Chapter 6. Evolution of the Brain and Behavior
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By Kelli Whitlock Burton Evolutionarily speaking, we are born to make babies. Our bodies—and brains—don’t fall apart until we come to the end of our child-bearing years. So why are grandmothers, who don’t reproduce and who contribute little to food production, still around and still mentally sound? A new study offers an intriguing genetic explanation. Scientists have proposed several explanations for why our species lives as long and as healthily as it does. One idea is that grandmothers help out with child rearing. A 1998 study found, for example, that a Hadza group of hunter-gatherers in Tanzania had more babies if grandmothers helped feed their newly-weaned young grandchildren. The researchers speculated this kind of care freed up young mothers to reproduce, and ensured that the caregiver grandmother’s genes were passed on to more young. They called their theory the “grandmother hypothesis.” But grandmothers need to have all their wits about them to help out in this way, and the new study may explain how this happens. Physician-scientist Ajit Varki and evolutionary biologist Pascal Gagneux of the University of California, San Diego, arrived at the findings accidentally. The pair was studying a gene that helps control the body’s inflammatory and immune response to injury or infection. Previous studies have linked two forms of the gene—CD33—to Alzheimer’s disease. While one CD33 variant, or allele, predisposes a person to the disease, the other appears to protect against it by preventing the formation of protein clumps in the brain. © 2015 American Association for the Advancement of Science.
By Diana Kwon The human brain is unique: Our remarkable cognitive capacity has allowed us to invent the wheel, build the pyramids and land on the moon. In fact, scientists sometimes refer to the human brain as the “crowning achievement of evolution.” But what, exactly, makes our brains so special? Some leading arguments have been that our brains have more neurons and expend more energy than would be expected for our size, and that our cerebral cortex, which is responsible for higher cognition, is disproportionately large—accounting for over 80 percent of our total brain mass. Suzana Herculano-Houzel, a neuroscientist at the Institute of Biomedical Science in Rio de Janeiro, debunked these well-established beliefs in recent years when she discovered a novel way of counting neurons—dissolving brains into a homogenous mixture, or “brain soup.” Using this technique she found the number of neurons relative to brain size to be consistent with other primates, and that the cerebral cortex, the region responsible for higher cognition, only holds around 20 percent of all our brain’s neurons, a similar proportion found in other mammals. In light of these findings, she argues that the human brain is actually just a linearly scaled-up primate brain that grew in size as we started to consume more calories, thanks to the advent of cooked food. Other researchers have found that traits once believed to belong solely to humans also exist in other members of the animal kingdom. Monkeys have a sense of fairness. Chimps engage in war. Rats show altruism and exhibit empathy. In a study published last week in Nature Communications, neuroscientist Christopher Petkov and his group at Newcastle University found that macaques and humans share brain areas responsible for processing the basic structures of language. © 2015 Scientific American
by Sarah Zielinski Call someone a “bird brain” and they are sure to be offended. After all, it’s just another way of calling someone “stupid.” But it’s probably time to retire the insult because scientists are finding more and more evidence that birds can be pretty smart. Consider these five species: We may call pigeons “flying rats” for their penchant for hanging out in cities and grabbing an easy meal. (Long before there was “pizza rat,” you know there had to be “pizza pigeons” flying around New York City.) But there may be more going on in their brains than just where to find a quick bite. Richard Levenson of the University of California, Davis Medical Center and colleagues trained pigeons to recognize images of human breast cancers. In tests, the birds proved capable of sorting images of benign and malignant tumors. In fact, they were just as good as humans, the researchers report November 18 in PLOS ONE. In keeping with the pigeons’ reputation, though, food was the reward for their performance. No one would suspect the planet’s second-best toolmakers would be small black birds flying through mountain forests on an island chain east of Australia. But New Caledonian crows have proven themselves not only keen toolmakers but also pretty good problem-solvers, passing some tests that even dogs (and pigeons) fail. For example, when scientists present an animal with a bit of meat on a long string dangling down, many animals don’t ever figure out how to get the meat. Pull it up with one yank, and the meat is still out of reach. Some animals will figure out how to get it through trial and error, but a wild New Caledonian crow solved the problem — pull, step on string, pull some more — on its first try. © Society for Science & the Public 2000 - 2015
Human DNA is 1 to 2% Neandertal, or more, depending on where your ancestors lived. Svante Pääbo, founder of the field of paleogenetics and winner of a 2016 Breakthrough Prize, explains why that matters © 2015 Scientific American
Jon Hamilton Patterns of gene expression in human and mouse brains suggest that cells known as glial cells may have helped us evolve brains that can acquire language and solve complex problems. Scientists have been dissecting human brains for centuries. But nobody can explain precisely what allows people to use language, solve problems or tell jokes, says Ed Lein, an investigator at the Allen Institute for Brain Science in Seattle. "Clearly we have a much bigger behavioral repertoire and cognitive abilities that are not seen in other animals," he says. "But it's really not clear what elements of the brain are responsible for these differences." Research by Lein and others provides a hint though. The difference may involve brain cells known as glial cells, once dismissed as mere support cells for neurons, which send and receive electrical signals in the brain. Lein and a team of researchers made that finding after studying which genes are expressed, or switched on, in different areas of the brain. The effort analyzed the expression of 20,000 genes in 132 structures in brains from six typical people. Usually this sort of study is asking whether there are genetic differences among brains, Lein says. "And we sort of flipped this question on its head and we asked instead, 'What's really common across all individuals and what elements of this seem to be unique to the human brain?' " he says. It turned out the six brains had a lot in common. © 2015
Ewen Callaway A long stretch of DNA called a supergene explains the variety of bizarre tactics that a wading bird species deploys to win mates, a pair of genome-sequencing studies concludes1, 2. Common to marshes and wet meadows in northern Europe and Asia, ruffs (Philomachus pugnux) are named after the decorative collars popular in Renaissance Europe. But the birds’ poufy plumage is not the only baroque aspect of their biology. Males gather at mass breeding grounds where they juke, jump and lunge toward other males, in hopes of winning females. Male ruffs belong to one of three different forms, each with a unique approach to mating. 'Independent' males, with hodgepodge of brown and black neck feathers, are territorial and defend their bit of the breeding ground. White-feathered 'satellite' males, by contrast, invade the turf of independents to steal nearby females. A third, rarer form, called 'faeders' (Old English for father), take advantage of their resemblance to female ruffs to interrupt coital encounters. “They dash in and jump on the female before the territorial males does,” says Terry Burke, an evolutionary biologist at University of Sheffield, UK. “My colleague describes this as the 'sandwich'. You end up with the territorial male jumping on the back of the mimic.” Burke was part of a team that, in 1995, found that the different approaches of male ruffs were caused by a single inherited factor3. But it seemed improbable that one gene could trigger such wide-ranging differences in behaviour and appearance. © 2015 Nature Publishing Group
By Virginia Morell Plunge a live crab into a pot of boiling water, and it’s likely to try to scramble out. Is the crab’s behavior simply a reflex, or is it a sign of pain? Many scientists doubt that any invertebrate (or fish) feels pain because they lack the areas in the brain associated with human pain. Others argue this is an unfair comparison, noting that despite the major differences between vertebrate and invertebrate brains, their functions (such as seeing) are much the same. To get around this problem, researchers in 2014 argued that an animal could be classified as experiencing pain if, among other things, it changes its behavior in a way that indicates it’s trying to prevent further injury, such as through increased wariness, and if it shows a physiological change, such as elevated stress hormones. To find out whether crabs meet these criteria, scientists collected 40 European shore crabs (Carcinus maenas), shown in the photo above, in Northern Ireland. They placed the animals into individual tanks, and gave half 200-millisecond electrical shocks every 10 seconds for 2 minutes in their right and left legs. The other 20 crabs served as controls. Sixteen of the shocked crabs began walking in their tanks, and four tried to climb out. None of the control crabs attempted to clamber up the walls, but 14 walked, whereas six didn’t move at all. There was, however, one big physiological difference between the 16 shocked, walking crabs and the 14 control walkers, the scientists report in today’s issue of Biology Letters: Those that received electrical jolts had almost three times the amount of lactic acid in their haemolymph, a fluid that’s analogous to the blood of vertebrates—a clear sign of stress. Thus, crabs pass the bar scientists set for showing that an animal feels pain. © 2015 American Association for the Advancement of Science.
By Jason G. Goldman When a monkey has the sniffles or a headache, it doesn't have the luxury of popping a few painkillers from the medicine cabinet. So how does it deal with the common colds and coughs of the wildlife world? University of Georgia ecologist Ria R. Ghai and her colleagues observed a troop of more than 100 red colobus monkeys in Uganda's Kibale National Park for four years to figure out whether the rain forest provides a Tylenol equivalent. Monkeys infected with a whipworm parasite were found to spend more time resting and less time moving, grooming and having sex. The infected monkeys also ate twice as much tree bark as their healthy counterparts even though they kept the same feeding schedules. The findings were published in September in the journal Proceedings of the Royal Society B. The fibrous snack could help literally sweep the intestinal intruder out of the simians' gastrointestinal tracts, but Ghai suspects a more convincing reason. Seven of the nine species of trees and shrubs preferred by sick monkeys have known pharmacological properties, such as antisepsis and analgesia. Thus, the monkeys could have been self-medicating, although she cannot rule out other possibilities. The sick individuals were, however, using the very same plants that local people use to treat illnesses, including infection by whipworm parasites. And that “just doesn't seem like a coincidence,” Ghai says. © 2015 Scientific American,
Angus Chen English bursts with consonants. We have words that string one after another, like angst, diphthong and catchphrase. But other languages keep more vowels and open sounds. And that variability might be because they evolved in different habitats. Consonant-heavy syllables don't carry very well in places like windy mountain ranges or dense rainforests, researchers say. "If you have a lot of tree cover, for example, [sound] will reflect off the surface of leaves and trunks. That will break up the coherence of the transmitted sound," says Ian Maddieson, a linguist at the University of New Mexico. That can be a real problem for complicated consonant-rich sounds like "spl" in "splice" because of the series of high-frequency noises. In this case, there's a hiss, a sudden stop and then a pop. Where a simple, steady vowel sound like "e" or "a" can cut through thick foliage or the cacophony of wildlife, these consonant-heavy sounds tend to get scrambled. Hot climates might wreck a word's coherence as well, since sunny days create pockets of warm air that can punch into a sound wave. "You disrupt the way it was originally produced, and it becomes much harder to recognize what sound it was," Maddieson says. "In a more open, temperate landscape, prairies in the Midwest of the United States [or in Georgia] for example, you wouldn't have that. So the sound would be transmitted with fewer modifications." © 2015 npr
Natasha Gilbert The eye-catching plumage of some male songbirds has long been explained as a result of sexual selection: brighter males compete more successfully for mates, so evolution favours their spread. Females, by contrast, remain drab. A new study turns this explanation on its head. Sexual-selection pressures drive females to evolve dull feathers more strongly than they drive males to become colourful, argues James Dale, an evolutionary ecologist at Massey University in Auckland, New Zealand. That surprising conclusion is based on a data set of plumage colour in nearly 6,000 songbirds, which Dale and his colleagues built. They used their data to ask how various potential evolutionary factors drive male and female plumage colour. If a particular songbird species was polygynous (that is, the males had more than one mate), displayed a large difference in size between males and females, and left care of the young mainly up to the females, then the researchers judged that sexual selection was likely to be an important factor in that species' evolution. The study, published in Nature1, found that sexual selection does play an important role in creating colour differences between male and female plumage. But the contrast is largely driven by females evolving to become drab. “Females are the chief architect of the difference,” says Dale. © 2015 Nature Publishing Group
By Hanae Armitage Schools of fish clump together for a very simple reason: safety in numbers. But for some, banding together offers more than just protection. It’s a way of getting to the head of the class. Schooling fish learn from each other, and new research shows that when they’re taken out of their normal social group, individuals struggle to learn on their own. Scientists have long known that schooling fish observe and learn from each other’s failures and successes, behaviors that stimulate neural development, especially in the part of the brain responsible for memory and learning. But this is the first time they have found evidence of that link in spatial learning. To test their theory, scientists divided a school of social cichlid fish into two categories: 14 social fish and 15 loners. Researchers kept the social fish grouped together while they partitioned the loners into single-fish isolation tanks. They ran both groups through a simple T-shaped maze, color coding the side that harbored food—a yellow mark for food, a green mark for no food. Seven of the 14 socialized fish learned to associate yellow with food (high marks for the cichlids, which are not the brightest fish in the animal kingdom), whereas only three of the 15 isolated fish successfully made the same association. Writing in this month’s issue of Applied Animal Behaviour Science, the researchers say this suggests fish in group settings are able to learn better and faster than their singled out counterparts. The moral? Simple: Fish should stay in school. © 2015 American Association for the Advancement of Science
Carl Zimmer In recent years, a peculiar sort of public performance has taken place periodically on the sidewalks of Seattle. It begins with a woman named Kaeli N. Swift sprinkling peanuts and cheese puffs on the ground. Crows swoop in to feed on the snacks. While Ms. Swift observes the birds from a distance, notebook in hand, another person walks up to the birds, wearing a latex mask and a sign that reads “UW CROW STUDY.” In the accomplice’s hands is a taxidermied crow, presented like a tray of hors d’oeuvres. This performance is not surreal street theater, but an experiment designed to explore a deep biological question: What do crows understand about death? Ms. Swift has been running this experiment as part of her doctoral research at the University of Washington, under the guidance of John M. Marzluff, a biologist. Dr. Marzluff and other experts on crow behavior have long been intrigued by the way the birds seem to congregate noisily around dead comrades. Dr. Marzluff has witnessed these gatherings many times himself, and has heard similar stories from other people. “Whenever I give a talk about crows, there’s always someone who says, ‘Well, what about this?’ ” he said. Dr. Marzluff and Ms. Swift decided to bring some scientific rigor to these stories. They wanted to determine whether a dead crow really does trigger a distinctive response from living crows and, if so, what the purpose of the large, noisy gatherings might be. To run the experiment, Ms. Swift began by delivering food to a particular spot each day, so that the crows learned to congregate there to eat. Then one of her volunteers would approach the feast with a dead crow, and Ms. Swift observed how the birds reacted. © 2015 The New York Times Company
By Jon Cohen A virus that long ago spliced itself into the human genome may play a role in amyotrophic lateral sclerosis (ALS), the deadly muscle degenerative disease that crippled baseball great Lou Gehrig and ultimately took his life. That’s the controversial conclusion of a new study, which finds elevated levels of human endogenous retrovirus K (HERV-K) in the brains of 11 people who died from the disease. “This certainly is interesting and provocative work,” says Raymond Roos, a neurologist at the University of Chicago in Illinois who treats and studies ALS but who was not involved with the finding. Still, even the scientists behind the work caution that more research is needed to confirm the link. “I’m very careful to say HERV-K doesn’t cause the disease but may play a role in the pathophysiology,” says study leader Avindra Nath, a neuroimmunologist at the National Institute of Neurological Disorders and Stroke in Bethesda, Maryland. “The darn thing is in the chromosomes to begin with. It’s going to be very hard to prove causation.” It was another retrovirus, HIV, that led Nath to first suspect a connection between viruses and ALS. In 2006, he was helping a patient control his HIV infection with antiretroviral drugs when he noticed that the man’s ALS also improved. “That intrigued me, and I looked in the ALS literature and saw that people had reported they could see reverse transcriptase in the blood.” Reverse transcriptase, an enzyme that converts RNA to DNA, is a hallmark of retroviruses, which use it to insert copies of their genes into chromosomes of their hosts. © 2015 American Association for the Advancement of Science
Are you good at picking someone out of a crowd? Most of us are better at recognising faces than distinguishing between other similar objects, so it’s long been suspected there’s something mysterious about the way the brain processes a face. Now further evidence has emerged that this is a special, highly evolved skill. A study of twins suggests there are genes influencing face recognition abilities that are distinct from the ones affecting intelligence – so it’s not that people who are good with faces just have a better memory, for instance. “The idea is that telling friend from foe was so important to survival that there was very strong pressure to improve that trait,” says Nicholas Shakeshaft of King’s College London. Previous studies using brain scanning have suggested there is a part of the brain dedicated to recognising faces, called the fusiform face area. But others have suggested this region may in fact just be used for discriminating between any familiar objects. Wondering if genetics could shed any light, Shakeshaft’s team tested more than 900 sets of UK twins – including both identical and non-identical pairs – on their face recognition skills. The ability turned out to be highly heritable, with identical twins having more similar abilities than fraternal ones. The same went for intelligence, which had earlier been tested as part of a long-running study. © Copyright Reed Business Information Ltd.
By Michael Balter Are some animals smarter than others? It’s hard to say, because you can’t sit a chimpanzee or a mouse down at a table for an IQ test. But a new study, in which scientists tested wild robins on a variety of skills, concludes that they do differ in the kind of “general intelligence” that IQ tests are supposed to measure. General intelligence is usually defined as the ability to do well on multiple cognitive tasks, from math skills to problem solving. For years, researchers have questioned whether measurable differences exist in humans and nonhumans alike. In humans, factors like education and socioeconomic status can affect performance. When it comes to animals, the problem is compounded for two main reasons: First, it is very difficult to design and administer tests that pick up on overall smarts instead of specific skills, such as the keen memories of food-hoarding birds or the fine motor skills of chimpanzees that make tools for finding insects in trees. Second, differences in animal test scores can depend on how motivated they are to perform. Because most experiments award would-be test-takers with food, an empty (or a full) stomach might be all it takes to skew the results. Thus, even studies that suggest variations in intelligence among mice, birds, and apes all carry the caveat that alternative explanations could be at play. To get around some of these limitations, a team led by Rachael Shaw, an animal behavior researcher at Victoria University of Wellington, turned to a population of New Zealand North Island robins for a new round of experiments. The robins live at the Zealandia wildlife sanctuary, a 225-hectare nature paradise in Wellington where more than 700 of the birds live wild and protected from predators in the middle of the city. © 2015 American Association for the Advancement of Science.
By JAMES GORMAN Among the deep and intriguing phenomena that attract intense scientific interest are the birth and death of the universe, the intricacies of the human brain and the way dogs look at humans. That gaze — interpreted as loving or slavish, inquisitive or dumb — can cause dog lovers to melt, cat lovers to snicker, and researchers in animal cognition to put sausage into containers and see what wolves and dogs will do to get at it. More than one experiment has made some things pretty clear. Dogs look at humans much more than wolves do. Wolves tend to put their nose to the Tupperware and keep at it. This evidence has led to the unsurprising conclusion that dogs are more socially connected to humans and wolves more self-reliant. Once you get beyond the basics, however, agreement is elusive. In order to assess the latest bit of research, published in Biology Letters Tuesday by Monique Udell at Oregon State University, some context can be drawn from an earlier experiment that got a lot of attention more than a decade ago. In a much publicized paper in 2003, Adam Miklosi, now director of the Family Dog Project, at Eotvos Lorand University in Budapest, described work in which dogs and wolves who were raised by humans learned to open a container to get food. Then they were presented with the same container, modified so that it could not be opened. Wolves persisted, trying to solve the unsolvable problem, while dogs looked back at nearby humans. At first glance it might seem to a dog lover that the dogs were brilliant, saying, in essence, “Can I get some help here? You closed it; you open it.” But Dr. Miklosi didn’t say that. He concluded that dogs have a genetic predisposition to look at humans, which could have been the basis for the intense but often imperfect communication that dogs and people engage in. © 2015 The New York Times Company
James Gorman If spiders had nightmares, the larvae of ichneumonid wasps would have to star in them. The wasp lays an egg on the back of an orb weaver spider, where it grows fat and bossy, and occupies itself with turning the spider into a zombie. As Keizo Takasuka and his colleagues point out in The Journal of Experimental Biology, this is a classic case of “host manipulation.” Using more colorful language, he described the larva turning the spider into a “drugged navvy.” The larva forces the spider to turn its efforts away from maintaining a sticky, spiral web to catch prey, and to devote itself to building a safe and sturdy web to serve as a home for the larva’s cocoon, in which it will transform itself into a wasp. This process was well known, but Dr. Takasuka and Kaoru Maeto at Kobe University, working with other Japanese researchers, wanted to explore how the wasp overlords controlled their spiders. They suspected that the larvae were co-opting a natural behavior of the spiders. Turning on a behavior already in the spiders’ repertoire would be much easier than controlling every step of modifying a sticky web. So they compared the cocoon web to one that the spiders themselves build to rest in when they are molting. It’s called a resting web. The similarities were striking. In both the resting and cocoon webs, the sticky, spiraling threads that make the webs of orb weavers so appealing were gone. Instead, the spokes of the web remained, decorated with fibrous spider silk that the researchers found reflected ultraviolet light. That would be a highly useful quality to warn away birds and some large insects from flying into the web because those creatures can see in the ultraviolet spectrum. The strength of the two silk webs was also similar. © 2015 The New York Times Company
Mo Costandi At some point back in deep time, a group of fish were washed into a limestone cave somewhere in northeastern Mexico. With no way out and little more than bat droppings to eat, the fish began to adapt to their new troglodytic lifestyle. Unable to see other members of their group in the dark, they lost their colourful pigmentation. Then they lost their eyesight, their eyes gradually got smaller, and then disappeared altogether. This was accompanied by a dramatic reduction in the size of the brain’s visual system. Yet, the question of why the blind cave fish lost its eyes and a large part of its brain remains unresolved. Now, biologists in Sweden believe they have found the answer. In new research published today, they report that loss of the visual system saves the fish a substantial amount of energy, and was probably key to their stranded ancestors’ survival. The blind cave fish Astyanax mexicanus is adapted to its subterranean environment in other ways. As its vision regressed, it became more reliant on smell and taste, and its taste buds grew larger and more numerous. They also developed an enhanced ability to detect changes in mechanical pressure, which made them more sensitive to water movements. Last year, Damian Moran of Lund University and his colleagues reported that blind cave fish eliminated the circadian rhythm in their metabolism during their course of evolution, and that this leads to a massive 27% reduction in their energy expenditure. This new study was designed test whether or not they lost their visual system for the same reason. © 2015 Guardian News and Media Limited
By JOHN NOBLE WILFORD Acting on a tip from spelunkers two years ago, scientists in South Africa discovered what the cavers had only dimly glimpsed through a crack in a limestone wall deep in the Rising Star Cave: lots and lots of old bones. The remains covered the earthen floor beyond the narrow opening. This was, the scientists concluded, a large, dark chamber for the dead of a previously unidentified species of the early human lineage — Homo naledi. The new hominin species was announced on Thursday by an international team of more than 60 scientists led by Lee R. Berger, an American paleoanthropologist who is a professor of human evolution studies at the University of the Witwatersrand in Johannesburg. The species name, H. naledi, refers to the cave where the bones lay undisturbed for so long; “naledi” means “star” in the local Sesotho language. In two papers published this week in the open-access journal eLife, the researchers said that the more than 1,550 fossil elements documenting the discovery constituted the largest sample for any hominin species in a single African site, and one of the largest anywhere in the world. Further, the scientists said, that sample is probably a small fraction of the fossils yet to be recovered from the chamber. So far the team has recovered parts of at least 15 individuals. “With almost every bone in the body represented multiple times, Homo naledi is already practically the best-known fossil member of our lineage,” Dr. Berger said. The finding, like so many others in science, was the result of pure luck followed by considerable effort. Two local cavers, Rick Hunter and Steven Tucker, found the narrow entrance to the chamber, measuring no more than seven and a half inches wide. They were skinny enough to squeeze through, and in the light of their headlamps they saw the bones all around them. When they showed the fossil pictures to Pedro Boshoff, a caver who is also a geologist, he alerted Dr. Berger, who organized an investigation. © 2015 The New York Times Company
Link ID: 21396 - Posted: 09.11.2015
Bill McQuay and Christopher Joyce Acoustic biologists who have learned to tune their ears to the sounds of life know there's a lot more to animal communication than just, "Hey, here I am!" or "I need a mate." From insects to elephants to people, we animals all use sound to function and converse in social groups — especially when the environment is dark, or underwater or heavily forested. "We think that we really know what's going on out there," says Dartmouth College biologist Laurel Symes, who studies crickets. But there's a cacophony all around us, she says, that's full of information still to be deciphered. "We're getting this tiny slice of all of the sound in the world." Recently scientists have pushed the field of bioacoustics even further, to record whole environments, not just the animals that live there. Some call this "acoustic ecology" — listening to the rain, streams, wind through the trees. A deciduous forest sounds different from a pine forest, for example, and that soundscape changes seasonally. Neuroscientist Seth Horowitz, author of the book The Universal Sense: How Hearing Shapes the Mind, is especially interested in the ways all these sounds, which are essentially vibrations, have shaped the evolution of the human brain. "Vibration sensitivity is found in even the most primitive life forms," Horowitz says — even bacteria. "It's so critical to your environment, knowing that something else is moving near you, whether it's a predator or it's food. Everywhere you go there is vibration and it tells you something." © 2015 NPR