adulthood.

The authors report in Evolutionary Ecology that in free-living nests of Temnothorax ants, 85 percent of young pupae grew up happy and healthy. But in P. americanus nests—where the young were nursed by the same Temnothorax ants, but this time as slaves—only 45 percent of the young lived, on average.

At least part of this difference in survivorship, the researchers believe, is thanks to enslaved ants allowing some of their charges to die (or killing them outright). Rebellion seems to be more common in some populations than in others, because the death rate of young P. americanus ants varied from site to site. But in all cases, it was higher than the death rate when Temnothorax ants raised their own young.

Additionally, “[Slave] workers attack and kill slavemaker queen pupae more often than male or worker pupae,” Foitzik says. Caretakers may be able to detect which young are destined to be queens by the particular molecules on the outsides of their bodies.

P. americanus ants are stuck with their slavemaking strategy. They rely on their enslaved workers, so they can’t simply free their captives and take care of their own young. As for the enslaved Temnothorax ants, it’s too late for them: whether they obediently raise their captors’ young or not, they can’t return to their own nests. And presumably they don’t have big enough brains to house thoughts of revenge. So why bother with the baby-killing?

The acts of rebellion by Temnothorax ants might protect their relatives in the future, the researchers say. By killing some of their captors’ young, especially the queens, the slaves ensure that the next generation of slavemakers will be a little smaller. The other Temnothorax nests in the neighborhood—which may include relatives who share the enslaved ants’ genes—will be that much safer from attack. The war will continue, but neither side is conceding.

Tobias Pamminger, Annette Leingärtner, Alexandra Achenbach, Isabelle Kleeberg, Pleuni S. Pennings, & Susanne Foitzik (2012). Geographic distribution of the anti-parasite trait “slave rebellion” Evolutionary Ecology DOI: 10.1007/s10682-012-9584-0

Image: Achenbach and Foitzik 2009.