Chapter 6. Evolution of the Brain and Behavior
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By CARL ZIMMER There are many things that make humans a unique species, but a couple stand out. One is our mind, the other our brain. The human mind can carry out cognitive tasks that other animals cannot, like using language, envisioning the distant future and inferring what other people are thinking. The human brain is exceptional, too. At three pounds, it is gigantic relative to our body size. Our closest living relatives, chimpanzees, have brains that are only a third as big. Scientists have long suspected that our big brain and powerful mind are intimately connected. Starting about three million years ago, fossils of our ancient relatives record a huge increase in brain size. Once that cranial growth was underway, our forerunners started leaving behind signs of increasingly sophisticated minds, like stone tools and cave paintings. But scientists have long struggled to understand how a simple increase in size could lead to the evolution of those faculties. Now, two Harvard neuroscientists, Randy L. Buckner and Fenna M. Krienen, have offered a powerful yet simple explanation. In our smaller-brained ancestors, the researchers argue, neurons were tightly tethered in a relatively simple pattern of connections. When our ancestors’ brains expanded, those tethers ripped apart, enabling our neurons to form new circuits. Dr. Buckner and Dr. Krienen call their idea the tether hypothesis, and present it in a paper in the December issue of the journal Trends in Cognitive Sciences. “I think it presents some pretty exciting ideas,” said Chet C. Sherwood, an expert on human brain evolution at George Washington University who was not involved in the research. Dr. Buckner and Dr. Krienen developed their hypothesis after making detailed maps of the connections in the human brain using f.M.R.I. scanners. When they compared their maps with those of other species’ brains, they saw some striking differences. © 2013 The New York Times Company
By Melissa Hogenboom Science reporter, BBC News An analysis of a Neanderthal's fossilised hyoid bone - a horseshoe-shaped structure in the neck - suggests the species had the ability to speak. This has been suspected since the 1989 discovery of a Neanderthal hyoid that looks just like a modern human's. But now computer modelling of how it works has shown this bone was also used in a very similar way. Writing in journal Plos One, scientists say its study is "highly suggestive" of complex speech in Neanderthals. The hyoid bone is crucial for speaking as it supports the root of the tongue. In non-human primates, it is not placed in the right position to vocalise like humans. An international team of researchers analysed a fossil Neanderthal throat bone using 3D x-ray imaging and mechanical modelling. This model allowed the group to see how the hyoid behaved in relation to the other surrounding bones. Stephen Wroe, from the University of New England, Armidale, NSW, Australia, said: "We would argue that this is a very significant step forward. It shows that the Kebara 2 hyoid doesn't just look like those of modern humans - it was used in a very similar way." He told BBC News that it not only changed our understanding of Neanderthals, but also of ourselves. "Many would argue that our capacity for speech and language is among the most fundamental of characteristics that make us human. If Neanderthals also had language then they were truly human, too." BBC © 2013
By JOHN NOBLE WILFORD Early in the 20th century, two brothers discovered a nearly complete Neanderthal skeleton in a pit inside a cave at La Chapelle-aux-Saints, in southwestern France. The discovery raised the possibility that these evolutionary relatives of ours intentionally buried their dead — at least 50,000 years ago, before the arrival of anatomically modern humans in Europe. These and at least 40 subsequent discoveries, a few as far from Europe as Israel and Iraq, appeared to suggest that Neanderthals, long thought of as brutish cave dwellers, actually had complex funeral practices. Yet a significant number of researchers have since objected that the burials were misinterpreted, and might not represent any advance in cognitive and symbolic behavior. Now an international team of scientists is reporting that a 13-year re-examination of the burials at La Chapelle-aux-Saints supports the earlier claims that the burials were intentional. The researchers — archaeologists, geologists and paleoanthropologists — not only studied the skeleton from the original excavations, but found more Neanderthal remains, from two children and an adult. They also studied the bones of other animals in the cave, mainly bison and reindeer, and the geology of the burial pits. The findings, in this week’s issue of Proceedings of the National Academy of Sciences, “buttress claims for complex symbolic behavior among Western European Neanderthals,” the scientists reported. William Rendu, the paper’s lead author and a researcher at the Center for International Research in the Humanities and Social Sciences in New York, said in an interview that the geology of the burial pits “cannot be explained by natural events” and that “there is no sign of weathering and scavenging by animals,” which means the bodies were covered soon after death. © 2013 The New York Times Company
Link ID: 19041 - Posted: 12.17.2013
By CARL ZIMMER Scientists have found the oldest DNA evidence yet of humans’ biological history. But instead of neatly clarifying human evolution, the finding is adding new mysteries. In a paper in the journal Nature, scientists reported Wednesday that they had retrieved ancient human DNA from a fossil dating back about 400,000 years, shattering the previous record of 100,000 years. The fossil, a thigh bone found in Spain, had previously seemed to many experts to belong to a forerunner of Neanderthals. But its DNA tells a very different story. It most closely resembles DNA from an enigmatic lineage of humans known as Denisovans. Until now, Denisovans were known only from DNA retrieved from 80,000-year-old remains in Siberia, 4,000 miles east of where the new DNA was found. The mismatch between the anatomical and genetic evidence surprised the scientists, who are now rethinking human evolution over the past few hundred thousand years. It is possible, for example, that there are many extinct human populations that scientists have yet to discover. They might have interbred, swapping DNA. Scientists hope that further studies of extremely ancient human DNA will clarify the mystery. “Right now, we’ve basically generated a big question mark,” said Matthias Meyer, a geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and a co-author of the new study. Hints at new hidden complexities in the human story came from a 400,000-year-old femur found in a cave in Spain called Sima de los Huesos (“the pit of bones” in Spanish). The scientific team used new methods to extract the ancient DNA from the fossil. “This would not have been possible even a year ago,” said Juan Luis Arsuaga, a paleoanthropologist at Universidad Complutense de Madrid and a co-author of the paper. © 2013 The New York Times Company
Barn owl nestlings recognise their siblings' calls, according to researchers. Instead of competing aggressively for food, young barn owls are known to negotiate by calling out. A team of scientists in Switzerland discovered that the owlets have remarkably individual calls. They suggest this is to communicate each birds' needs and identity in the nest. The findings were announced in the Journal of Evolutionary Biology by Dr Amelie Dreiss and colleagues at the University of Lausanne, Switzerland. Barn owls (Tyto alba) are considered one of the most widespread species of bird and are found on every continent except Antarctica. An average clutch size ranges between four and six eggs but some have been known to contain up to 12. Previous studies have highlighted how barn owl nestlings, known as owlets, negotiate with their siblings for food instead of fighting. While their parents search for food the owlets advertise their hunger to their brothers and sisters by calling out. "These vocal signals deter siblings from vocalizing and from competing for the prey at parental return," explained Dr Dreiss. "If there is a disagreement, they can escalate signal intensity little by little, always without physical aggression, until less hungry siblings finally withdraw from the contest." BBC © 2013
By Victoria Gill Science reporter, BBC News Great tits use different alarm calls for different predators, according to a scientist in Japan. The researcher analysed the birds' calls and found they made "jar" sounds for snakes and "chicka" sounds for crows and martens. This, he says, is the first demonstration birds can communicate vocally about the type of predator threatening them. The findings are published in the journal Animal Behaviour. From his previous observations, the researcher, Dr Toshitaka Suzuki, from the Graduate University for Advanced Studies in Kanagawa, found great tits appeared to be able to discriminate between different predators. To test whether they could also communicate this information, he placed models of three different animals that prey on nestlings - snakes, crows and martens - close to the birds' nest boxes. He then recorded and analysed the birds' responses. "Parents usually make alarm calls when they approach and mob the nest predators," said Dr Suzuki. "They produced specific 'jar' alarm calls for the snakes and the same 'chicka' alarm call in response to both the crows and martens," he said. But a closers analysis of the sounds showed the birds had used different "note combinations" in their crow alarm calls from those they had used for the martens. Dr Suzuki thinks the birds might have evolved what he called a "combinatorial communication system" - combining different notes to produce calls with different meanings. Since snakes are able to slither into nest boxes, they pose a much greater threat to great tit nestlings than other birds or mammals, so Dr Suzuki says it makes sense that the birds would have a specific snake alarm call. BBC © 2013
Ewen Callaway New genome sequences from two extinct human relatives suggest that these ‘archaic’ groups bred with humans and with each other more extensively than was previously known. The ancient genomes, one from a Neanderthal and one from a different archaic human group, the Denisovans, were presented on 18 November at a meeting at the Royal Society in London. They suggest that interbreeding went on between the members of several ancient human-like groups living in Europe and Asia more than 30,000 years ago, including an as-yet unknown human ancestor from Asia. “What it begins to suggest is that we’re looking at a ‘Lord of the Rings’-type world — that there were many hominid populations,” says Mark Thomas, an evolutionary geneticist at University College London who was at the meeting but was not involved in the work. The first Neanderthal1 and the Denisovan2 genome sequences revolutionized the study of ancient human history, not least because they showed that these groups interbred with anatomically modern humans, contributing to the genetic diversity of many people alive today. All humans whose ancestry originates outside of Africa owe about 2% of their genome to Neanderthals; and certain populations living in Oceania, such as Papua New Guineans and Australian Aboriginals, got about 4% of their DNA from interbreeding between their ancestors and Denisovans, who are named after the cave in Siberia’s Altai Mountains where they were discovered. The cave contains remains deposited there between 30,000 and 50,000 years ago. © 2013 Nature Publishing Group
Link ID: 18946 - Posted: 11.20.2013
By EMILY ANTHES Humans have no exclusive claim on intelligence. Across the animal kingdom, all sorts of creatures have performed impressive intellectual feats. A bonobo named Kanzi uses an array of symbols to communicate with humans. Chaser the border collie knows the English words for more than 1,000 objects. Crows make sophisticated tools, elephants recognize themselves in the mirror, and dolphins have a rudimentary number sense. Anolis evermanni lizards normally attack their prey from above. The lizards were challenged to find a way to access insects that were kept inside a small hole covered with a tightfitting blue cap. And reptiles? Well, at least they have their looks. In the plethora of research over the past few decades on the cognitive capabilities of various species, lizards, turtles and snakes have been left in the back of the class. Few scientists bothered to peer into the reptile mind, and those who did were largely unimpressed. “Reptiles don’t really have great press,” said Gordon M. Burghardt, a comparative psychologist at the University of Tennessee at Knoxville. “Certainly in the past, people didn’t really think too much of their intelligence. They were thought of as instinct machines.” But now that is beginning to change, thanks to a growing interest in “coldblooded cognition” and recent studies revealing that reptile brains are not as primitive as we imagined. The research could not only redeem reptiles but also shed new light on cognitive evolution. Because reptiles, birds and mammals diverged so long ago, with a common ancestor that lived 280 million years ago, the emerging data suggest that certain sophisticated mental skills may be more ancient than had been assumed — or so adaptive that they evolved multiple times. © 2013 The New York Times Company
By Tanya Lewis and LiveScience SAN DIEGO — Being a social butterfly just might change your brain: In people with a large network of friends and excellent social skills, certain brain regions are bigger and better connected than in people with fewer friends, a new study finds. The research, presented here Tuesday (Nov. 12) at the annual meeting of the Society for Neuroscience, suggests a connection between social interactions and brain structure. "We're interested in how your brain is able to allow you to navigate in complex social environments," study researcher MaryAnn Noonan, a neuroscientist at Oxford University, in England, said at a news conference. Basically, "how many friends can your brain handle?" Noonan said. Scientists still don't understand how the brain manages human behavior in increasingly complex social situations, or what parts of the brain are linked to deviant social behavior associated with conditions like autism and schizophrenia. Studies in macaque monkeys have shown that brain areas involved in face processing and in predicting the intentions of others are larger in animals living in large social groups than in ones living in smaller groups. To investigate these brain differences in humans, Noonan and her colleagues at McGill University, in Canada, recruited 18 participants for a structural brain-imaging study. They asked people how many social interactions they had experienced in the past month, in order to determine the size of their social networks. As was the case in monkeys, some brain areas were enlarged and better connected in people with larger social networks. In humans, these areas were the temporal parietal junction, the anterior cingulate cortex and the rostral prefrontal cortex, which are part of a network involved in "mentalization" — the ability to attribute mental states, thoughts and beliefs to another. © 2013 Scientific American
by Bob Holmes When it comes to evolution, there is no such thing as perfection. Even in the simple, unchanging environment of a laboratory flask, bacteria never stop making small tweaks to improve their fitness. That's the conclusion of the longest-running evolutionary experiment carried out in a lab. In 1988, Richard Lenski of Michigan State University in East Lansing began growing 12 cultures of the same strain of Escherichia coli bacteria. The bacteria have been growing ever since, in isolation, on a simple nutrient medium – a total of more than 50,000 E. coli generations to date. Every 500 generations, Lenski freezes a sample of each culture, creating an artificial "fossil record". This allows him to resurrect the past and measure evolutionary progress by comparing how well bacteria compete against each other at different points in the evolutionary process. No upper limit After 10,000 generations, Lenski thought that the bacteria might approach an upper limit in fitness beyond which no further improvement was possible. But the full 50,000 generations of data show that isn't the case. When pitted against each other in an equal race, new generations always grew faster than older ones. In other words, fitness never stopped increasing. Their results fit a mathematical pattern known as a power law, in which something can increase forever, but at a steadily diminishing rate. "Even if we extrapolate it to 2.5 billion generations, there's no obvious reason to think there's an upper limit," says Lenski. © Copyright Reed Business Information Ltd.
Link ID: 18937 - Posted: 11.16.2013
Helen Shen To researchers who study how living things move, the octopus is an eight-legged marvel, managing its array of undulating appendages by means of a relatively simple nervous system. Some studies have suggested that each of the octopus’s tentacles has a 'mind' of its own, without rigid central coordination by the animal’s brain1. Now neuroscientist Guy Levy and his colleagues at the Hebrew University in Jerusalem report that the animals can rotate their bodies independently of their direction of movement, reorienting them while continuing to crawl in a straight line. And, unlike species that use their limbs to move forward or sideways relative to their body's orientation, octopuses tend to slither around in all directions. The team presented its findings on 10 November at the annual meeting of the Society for Neuroscience in San Diego, California. The new description of octopus movement is “not how one would imagine that would happen, but it seems to give a lot of control to the animal", says Gal Haspel, a neuroscientist at the New Jersey Institute of Technology in Newark. Haspel studies worm locomotion, and he was also surprised by the researchers’ report that the octopus pushes itself with worm-like contractions of its tentacles. Different combinations flex together to produce movement in different directions. Levy, who began the research as part of a project to design and control flexible, octopus-like robots, says that the work could also help to uncover basic biological principles of locomotion. Levy’s team deconstructed octopus movement using a transparent tank rigged with a system of mirrors and video cameras, in which they tested nine adult common octopuses (Octopus vulgaris). © 2013 Nature Publishing Group
Ed Yong Humanity's success depends on the ability of humans to copy, and build on, the works of their predecessors. Over time, human society has accumulated technologies, skills and knowledge beyond the scope of any single individual. Now, two teams of scientists have independently shown that the strength of this cumulative culture depends on the size and interconnectedness of social groups. Through laboratory experiments, they showed that complex cultural traditions — from making fishing nets to tying knots — last longer and improve faster at the hands of larger, more sociable groups. This helps to explain why some groups, such as Tasmanian aboriginals, lost many valuable skills and technologies as their populations shrank. “For producing fancy tools and complexity, it’s better to be social than smart,” says psychologist Joe Henrich of the University of British Columbia in Vancouver, Canada, the lead author of one of the two studies, published today in Proceedings of the Royal Society B1. “And things that make us social are going to make us seem smarter.” “There were some theoretical models to explain these phenomena but no one had done experiments,” says evolutionary biologist Maxime Derex of the University of Montpellier, France, who led the other study, published online today in Nature2. Derex’s team asked 366 male students to play a virtual game in which they gained points — and eventually money — by building either an arrowhead or a fishing net. The nets offered greater rewards, but were also harder to make. The students watched video demonstrations of the two tasks in groups of 2, 4, 8 or 16, before attempting the tasks individually. Their arrows and nets were tested in simulations and scored. After each trial, they could see how other group members fared, and watch a step-by-step procedure for any one of the designs. © 2013 Nature Publishing Group
by Jennifer Viegas Music skills evolved at least 30 million years ago in the common ancestor of humans and monkeys, according to a new study that could help explain why chimpanzees drum on tree roots and monkey calls sound like singing. The study, published in the latest issue of Biology Letters, also suggests an answer to this chicken-and-egg question: Which came first, language or music? The answer appears to be music. "Musical behaviors would constitute a first step towards phonological patterning, and therefore language," lead author Andrea Ravignani told Discovery News. For the study, Ravignani, a doctoral candidate at the University of Vienna's Department of Cognitive Biology, and his colleagues focused on an ability known as "dependency detection." This has to do with recognizing relationships between syllables, words and musical notes. For example, once we hear a certain pattern like Do-Re-Mi, we listen for it again. Hearing something like Do-Re-Fa sounds wrong because it violates the expected pattern. Normally monkeys don't respond the same way, but this research grabbed their attention since it used sounds within their frequency ranges. In the study, squirrel monkeys sat in a sound booth and listened to a set of three novel patterns. (The researchers fed the monkeys insects between playbacks, so the monkeys quickly got to like this activity.) Whenever a pattern changed, similar to our hearing Do-Re-Fa, the monkeys stared longer, as if to say, "Huh?" © 2013 Discovery Communications, LLC.
by Sarah Zielinski If you put two birds together and gave them a problem, would they be any better at solving it than if they were alone? A study in Animal Behaviour of common mynas finds that not only are they no better at problem solving when in a pair than when on their own, the birds actually get a lot worse when put in a group. Andrea S. Griffin and her research team from the University of Newcastle in Callaghan, Australia, began by using dog food pellets as bait to capture common mynas (a.k.a. the Indian mynah, Acridotheres tristis) from around Newcastle. Then they gave each of the birds an innovation test, consisting of a box containing a couple of drawers and some Petri dishes. To get to the food hidden in spots in the box, the birds would have to get creative and figure out how to open one of the four containers by doing things like levering up a lid or pushing open a drawer. The scientists then ranked the birds by innovative ability before pairing them up. Half the pairs consisted of a high-innovation and a low-innovation myna, and the other half were pairs of medium-innovation birds. Then the pairs each received an innovation test similar to the one with boxes. Another experiment tested the birds in same-sex groups of five. On their own, 29 of 34 birds were able to access at least one container. But in pairs, only 15 of the 34 birds did so, and they took a lot longer. Performance dropped for both high- and medium-innovation birds, and it didn’t improve for the low-ranked ones, which had done so poorly the first time around that their results couldn’t get any worse. In groups of five, birds’ results fell even further: No mynas solved any of those tasks. © Society for Science & the Public 2000 - 2013
Where, exactly, does the sand flea have sex? On the dusty ground, where it spends the first half of its life? Or already nestled snugly in its host—such as in a human foot—where it can suck the blood it needs to nourish its eggs? The answer to this question, which has long puzzled entomologists and tropical health experts, seems to be the latter. A new study, in which a researcher let a sand flea grow inside her skin, concludes that the parasites most likely copulate when the females are already inside their hosts. Tunga penetrans, also known as the chigger flea, sand flea, chigoe, jigger, nigua, pique, or bicho de pé, is widespread in the Caribbean, South America, and sub-Saharan Africa. The immature female burrows permanently into the skin of a warm-blooded host—it also attacks dogs, rats, cattle, and other mammals—where over 2 weeks it swells up to many times its original size, reaching a diameter of up to 10 mm. Through a small opening at the end of its abdominal cone, the insect breathes, defecates, and expels eggs. The female usually dies after 4 to 6 weeks, still embedded in the skin. Native to the Caribbean, sand fleas infected crewmen sailing with Columbus on the Santa Maria after they were shipwrecked on Haiti. They and others brought the parasite back to the Old World, where it eventually became endemic across sub-Saharan Africa. Even today it is an occasional stowaway, showing up in European and North American travel clinics in the feet of tourists who have gone barefoot on tropical beaches. For people living in infested regions, however, the flea is a serious public health issue. What starts as a pale circle in the skin turns red and then black, becoming painful, itchy—and often infected, a condition called tungiasis. One flea seems to attract others, and people can be infested with dozens at once. © 2013 American Association for the Advancement of Science
by Tina Hesman Saey BOSTON— Siberians may use genes to stay warm, a new study shows. As part of an effort to catalog genetic diversity in Siberia, Alexia Cardona of the University of Cambridge and collaborators sampled DNA from 200 Siberians representing 10 native groups. The team looked for genes that have more changes in Siberians than would be expected by chance — a sign that the genes evolved rapidly in the 24,000 years since people settled the frigid land. Rapid changes suggest that a gene is important for adapting to an environment. Several of the Siberians’ genes have variants that may help keep Arctic dwellers warm during the long winters, Cardona reported October 24 at the annual meeting of the American Society of Human Genetics. Among the candidates for genetic heaters are genes involved in metabolizing fats. Some Siberian groups eat mostly meat, so genes that help convert animal fat to energy are important for creating heat. Another gene with variants unique to Siberians is called PRKG1; it helps regulate body heat by controlling muscle contraction and the constriction and dilation of blood vessels. Muscle contractions are an important part of shivering, which can raise body temperature. The researchers also identified variants in genes involved in thyroid function, which plays a role in temperature regulation. A. Cardona et al. Genome-wide analysis of cold adaption in indigenous Siberian populations. American Society of Human Genetics annual meeting, Boston, October 24, 2013. © Society for Science & the Public 2000 - 2013
Think fast. The deadly threat of snakes may have driven humans to develop a complex and specialized visual system. The sinuous shape triggers a primal jolt of recognition: snake! A new study of the monkey brain suggests that primates are uniquely adapted to recognize the features of this slithering threat and react in a flash. The results lend support to a controversial hypothesis: that primates as we know them would never have evolved without snakes. A tussle with a snake meant almost certain death for our preprimate ancestors. The reptiles slithered through the forests of the supercontinent Gondwana roughly 100 million years ago, squeezing the life out of the tiny rodent-sized mammalian ancestors of modern primates. About 40 million years later, likely after primates had emerged, some snakes began injecting poison, which made them an even deadlier and more immediate threat. Snakes were “the first and most persistent predators” of early mammals, says Lynne Isbell, a behavioral ecologist the University of California, Davis. They were such a critical threat, she has long argued, that they shaped the emergence and evolution of primates. By selecting for traits that helped animals avoid them, snakes ultimately endowed us with forward-facing eyes, for example, and enlarged visual centers deep in our brains that are specialized for picking out specific features in the world around us, such as the general shape of a snake’s body camouflaged among leaves. Isbell published her “Snake Detection Theory” in 2006. To support it, she showed that the rare primates that have not encountered venomous snakes in the course of their evolution, such as lemurs in Madagascar, have poorer vision than those that evolved alongside snakes. © 2013 American Association for the Advancement of Science
by Bethany Brookshire There are many animal species out there that exhibit same-sex mating behavior. This can take the form of courtship behaviors, solicitation, all the way through to mounting and trading off sperm). In some species, it’s clear that some of this behavior is because the animals involved have pair bonded. But what about insects? Many insects mate quickly, a one and done approach, with very little bonding involved beyond what’s needed to protect against other potential suitors. When it comes to bugs, is it intentional same-sex behavior? Or is it all a mistake? Hypotheses are out there, but in the end, we need science. A new study in the November Behavioral Ecology and Sociobiology wants to answer these questions. The authors did a meta-analysis of papers looking at same-sex sexual activity in male insects and arachnids. They tried to tease out why same-sex sexual behavior might occur in insects. What are the benefits? The potential downsides? And from that, to hypothesize why it might occur. Some of it, it turns out, could be due to context. A lot of observed same-sex mating behavior in insects is observed, for example, when the males are all housed together, away from the females. Partially because of this (but possibly for other reasons as well), same-sex sexual behavior in insects tends to occur much more frequently in the lab than in the wild. But it’s still often documented in the field. Why does it happen? Some say that by mating with a “passive” male and transferring sperm, that sperm then gets passed over to the female when the passive male mates. Sneaky. But does it really happen? And if it does, is it effective? So far, it doesn’t appear that it is; less than 0.5% of the offspring resulted from the transfer of sperm when these cases were documented. © Society for Science & the Public 2000 - 2013
If you were stung by a bark scorpion, the most venomous scorpion in North America, you’d feel something like the intense, painful jolt of being electrocuted. Moments after the creature flips its tail and injects venom into your skin, the intense pain would be joined by a numbness or tingling in the body part that was stung, and you might experience a shortness of breath. The effect of this venom on some people—small children, the elderly or adults with compromised immune systems—can even trigger frothing at the mouth, seizure-like symptoms, paralysis and potentially death. Based solely on its body size, the four-inch-long furry grasshopper mouse should die within minutes of being stung—thanks to the scorpion’s venom, which causes temporary paralysis, the muscles that allow the mouse to breathe should shut down, leading to asphyxiation—so you’d think the rodent would avoid the scorpions at all costs. But if you put a mouse and a scorpion in the same place, the rodent’s reaction is strikingly brazen. If stung, the four-inch-long rodent might jump back for a moment in surprise. Then, after a brief pause, it’ll go in for the kill and devour the scorpion piece by piece: This predatory behavior isn’t the result of remarkable toughness. As scientists recently discovered, the mouse has evolved a particularly useful adaptation: It’s immune to both the pain and paralytic effects that make the scorpion’s venom so toxic. Although scientists long knew that the mouse, native to the deserts of the American Southwest, preys upon a range of non-toxic scorpions, “no one had ever really asked whether they attack and kill really toxic scorpions,” says Ashlee Rowe of Michigan State University, who led the new study published today in Science.
Who would win in a fight: a bark scorpion or a grasshopper mouse? It seems like an easy call. The bark scorpion (Centruroides sculpturatus) delivers one of the most painful stings in the animal kingdom—human victims have compared the experience to being branded. The 25-gram grasshopper mouse (Onychomys torridus) is, well, a mouse. But as you can see in the video above, grasshopper mice routinely kill and eat bark scorpions, blissfully munching away even as their prey sting them repeatedly (and sometimes right in the face). Now, scientists have discovered why the grasshopper mice don’t react to bark scorpion stings: They simply don’t feel them. Evolutionary neurobiologist Ashlee Rowe at the University of Texas, Austin, has been studying the grasshopper mice’s apparent superpower since she was in graduate school. For the new study, she milked venom from nearly 500 bark scorpions and started experimenting. When she injected the venom into the hind paws of regular laboratory mice, the mice furiously licked the site for several minutes. But when she injected the same venom into grasshopper mice, they licked their paws for just a few seconds and then went about their business, apparently unfazed. In fact, the grasshopper mice appeared to be more irritated by injections of the saline solution Rowe used as a control. Rowe knew that grasshopper mice weren’t entirely impervious to pain—they reacted to injections of other painful chemicals such as formalin, just not the bark scorpion venom. To find out what was going on, she and her team decided to determine how the venom affects the grasshopper mouse’s nervous system, in particular the parts responsible for sensing pain. © 2013 American Association for the Advancement of Science