Lizard tails, gecko toes, and Mission: Impossible

In Mission: Impossible! Ghost Protocol (the fourth installment of the series), a memorable scene has the heroic--and nigh-indestructible--Ethan Hunt (Tom Cruise) climbing the outside of the Burj Khalifa, more than 1700 feet above the ground. Because it's Mission: Impossible, Hunt of course has no harness, rope, or other climbing gear--only a pair of adhesive gloves and some rubber-soled shoes. 

When tech wizard Benji (Simon Pegg) is giving Ethan the demo of how the gloves work, he emphasizes that to un-stick the glove from the window, Ethan needs to use a "rolling-off motion." Ethan, being the super-agent he is, quickly masters this rolling motion and proceeds to use the gloves to pull himself up the sheer glass wall of the skyscraper. 

(It probably goes without saying that, of course, one of the gloves' batteries die once he is past the point of no return, requiring him to finish the climb with only one miracle sticky glove, which manages to catch his entire body weight from a free-fall a short time later.)

As with most action movies, willing suspension of disbelief is required to truly appreciate the stunts that punctuate Ghost Protocol at predictable intervals. But in this case, the physics isn't actually that far-fetched: it's likely that the gloves are biomemetic--technology and engineering inspired by the design and/or function of biological systems. In this case, judging by their appearance and Benji's description of how they work, it's a safe assumption that the gloves are based on the biomechanics and biophysics that help geckos, flies, and other small animals scale walls, dangle from ceilings, and perform other impossible missions. 

The climbing abilities of various lizards have been an object of fascination at least since the time of Aristotle, but it's only within the last century or so that we've been able to start building a detailed understanding of how the structure of their feet enables those abilities. With the advent of advanced microscopy, computer modeling, and incredibly sensitive force detection tools, the mechanism of lizard toe adhesion has become more well understood.

A lizard climbing up a wall or across a ceiling has to have a way to "turn on" and "turn off" the stickiness of its feet. Always "on" and the lizard can't move; always "off" and it's stuck (heh) with the limitations of gravity like we poor bipeds. So how does it work? 

As with many seemingly miraculous everyday occurrences, lizards can thank physics for their selectively sticky feet. Like Benji's magic sticky gloves, lizard feet have a differential stress response: they respond differently to tension than to shear forces. When under tension, the adhesive force is strong; but subject it to a shearing or bending force and it weakens enough to detach. The "rolling motion" Ethan uses to un-stick his hand from the Burj Khalifa's glass produces that shearing force, and lizards create a similar type of motion in their feet when they walk. 

The notion of a material that responds differently to different kinds of force shouldn't be too unfamiliar: rope is strong under tension (pulling) but weak under compression or shear (sideways) forces, whereas many kinds of structural materials (girders etc.) are strong under compression but weaker under shear or tension forces. This is one of the reasons why earthquakes, tidal waves, and other disturbances that produce strong shear forces can cause massive amounts of structural damage, even though they generate less total force than the weight of the building itself pressing downward every day. 

But these are all examples of solid objects demonstrating internal strength or weakness, which is a far cry from adhesion (stickiness) between two different objects--and this is where the physics comes in. As it turns out, gecko feet owe their stickiness to the millions of tiny setae (projections) that make up the bottom surface of each toe--more than 14,000 per square millimeter. Because of their microscopic size, each seta experiences tiny, transient molecular forces known as van der Waals forces when it contacts another surface. 

Although van der Waals forces are among the weakest interactions between molecules, they nonetheless can provide a significant amount of adhesive force when applied to the millions of individual setae on each foot. In one clever study, Autumn et al calculated an adhesive force of more than five atmospheres--more than enough to hold a tiny lizard against the ceiling. 

What's especially fascinating about Autumn et al's results is that they were able to demonstrate that the adhesive forces were primarily the result of the size and shape of the setae, rather than their chemical composition: when they created simulated setae out of rubber and polyester resin, they observed the same adhesive properties as were present in the gecko toes. They were also able to demonstrate that the adhesive force increases significantly as the setae get smaller--which may be part of the reason this type of setal structure has been evolutionarily selected in so many climbing lizards.

The idea that large numbers of setae may have a survival advantage in some species is supported by the genomic work of Liu et al, who used genetic data to construct phylogenetic trees showing the evolution of a number of different lizard species. Their results showed that Gekko japonicus, the Schlegel's Japanese gecko, contain more copies of genes for the proteins that form setae than do other lizard species who have fewer setae per unit area. 

In other words, the species that have lots of very tiny setae have more copies of the gene than the species with fewer, larger setae or no setae at all, and the density of setae is closely related to the species' habit: G. japonicus is a typical gecko, with sticky feet that it uses to climb trees and walls and catch prey, and it has the most copies of the gene and the largest setal density; Anolis carolinensis, the green anole, has fewer copies of the gene and a lower setal density, but still demonstrates some wall-climbing ability; and Alligator sinensis, the Yangtze alligator, has only two copies of the gene and no setae--it lives primarily in water and does not climb. These patterns, together with the inferred timing of the genetic differentiation between the species, support the hypothesis that selection pressure drove the increase in the number of setae (and correspondingly stickier feet).  

Liu et al also investigated the evolutionary history of another well-known trait of many lizard species: caudal autotomy, or the ability to "shed" a still-wriggling tail to distract predators and buy time for an escape. Species that have this ability, such as G. japonicus and An. carolinensis, show positive selection pressure for genes associated with wound healing and cell growth, whereas species that don't shed their tails lack those positively selected genes. 

And now we come full-circle, because it turns out that there are a lot of similarities between the physics of gecko toes and the physics of tail autotomy: both rely on the characteristic strong-under-tension-but-weak-under-shear strain responses associated with the microscopic interfaces between surfaces. 

Just as a gecko's foot is covered in millions of tiny setae, which generate adhesive forces, the point at which its tail connects to its body also contains large numbers of tiny pillars of muscle that hold the tail in place but also allow for its quick release. At the point where it detaches during autotomy, the tail contains a roughly conical arrangement of tiny, mushroom-shaped pillars. The "cone" shape at the end of the tail interfaces with a corresponding "socket" shape on the lizard's body--in other words, the tail attaches to the body the way a phone cable attaches to its charging socket. 

Unlike a charging cable, though, the lizard's tail attachment is quite strong under tension--it won't detach if it's pulled straight backward away from its body. This helps prevent the tail from falling off under regular lizard behavior. But if the lizard moves its tail in just the right type of side-to-side motion, its tail separates from its body. 

Baban et al used a biomemetic fracture model to elucidate how this works in detail. They demonstrated that the geometry of the tail-body attachment, along with the specific type of motion the lizard uses to initiate detachment, allows a "fracture plane" to propagate through the tail attachment surface. As the plane propagates, the tail muscles detach from their corresponding partners in the lizard's body, and eventually the entire tail has detached.

By creating silicon-based models of the tail and its attachment socket, Baban et al were able to demonstrate that the arrangement of the micropillars contributes to the tail's behavior under different kinds of stress. In most situations, the flexibility and nanoscale adhesion between the micropillars and their corresponding sockets helps to stop any incipient fractures from propagating. As Ghatak describes in a summary of the Baban article, the flexibility ensures the stresses never build up enough to cause the tail to detach completely.

It's probably unlikely that Ethan Hunt will ever dangle from the Tokyo Skytree by a detachable rope-tail...but one never knows. 

Autumn, Kellar, Metin Sitti, Yiching A. Lang, Anne M. Peattie, Wendy R. Hansen, Simon Sponberg, Thomas W. Kenny, Ronald Fearing, Jacob N. Israelachvili, and Robert J. Full. 2002. "Evidence for van der Waals adhesion in gecko setae." Proc. Natl. Acad. Sci. 99 (19): 12252-12256. https://doi.org/10.1073/pnas.192252799.

Baban, Navajit S., Ajymurat Orozaliev, Sebastian Kirchhof, Christopher J. Stubbs, and Yong-Ak Song. 2022. "Biomimetic fracture model of lizard tail autotomy." Science 375 (6582): 770-774. https://doi.org/10.1126/science.abh1614.  

Ghatak, Animangsu. 2022. "How does a lizard shed its tail?" Science 375 (6582): 721-722. https://doi.org/10.1126/science.abn4949.

Liu, Yan, Qian Zhou, Yonjun Wang, Longhai Luo, Jian Yang, Linfeng Yang, Mei Liu, Yingrui Li, Tianmei Qian, Yuan Zheng, et. al. 2015. "Gekko japonicus genome reveals evolution of adhesive toe pads and tail regeneration." Nature Communications 6, 10033. https://doi.org/10.1038/ncomms10033.

On Unintentional Literary Alignment

Like many chronic readers, I'm frequently surprised--and grateful--when random reading selections conspire to give me exactly what I was looking for, even when I wasn't consciously aware of what that was. Sometimes, it's an excuse to grieve, laugh, or reflect; sometimes, it's exactly the advice or information I needed to make a big decision. And sometimes, it's just the articulation of an idea that's been subconsciously percolating, waiting for a chance to surface.

I recently finished three books (Finding the Mother Tree by Suzanne Simard, Braiding Sweetgrass by Robin Wall Kimmerer, and Walk Out, Walk On by Margaret J. Wheatley and Deborah Frieze) that I certainly did not select with any intention of a common theme. In fact, one of the three was a gift--a book I probably would not have found on my own. And yet, as I worked my way through them, it was impossible to escape the obvious: these books, taken together, tell a clear story: a story of resilience and hope out of grief, and a lesson and warning about the dangers of human arrogance, all of which feel especially critical in light of the various events unfolding in our world today.

Interconnectedness and Systems Thinking

The students come already knowing a lot about ecosystems and can identify an impressive list of plants. But when I ask how these plants take care of them, they cannot say...Laying open the soil is like a careful dissection and there is the same astonishment among the students at the orderly beauty of the organs, the harmony of how they rest against one another, form to function. --Robin Wall Kimmerer, Braiding Sweetgrass

In Braiding Sweetgrass, Robin Wall Kimmerer weaves her exploration of her Native American heritage with her work as a botanist and educator, sharing anecdotes and experiences that highlight the interdependence of the systems in which we live. Throughout her beautifully written narrative, she returns time and again to the idea that "all flourishing is mutual"--that no action is without consequence and no being (whether human, eagle, or maple tree) is truly independent. 

In one story, Kimmerer describes the change in her students' perspectives as they go "shopping" in a wetland and forest--gathering wood and bark with which to construct a wigwam, collecting fruit and water plants for food and fuel, and exploring the network of roots, fungal hyphae, and humus that make up the forest floor beneath their feet. Over the course of the trip, they build understanding of the relationships between all the members of an ecosystem, humans included, and start to see themselves as part of that network.

Suzanne Simard likewise explores how she came to understand this concept of interconnectedness in her powerful memoir Finding the Mother Tree. Combining scientific narrative with personal vignettes, she recounts her journey to understanding the importance of soil and fungi to the health of forests--from intuitive belief to robust, evidence-based principle by way of tenacious research and data analysis. 

Kimmerer and Simard both emphasize the idea that ecosystems, and the human societies that depend on them, cannot be truly understood by examining each part in isolation. While we certainly can learn a lot by studying an individual tree or fungus or bacterium, put them together in a forest and they become so much more than the sum of their parts. Unless we study the system as a whole, we miss critical understandings about the implications of our actions. This "systems approach," they argue, is equally essential to understanding the functioning of the natural world and to recognizing the impacts of human behaviors and choices. 

Both Kimmerer and Simard have scientific training and experience in botany and ecology, so it is perhaps not surprising that they would share some common experiences and revelations. For example, both describe how observing the networks and connections among fungal hyphae in forest soil contributed to their conception of the connections between ecosystem components. But what I found especially interesting was how this same theme of "threads of connection" showed up in a seemingly unrelated book I was reading at the same time: Walk Out, Walk On by Margaret J. Wheatley and Deborah Frieze. 

Walk Out, Walk On is presented as a series of case studies of people who choose to "walk out" of oppressive, exploitative, limiting systems and "walk on" to create thriving, dynamic, resilient communities that make optimal use of their available resources (natural and human). While these communities vary widely in location, resources, and challenges, a theme common to all is "start anywhere, follow it everywhere": just as you can map and understand the connections in an ecosystem by starting with a fungus or a tree or an insect and following its relationships, you can start to understand and address the challenges in a community by starting with one small piece and following where it leads. 

Taken together, these three narratives convey the clear message that we cannot survive alone--none of us is truly independent, no matter what our modern mythology would have us believe. "All flourishing is mutual": either we all thrive, or none of us truly can. 

Arrogance and Oversimplification

We emphasize domination and competition in the management of trees in forests. And crops in agricultural fields. And stock animals on farms. We emphasize factions instead of coalitions.  --Suzanne Simard, Finding the Mother Tree

Given the emphasis on relationship, connection, and complexity in these three books, it's probably not surprising that another common theme relates to the dangers of oversimplifying, of arrogantly assuming we understand a system when we look only at its surface. This is the harm that comes from assuming we know the solution to a problem when we haven't taken the time to listen or study with an open mind or to fully explore the complexity of an ecosystem or community. 

In Finding the Mother Tree, Simard shares her growing discomfort with the then-common forestry practice of focusing solely on tree counts when determining appropriate restoration practices for logging areas. By (over)simplifying a forest to the number of individual trees present, these practices emphasized simply replacing harvested trees with an equivalent number of seedlings--with those seedlings being selected for economic value and regrowth potential rather than compatibility with the local microbiome or ecosystem. These practices, if they didn't quite cause more harm than good, certainly didn't live up to the goals of "restoring" the forest to a sustainable state--Simard recounts visiting countless "replanted" sites only to find the seedlings dead, dying, or barely surviving. 

As a member of the Citizen Potawatomi Nation, Robin Wall Kimmerer has personal and generational experience with the dangers of human arrogance; the history of Native peoples in North America is a testament to the irreparable harm that comes from assuming there is one "right" way to live or to be and that we know what that is. And her training as a botanist--like Suzanne Simard's training in forestry--gave her insight into the oversimplification and essentialism that can dominate many scientific endeavors. In one vignette, she describes how her freshman advisor, hearing that she wanted to study botany to understand why asters and goldenrod look beautiful together, lectures her that botany is "not about that." Botany is science--and science, even when it is studying interconnected, living systems--is not about the beauty or complexity of those systems, but about breaking them down into their component parts. That's the "right" way to do science--the right reason to do science--and therefore surely the superior mentality to take toward living systems. 

And of course this arrogance, and the harm it causes, is not limited to scientific endeavors or ecological study. Throughout Walk Out, Walk On, Margaret J. Wheatley and Deborah Frieze illustrate the value of local, community-driven innovation in addressing seemingly intractable problems. Indeed, the examples they share--from "arborloo" toilets in Zimbabwe that use waste to fertilize food-giving trees to "art of hosting" shared leadership communities in Columbus, Ohio--dramatically illustrate the limitations of looking for top-down, universal solutions. Time and again, the communities they highlight have faced initiatives imposed from above or outside, without consideration of the nuances or needs of the community, and time and again those initiatives have failed to lead to lasting change or have even made things worse. 

Diversity and Resilience

We create healthy and resilient communities by relying on the wisdom and wealth available in our people, traditions and environment...[we] follow eight principles [that form] a powerful and coherent theory for how to foster systemic change and create healthy and resilient community: start anywhere, follow it everywhere; we make our path by walking it; we have what we need; the leaders we need are already here; we are living the worlds we want today; we walk at the pace of the slowest; we listen, even to the whispers; we turn to one another. --Margaret J. Wheatley and Deborah Frieze, Walk Out, Walk On

Suzanne Simard provides a vivid illustration of the importance of looking beyond limited, short-term outcomes in her discussion of the "free to grow" policy guiding Canadian forestry practice for most of her career. The "free to grow" approach prioritizes short-term benefit according to a limited number of quantitative metrics over long-term sustainability. 

In the free-to-grow perspective, competition is the dominant influence on the growth of (economically valuable) conifers. Underbrush, "weeds," broadleaf trees such as aspens (which are less useful industrially), and dead wood in a forest are at best useless and at worst detrimental to the success of the "important" conifers. The free-to-grow practice therefore encouraged clear-cutting, weeding, thinning, and otherwise imposing uniformity on previously diverse forest ecosystems--all in the name of (short-term) productivity. 

Through her research, Simard compelling demonstrated that while free-to-grow plots can, under certain circumstances, produce higher yields in the short term, overall the practice reduces the health and stability of the ecosystems. Broadleaf aspens provide shade and protection from both sunlight and frost; dead wood restores soil nutrients and supports mycorrhizal networks that are critical for soil stability and tree health; understory growth enriches the soil and stabilizes microbial systems. Diversity is sustainable and resilient--and ultimately, therefore, more valuable in the long term. 

The idea that a diverse forest ecosystem would be more resilient and stable than a monoculture should be unsurprising to anyone with even a basic familiarity with ecology, but as with other crosscutting themes in these books, it's not limited to one context. 

In Braiding Sweetgrass, Robin Wall Kimmerer describes the "three sisters" gardening style common to many Native and Indigenous cultures in the Americas: by planting beans, corn, and pumpkins or squash together in the same plot, we can take advantage of their complementary growth habits and structures. Corn sprouts quickly and grows tall and straight, forming a scaffold for the beans. Squash grows low to the ground, shading the soil to retain moisture and reduce weed growth. The nitrogen-fixing bacterial symbiotes that live within the beans' root structures contribute essential nitrogen to the soil, benefiting all three plants. This ancient technique would never work in a monoculture- and efficiency-focused industrial agriculture setting--but it also doesn't require as much energy, water, or fertilizer as those monocultures do. 

The communities highlighted in Walk Out, Walk On--from Mexico to Greece, from India to South Africa--likewise demonstrate the power and value of honoring diversity in perspectives, skills, gifts, and traditions. In Brazil, for example, the Elos Institute uses the concept of "serious play" to harness the creativity and talents of community members--Guerreros Sem Armas or "Warriors Without Weapons"--to convert dangerous, decaying warehouses in Paquetá into a joy-filled community center. This project involves children, grandparents, architects, teachers, students, and middle-school dropouts, and that diversity allows the development of a richer and more robust design. 

If all three of these narratives illustrate the harms caused by arrogance and oversimplification, they equally demonstrate how to solve those harms: through respect for and investment in diverse, complex systems. These systems provide resilience in the rapidly changing world we face today. I hope you'll find the time to read or listen to each of these books and explore the many other common themes therein.

Links for the week of 2/8/2010

Physical sciences:
Scientists from the Weizmann Institute of Science in Rehovot, Israel figure out how to get water to freeze at different temperatures by modifying the electric charge on the surface it is sitting on. (ScienceNews)

Plastic water? misc.ience describes how scientists are able to make hydrogels that retain their shape, but are made almost entirely out of water.

Biology:
Researchers at the University of Maryland shed light on how Egyptian bats track their prey. Rather than firing sound waves directly at it, they shoot to either side. This makes them less likely to locate prey, but once they have found it, they can follow it more accurately. (EcoTone)

Beware mussels bearing "gifts": Neuroskeptic describes a study of amnesia caused by toxins in shellfish.

It's now fairly common knowledge that bees dance to tell other bees where to find food. But a recent study in Current Biology shows that they also use short buzzes to tell each other not to go to a dangerous location. Ed Yong at Not Exactly Rocket Science describes the study.

Female crickets can apparently warn their young of environmental dangers: baby crickets born to mothers hunted by wolf spiders are more likely to freeze and hide when they detect the spiders. (Not Exactly Rocket Science)

Sociology/human psychology:
Also from Neuroskeptic: A study of whether antipsychotic medication can reduce psychotic experiences in marijuana users.

If you want to encourage altruism, lead by example...and cleaning that bathroom might not hurt, either. A recent study suggests that watching other people perform good deeds increases the observer's altruistic tendencies. Interestingly, smells associated with cleaning also seem to increase altruism. (Psych Central; Not Exactly Rocket Science)

Daniel Hawes at Ingenious Monkey-20 two 5 has an excellent pair of posts on factors affecting girls' success in math.

A number of bloggers have written about Inuk, an ancient Greenlander whose entire genome was recently sequenced. Gene Expression describes the genetic relationships between Inuk's people (which anthropologists call the Saqqaq) and other human groups. Ed Yong describes what we know about his appearance, and how we know it.

John Tierney of The New York Times describes a sociological study conducted using the Times' own records. As it turns out, articles that inspire awe and those that deal with complex topics are the most likely to be forwarded on. (I wonder if there might be selection bias--perhaps readers of The New York Times are more likely to be interested in complex or awesome topics?)

Is religion necessary for morality? A common belief (for lack of a better word) is that religions originally developed to provide a basis for morality--i.e., to give the members of the society rules to follow to keep the society functioning. A recent analysis of studies in moral psychology, however, suggest that religious training and beliefs do not affect how people make moral decisions. Instead, the authors suggest, religion may have filled other needs in early society, such as the need to feel in control of one's surroundings. (björn brembs blog)

Links for the week of 2/1/2010

Since I don't seem able to put together a daily links post, maybe weekly will be more manageable. Within each group, links are posted in approximately reverse chronological order (most recent first). (Yes, I know some of these are from before Feb. 1. I never said what the error bars were on that date.)

First, DINOSAURS!:
Fossil Feather Colors Really ARE Written in Stone (Living the Scientific Life)
The renaissance of technicolour dinosaurs continues (and the gloves come off...) (Not Exactly Rocket Science)
Oldest feathered dino shows its colors (Science News)
Newly Described Bird-Like Dinosaur Predates Archaeopteryx by 15-20 Million Years (Living the Scientific Life)

Next: Running...ur doin it rong...(maybe):
Evo. Anthro. Study Suggests You Might Be Running Wrong (Laelaps)
New Nature Magazine Cover Story Shines More Light on Barefootin' (Runner's World Peak Performance)

How to not be annoying at the gym, courtesy of Peter at Obesity Panacea:
Appropriate Gym Etiquette
Annoying Gym Personalities
What to Wear


And finally, assorted other interesting things:
Seven habits of highly successful toads (Not Exactly Rocket Science)
Friday Weird Science: Preserving the Species (Neurotopia)
Un-Natural Disasters (In Terra Veritas)
Backyard Chickens: An Art, A Science, A Social Movement (Food Politics)
Dave Munger (formerly of the Cognitive Daily) has launched a new blog, The Daily Monthly. It's awesome.
Bees can learn to discriminate human faces (Arthropoda)
Playing to Learn (NYTimes Op/Ed)
Looking inside the structure of the Yellowstone caldera (Eruptions)
Power source for a light saber (Dot Physics)

Why migrate?

Growing up in New Hampshire, I took the yearly migration of ducks, geese, and (less obviously, but more impressively) Monarch butterflies pretty much for granted. It never really occurred to me to ask why all of these animals migrate. If I had been asked, I probably would have made the (common) anthropomorphic fallacy and said that they migrate because they "want to," or because they "like it better" at their ultimate destination.

Think about it: migration--especially very long-distance migration, such as that performed by some shorebirds, which can migrate from the southern tip of Africa all the way to the Arctic--is incredibly resource-intensive. It takes a long time, requires enormous amounts of energy, and is pretty dangerous. For a behavior like that to survive and develop in a population, it must provide significant survival or reproductive benefits. There are three main hypothesis about what those benefits might be:

1. Increases in food resources. For example, migrating south might allow birds to avoid competing for the limited food available in New England in the winter.
2. Reduction in parasite load. For example, migrating out of an area during a parasite's main breeding season might allow a bird to avoid infestation by the parasite.
3. Reduction in predation pressure. For example, migrating to different areas during different times of year might allow birds (or their eggs or hatchlings) to avoid attacks by predators that are common during those times.

It's possible to test these different hypotheses by looking at exactly where different populations migrate to. For example, consider those shorebirds I just mentioned. They migrate to high northern latitudes from the southern parts of Africa. The range of latitudes to which they migrate is wide; some stop just below the Arctic circle, but others keep going nearly to the North Pole. Previous studies have shown increased food availability and reduced parasite loads at these high latitudes. However, until now, there have been few studies that produced quantitative data on how migration site affects predation risk. In the 15 January issue of Science, Gilg and Yoccoz and McKinnon et al offer substantial evidence to support the idea that migrating to high latitudes offers significant benefits in terms of avoiding predators.

McKinnon et al placed more than 1500 artificial shorebird nests at various locations in northern Canada, over a latitude range of about 3350 km. They monitored the nests for two or more summers and recorded how well they survived predation.

The result? For every one degree further northward a nest was placed, the risk of predation on the nest decreased by about 3.6%. Over the range of latitudes they studied, that translates to 65% lower predation on the northernmost sites than on the southernmost sites. This is a significant reduction, and suggests that predation may indeed play an important role in driving bird migration.

However, other studies of predation risk at different latitudes didn't show such a clear trend. McKinnon et al suggest that this might be because those other studies used real nests, which vary in size, health, etc. In other words, previous trials were not as well controlled as was this investigation. By using artificial nests, the researchers reduced other potentially confounding factors.

Gilg and Yoccoz add to the story by suggesting that an important factor influencing predation on the nests is the distribution of another common prey species, the lemming. Lemmings and shorebirds (and shorebird eggs) are common prey for the Arctic fox. By comparing the distributions of the lemmings and the shorebirds, Gilg and Yoccoz show that the shorebirds most commonly hunted by foxes are typically common only where lemmings also occur. They hypothesize that, in areas without lemming populations, the predation pressure on the shorebirds is too high for their populations to survive.

Together, these two articles indicate that there are many more factors influencing bird migration patterns than simply "because the birds like it better there."

Gilg, O., & Yoccoz, N. (2010). Explaining Bird Migration Science, 327 (5963), 276-277 DOI: 10.1126/science.1184964

McKinnon, L., Smith, P., Nol, E., Martin, J., Doyle, F., Abraham, K., Gilchrist, H., Morrison, R., & Bety, J. (2010). Lower Predation Risk for Migratory Birds at High Latitudes Science, 327 (5963), 326-327 DOI: 10.1126/science.1183010

New NSF-funded site on science education

I just got this update from a friend (via LinkedIn):

A new NSF-funded site has just launched. According to the release:

"[The site is] dedicated to cataloging best practices in media-based science education and getting the word out about--and discussing--innovative new media-based science education programs and concepts (where media includes both traditional and new media). The site is located at:

http://www.mediasciencelearning.com/

To celebrate its launch (and help get its discussion areas quickly up to critical mass), the grantees (at their own expense, not NSF's) are giving away Zingerman's gift certificates to the best (and most prolific) contributors to the site's case discussions, located at:

http://www.mediasciencelearning.com/CaseStudies

Related to this, if you know of any great media-based science education programs that ought to be highlighted by this site (or you run one yourself and are looking for feedback, publicity, collaborators, or funders), you can submit it at:

http://www.mediasciencelearning.com/addacase/
"

Some more useful science online resources

Scivee--share your science online!

SciNet--a new social network for scientists of all stripes, from AAAS.

Periodic Table--interactive periodic table. h/t Staten Island Academy student in ScienceOnline session. (Play with the temperature slider and see how the states of different elements change!)

Miss Baker's Biology Class--some really amazing student projects, blogs, etc.

Staten Island Academy's blog community--a great role model for student blogging.

Some cool science and education web sites, courtesy of ScienceOnline2010

FieldTripEarth--free data, plus info from real, live research programs going on all over the world.

NESCent--the National Evolutionary Synthesis Center. More (lots more) free data, plus educational materials on cutting-edge evolution research. (I think when the speaker demonstrated this, half the room started drooling at the data that are available...)

EduWeb--a source for online/digital, educational games on science, technology, history, and art.

Dryad--another open access data depository.

PRI's The World Science--weekly science podcasts, news stories, etc.

Scitopia--a peer-reviewed-research search engine.

Fold.it--play computer games for SCIENCE!

Spot.us--donate to journalists needing funding to cover their stories.

Science for Citizens--find research projects looking for volunteers.

Pandemic II (game)--play the bad guy. Design germs, infect people, get points!

Science Cheerleaders--learn basic science facts, find citizen science projects, and take a brain makeover quiz!

Good news for paleontologists?

Paleontologists, as most folks know, study fossils (or, more generally, the evidence of past life of any kind). By examining the types and distributions of fossils in rocks of various ages, paleontologists can give us insight into how life on Earth has evolved. Thanks to the study of fossils, we know, for example, that Cambrian oceans were full of trilobites, that the Mesozoic Era was dominated by giant reptiles, and that giant "terror birds" once roamed South America.

Yes, fossils are undoubtedly vital to our understanding of life on Earth. However, although fossils are the only evidence we have for the existence of past life, they have--like all evidence--limitations. Foremost among these is the preservation bias. There's a reason nearly all the fossils you'll see in a museum or private collection are fossils of shells, bones, and teeth: hard parts are much more likely to fossilize than are soft parts.

This means that critters like the sea squirt and the cuttlefish, cute though they may be, are unlikely to appear in the fossil record. Their bodies are entirely (or almost entirely) made of soft tissue, which decays rapidly once they die. About the only way soft tissue can be preserved is through mummification or other direct preservation methods; and they are pretty darned uncommon.

Size and depositional environment also play a role in preservation bias. Larger body parts may be more likely to be preserved and fossilized than are smaller body parts, because it takes large parts longer to break down (thus allowing them more time to be buried and mineralized--although this isn't a hard-and-fast rule). Similarly, critters that die in the water are much more likely to be preserved, because they're more likely to be buried before they decay completely.

Ultimately, preservation bias means that our understandings of life on Earth are inevitably biased toward largeish, ocean-dwelling animals with shells, bones, and/or teeth. This is why we know so much more about trilobites than we do about, say, ancient jellyfish.

Of course, paleontologists acknowledge this problem, and make attempts to compensate for it. One way to try to compensate for preservation bias is to use so-called "live:dead" ratios. For example, suppose in a particular ocean ecosystem 30% of the animals are bivalves, 25% are bony fish, 35% are crustaceans, and 10% are "squishies" such as anemones and jellyfish. That critter composition is known as a "live assemblage" or a "life assemblage" for that ecosystem. (I made up those numbers. They probably bear almost no relation to realistic numbers--and those particular types of critters may not occur together. Just bear with me for the sake of demonstration.) To try to correct for preservation bias, a scientist might count the number of dead critters in each category. (I should note that this type of analysis would be based on numbers of individuals, not numbers of remains--so two clam shells would count as one clam, for example.) This "dead assemblage" or "death assemblage" can then be compared to the life assemblage to figure out relative preservation rates. If, for example, 30% of the living critters are bivalves, but 40% of the remains are bivalve remains, then bivalves would have a higher preservation rate than other critters in the ecosystem.

Potential problems with this method are probably obvious: How do you know which types of modern environments to compare ancient remains to? How do you know that preservation rates in remains are the same as fossilization rates? How do you know preservation rates for different types of critters are the same today as they were then? What happens if the ecosystem changes rapidly--do the death assemblages still accurately reflect the life assemblages?

In the May 22 issue of Science, Western and Behrensmeyer present data that may help to address the last two of these questions. They used a 40-year record from the Amboseli ecosystem in Kenya to study the relative preservation rates for large mammal (15 kg-4000 kg) bones. Previous studies have shown that the life and death assemblages for these mammals are similar at specific points in time; that is, at a given time, the proportions of different species in the life assemblage are similar to those in the death assemblage.

A variety of factors have caused the Amboseli environment to change quite rapidly since the 1960s. Woodlands have shrunk, grasslands have expanded, and swamps have doubled in size. These environmental changes, in addition to direct human actions, have substantially affected the mammal populations in Amboseli during that time. The ratios of different types of organisms--grazers vs. browsers, for example--have changed as a result, and overall species diversity has declined.

Bone and live animal surveys were conducted during two time periods: 1975-1976 and 2002-2004. The bones studied during those times could be separated into subintervals based on how long ago the animal died; this allowed the researchers to divide the samples into four subintervals (1964-1969, 1970-1976, 1993-1998, and 1999-2004). They also used census data to determine the numbers of live animals in various groups during those same time intervals.

For each of the time periods, the researchers compared the proportions of different organisms in the life assemblages with those in the death assemblages. They used these data to determine how well the death assemblages "track" or represent the life assemblages. What they found is pretty interesting:

Statistically significant correlations between live populations and bone counts for the different time intervals indicate that organisms that make up a larger fraction of a living community also make up a proportionally larger fraction of the bone assemblage for that community. In other words, at least for this ecosystem, you can use the death assemblage as a pretty direct proxy for the life assemblage--if 50% of the individuals represented by the death assemblage are medium-sized grazers, then you can infer that about 50% of the organisms in the ecosystem (on average) over the time period you're looking at were medium-sized grazers. You can also use the death assemblages to study how populations in the ecosystem changed over time; the ratios of grazers to browsers in the death assemblages roughly paralleled those in the life assemblages for the same time period. They were able to distinguish changes in population composition over time scales as small as 5 years; they were even able to "predict" ecological structure from the death assemblages (and those predictions were largely confirmed by the life assemblages).

Western & Behrensmeyer's data could be very useful for paleontologists, particularly large-vertebrate paleontologists; the data suggest that bone distributions in death assemblages can be used to infer population and community structures for ancient ecosystems. With some assumptions about ecolosystem properties, bone assemblages can also be used to infer other properties of ancient ecosystems, such as species richness and productivity.

Obviously, these data have limitations; Amboseli is a relatively dry terrestrial ecosystem populated by relatively large mammals, so it's not clear whether the same correlations apply to marine ecosystems, wetter (or drier) terrestrial ecosystems, or to those inhabited primarily by smaller organisms or invertebrates. Additionally, because all of the remains studied were relatively recent (40 years isn't long enough to produce fossilization or even significant burial in most terrestrial ecosystems), it's not clear how the processes of preservation, burial, and fossilization might affect the death assemblages. (Although they do note that partially buried bones--a "pre-fossil" assemblage--seem to show the same correlations as unburied remains.) But studies like these are still very important in determining the error bars (accuracy) of ecosystem studies based on fossil assemblages.

Their data also suggest that studies of death assemblages in modern ecosystems can be of use to scientists studying the effects of human actions and other phenomena, as well as to those wishing to confirm (or obtain) estimates of vertebrate population sizes and compositions.

Western, D., & Behrensmeyer, A. (2009). Bone Assemblages Track Animal Community Structure over 40 Years in an African Savanna Ecosystem Science, 324 (5930), 1061-1064 DOI: 10.1126/science.1171155

It's all in your head

I think most of us are pretty willing to accept that the "will" or "urge" to move originates in the brain, and that the nerve stimulus that initiates the movement also originates in the brain.

What you might not know (I didn't) is that those two impulses--wanting to move, and initiating the movement--may actually happen in different parts of the brain.

I suppose it's not really surprising that this should be the case; the brain is, after all, a pretty big place (from a neuron's perspective), and obviously everything doesn't happen all in one spot. But in the May 8 issue of Science, Desmurget et al give pretty good evidence that the area that starts your body moving is distinct from the area that actually generates the urge to move.

The researchers studied seven human patients undergoing brain surgery for tumors. All seven were conscious during the surgery (possible because the brain, although the largest concentration of nervous tissue in the body, has no actual pain receptors on its surface), so they were able to answer questions. (Although it's not made clear in the article, presumably the patients were on several medications to relax them, but they were still conscious.)

In brain surgeries like this, doctors sometimes stimulate areas of the brain near the tumor to identify what parts of the body (or personality) may be affected by the surgery. In this case, the researchers used a similar technique to learn more about how the brain works.

During each surgery, several different regions of the patient's brain were stimulated with a small electrical probe. The shocks varied in intensity and duration. The researchers repeated the stimulations up to four times for each location, to check for reproducibility.

What they found out strikes me as pretty interesting. It turns out that, for several of the patients, when parts of the inferior posterior parietal cortex were stimulated, the patients felt an urge to move one or more body parts (arm, lips, chest, etc). If the stimulation was repeated with a higher intensity, the patients thought that they had actually moved that body part, even though no movement actually occurred. (The researchers report that one patient even said "I moved my mouth, I talked, what did I say?", although no mouth movement or speech was observed.)

Additionally, when portions of the premotor cortex were stimulated, the patients did actually move some of their body parts. When the stimulation was increased, the movement became more pronounced. However, and this was the part that I thought was kind of neat, the patients were completely unaware that they had moved at all. In fact, when they were specifically asked, the patients denied that they had moved, even when the movement was quite significant (e.g., raising an arm, or making a fist).

During the procedures, the researchers monitored the electrical signals in the patients' muscles as well. They saw no evidence of muscle movement when the parietal cortex was stimulated, even when patients were sure they had moved.

As an interesting side note, Desmurget et al report that stimulation of the right inferior parietal cortex caused patients to want to move their left limbs--hands, arms, feet, etc. However, stimulation of the left inferior parietal cortex seemed to prompt a desire to move the lips, or to talk.

References:
Desmurget, M., et al, 2009. "Movement Intention After Parietal Cortex Stimulation in Humans." Science 324: 811-813. doi 10.1126/science.1169896
Haggard, P., 2009. "The Sources of Human Volition." Science 324: 731-733. doi 10.1126/science.1173827

UPDATE: This post appears in the June 15 Scientia Pro Publica at Mauka to Makai.

Fascinating

Ed Yong at Not Exactly Rocket Science has a post up about the "bacterial zoo" living on your skin.

It's really pretty amazing how many different little critters are hanging around all the time--and what a huge fraction of them are actually perfectly harmless.

It makes me wonder--when you see those articles in magazines etc. talking about all the different kinds of bacteria living on your keyboard, phone, desk, etc, they never actually say how many of them are pathogenic. My guess is that, unless you've recently had a bacterial infection, most of those bacteria the media like to use for scare tactics are actually quite harmless. But I could be wrong. Anyone have any data on that?

UPDATE: Here's a link to the "official" summary of the article from Science.

Primary sources and creationism

Chad at Uncertain Principles has an interesting post up about the difference between the humanities and the sciences with respect to primary sources. I was all set to make a short comment, when it occurred to me that what I was about to comment on actually spawned a more interesting thought:

My comment was going to be that a critical difference between many (most? all?) humanities primary sources and those in science is that, in science, the primary sources (especially old ones, like Principia) are more than likely no longer totally valid. Once Nietzsche wrote down his ideas, they were there--it's not like someone could come along and "disprove" them. That's the whole point; they're subjective. Most humanities primary sources are--the point of them is to present a position and defend it, in one way or another, but the position and the defense are both subjective. They might be more or less well-supported or more or less relevant, but they're still opinions, and therefore can't be disproved.

The same can't be said for many (most? all?) science primary sources. No one who knows any better claims that The Origin of Species is completely in line with modern evolutionary theory, because we've made discoveries since it was written. (I.e., Darwin didn't have all the facts. Neither do we today, which is why biologists in 150 years probably won't be citing papers published today as definitive references.) Not having read Principia (or even Cliff's notes of it), I can't say that's the case for it as well, but I would imagine it would be. Even in my relatively specific field, there are a few "primary" references that a lot of people go back to, but only for certain things--because the rest of the article has since been replaced by something more specific. This constant reexamination, replacement, updating, etc., of the "going thing" is a fundamental part of science, and it's the reason that it's considered questionable in a lot of fields to cite papers that are more than a few years old: we might have learned something since then that totally overthrows the previous paper. (The time scale of "acceptably recent" varies field-to-field, but it's always there.)

This led me to a thought: I'm wondering what fraction of the struggles we have with creationists might be due to a fundamental difference in the perceived importance of primary sources. A lot of creationism "arguments" against evolution are based on Origin, even though any competent biologist (or, really, any intelligent person who's taken high school biology) should be able to tell you that a great deal of the text in Origin is only somewhat correct, if not flat-out wrong. But a lot of the people arguing against evolution come from backgrounds that are, shall we say, not steeped in the fundamental concepts of science. (This isn't to say there aren't scientists who are creationists; there are. But my impression is that the vast majority of creationists are not scientists and have very little scientific training.)

How much of the problem could be attributed to creationists being more familiar with the humanities "method", and therefore reading the "original" texts and interpreting them, without bothering to think about anything that's come after them? It's completely appropriate in, say, philosopy or literature to read a primary source and then draw your own conclusions and opinions about it. And your opinions are just as valid as those of others who have read the same text and drawn different opinions. (Which isnt't to say there aren't "accepted" interpretations of many famous works, or that dissenting with those interpretations won't open you to ridicule or censure.)

How many creationists who think evolution = Darwininsm read Origin, interpret it in light of common knowledge, and then view works based on Origin (i.e., most of modern biology) as simply others' opinions?

I don't think this is the primary problem or stumbling block; I think that's more likely to be a combination of a poor mainstream understanding of the nature of science and the tendency of creationists to be indoctrinated into an absolute belief system. But I think this also might be part of it.

Vaccine safety and creationist nonsense

Todd W at AntiAntiVax has an excellent post up addressing some of the most common "arguments" against vaccination. Check it out.

(h/t Phil from Bad Astronomy)

I also recently found an excellent source for refutations to creationist "arguments" against evolution. It's quite comprehensive in scope, although each specific anti-argument is a bit brief.

And it's not just the gammas...

(Sorry for all the insectoid posts; Science just seems to have the most interesting articles on our six-legged friends lately!)

It's not just gamma male beetles that benefit from fake-outs. According to an article by Barbero et al., some species of butterfly also benefit from mimicry--of ants, of all things.

Ant society is very complex; most species include a number of different "genders" and societal roles, all of which are rigidly defined. (Emancipation has not yet come to the ant world.) Certain castes of ants are more valuable, and therefore more highly protected, than others. The extreme of this is, of course, the queen, who receives the most care and attention.

Although most of the communication necessary to keeping such a complex society running is chemical (e.g., pheromones) and physical (i.e., physical contact), apparently some of it is acoustic. Adults in certain ant subfamilies can produce "stridulations" (which I assume sound something like scraping noises, although I could be wrong) to communicate. Within these subfamilies, different castes produce different sounds (and larvae and pupae produce no sounds at all).

A number of ant species are also parasitized by the larvae and pupae of other insects. Barbero et al focused specifically on the butterfly species Maculinea rebeli, which parasitizes the ant species Myrmica schencki. M. rebeli larvae and pupae can infiltrate M. schencki nests and fool the ant workers into taking care of them. A significant characteristic enabling the butterfly caterpillars to survive in the ant nest is their ability to produce chemicals that mimic the chemicals produced by the ant larvae.

However, M. rebeli larvae and pupae apparently show higher "social status" than would be expected simply from the chemical mimicry. For example, M. schencki workers will rescue M. rebeli larvae and pupae instead of "dummies" that have been painted with the same chemical mimics. In addition, M. schencki queens will sometimes treat the butterfly larvae and pupae as rivals; at the same time, the ant workers treat the butterfly larvae like queens. This discrepancy led Barbero et al to guess that perhaps the butterfly larvae and pupae are able to produce acoustic signals that increase their status in the ants' social heirarchy.

As it turns out, they may be right. M. schencki workers and queens do produce distinct stridulations (i.e., they sound different to the other ants), and M. rebeli larvae and pupae produce sounds that are more similar to the queen ant sounds than to the worker ant sounds.

To test their hypothesis, Barbero et al carried out a number of tests. First, they recorded the sounds produced by the ant workers and queens. They played those sounds to "naive" worker ants. (They also exposed control groups to white noise and to silent speakers.) The worker ants showed more interest in the ant noises than the white noise or the silent speakers. In addition, the noises from the queens caused the workers to become more alert and to assume postures that are associated with "serving" the queens. This test confirmed that worker ants do respond to acoustic signals from other ants.

Next, the researchers recorded sounds from butterfly larvae and pupae. They played those sounds to similarly "naive" worker ants. The ants responded to both larval and pupal calls in the same way they responded to the queen ant calls.

Based on their observations, Barbero et al conclude that M. rebeli larvae and pupae are first able to enter an M. schencki nest through chemical mimicry. Once they are inside, however, acoustic mimicry may also play an important role in preventing the ants from rejecting them.

Barbero, Francesca, Jeremy A. Thomas, Simona Bonelli, Emilio Balletto, and Karsten Schönrogge, 2009. "Queen ants make distinctive sounds that are mimicked by a butterfly social parasite." Science 323: 782-785. doi: 10.1126/science.1163583

More insect phenotypic funkiness

Like the male members of many species, male beetles fight over female beetles. And, like male deer and antelope, many male beetles sport formidable (for a beetle, anyway) horns, spines, and mandibles, which they use to intimidate (if not outright harm) other males. In the February 6 issue of Science, Rowland and Emlen report that some male beetles take a different tack: instead of fighting over the ladies, they pull an Achilles and "dress" like them.

Rowland and Emlen conducted statistical analyses of body size and horn, mandible, or spine length for several different species of beetles. Most previous analyses had assumed only two main male phenotypes for each species (e.g., big horns and small horns, with hornless males being classified in the "small horns" phenotype). Rowland and Emlen, however, found that several species of beetles actually show facultative male trimorphism--that is, the males actually demonstrate three distinct phenotypes (alpha, beta, and gamma). Alpha males have large bodies and large horns (or mandibles, or spines). Beta males have smaller bodies and smaller horns (mandibles/spines). Gamma males have the smallest bodies and no horns (you get the idea).

The differences in phenotype are facultative because they don't seem to be related specifically to genotype. Instead, the main factor determining whether a male is alpha, beta, or gamma seems to be his body size at maturity--beetles that get lots of food and good living conditions end up as alpha males (big bodies = big horns), while beetles that get less food and are smaller at maturity end up as betas or gammas.

Alpha, beta, and gamma males also seem to employ somewhat different mating strategies. Alpha males have the typical pissing contests for access to mates--for example, some alpha males will guard the burrows where the ladies are living, and fight off all comers. Beta or gamma males, though, are sneakier: they might dig side tunnels into the burrows and cuckold the alphas without ever having to fight them. In some other species (e.g., cuttlefish), "gamma"-type males actually get in with the ladies by "cross-dressing"--for example, some male cuttlefish can change color to mimic female cuttlefish, thus allowing them to get in close enough to mate with the females while the other males are busy showing off.

According to Rowland and Emlen, previous studies (and phenotypic analysis methods) have assumed that the majority of beetles display male dimorphism, rather than trimorphism. They suggest that alternative analyses may be necessary to detect trimorphisms--apparently, some of the traditional analyses did not detect all three male morphs in the beetles they studied.

I suppose the moral of the story is, if you're an alpha male, you might want to check the skirts of the ladies in your harem!

Rowland, J. Mark, and Douglas J. Emlen, 2009. "Two thresholds, three male forms result in facultative male trimorphism in beetles." Science 323: 773-776. doi 10.1126/science.1167345

If the Egyptians had only known...

According to legend, one of the way the Egyptians were punished in the time of Moses was with a swarm of locusts.

Locusts are insects that look a bit like big grasshoppers. They're a classic example of what's called phenotypic plasticity. An organism's phenotype is basically its observable characteristics--behavior, color, size, etc. (Phenotype is the outward expression of genotype; genotype is the specific group of alleles that an organism has. Most--all? I'm not sure--genes have at least two alleles, or "flavors." The classic example is, of course, Mendel's peas; the gene that controls flower color in pea plants has two alleles, purple and white. A pea plant's flower-color genotype is the particular combination of alleles that it has; its flower-color phenotype is the particular color of flower it produces.)

Organisms such as locusts that demonstrate phenotypic plasticity can undergo significant changes in behavior, appearance, etc due to changes in their surrounding environment. Locusts are a classic case because the change is so dramatic. If you take two locusts and put them in a box, they will pretty much avoid each other--that is, assuming they're demonstrating the "solitarious" phenotype. This is pretty much the default position for locusts; most of the time, they hang out by themselves (not a lot of singles bars in their neighborhoods, I guess).

Now if you had put, say, 20 or 30 solitarious locusts in that box and shut them in for a couple of hours, they would be quite changed when you opened the box. They would be swarming together, and they would have changed in appearance (from kind of boring and green to a rather striking, Steelers-like combination of yellow/tan and black...sorry, couldn't help it). They would be demonstrating the "gregarious" phenotype.

Solitarious (top) and gregarious (bottom) desert locusts. Image from Dr. Tim Matheson, University of Leicester

Locusts in the gregarious phase are the stuff of legend. These are the critters that mow crops down to the roots and blacken the skies. (Presumably, the Egyptians crowded their locusts.)

The cause of the transition from solitarious to gregarious has been known for a while, at least in broad strokes: being in the presence of lots of other locusts makes a locust more gregarious. In a recent article in Science, Anstey et al identify the mechanism that triggers the transformation.

There are two different sets of stimuli that can make a locust more friendly: mechanical and "cephalic." Mechanical stimulation involves being jostled by other locusts; in contrast to most humans, most locusts become more friendly when strangers stroke their legs. Cephalic stimulation involves the sight and smell of other locusts; locusts apparently have really great makeup and cologne. Both types of stimuli cause the locust's central nervous system (CNS) to produce (what else?) serotonin. (Yes, that serotonin.)

Previous researchers established that serotonin levels are higher in locusts undergoing the solitarious-to-gregarious transition. Anstey et al set out to determine the limits of this relationship. They did four main experiments: first, they figured out whether artificial stimulation of the individual sensory pathways could stimulate serotonin production and gregariousness. Then, they tested whether serotonin antagonists (i.e., chemicals that block the action of serotonin) could prevent the onset of gregarious behavior. Third, they determined whether artificially increasing serotonin levels was enough to induce gregariousness. Finally, they determined whether giving the locusts a serotonin precursor (i.e., a chemical that is easily converted to serotonin) increased their sensitivity to environmental stimuli.

In the first experiment, the researchers either stroked the hind legs of solitarious locusts, stimulated the nerve connecting the legs to the CNS directly, or put the locusts in a cage that allowed them to see and smell (but not touch) other locusts. In all cases, the locusts switched from solitarious to gregarious, and serotonin levels increased, suggesting that either type of stimulation is sufficient to induce gregariousness.

Next, they injected some of the locusts with serotonin antagonists (they also, of course, injected others with just saline--this was the control group). After the injections, the treated locusts (the ones that received the antagonist) were significantly less responsive to stimuli than the control locusts; treated locusts did not become gregarious, even when exposed to stimuli that caused the control locusts to boogie down. This relationship showed that inhibiting the action of serotonin prevents the "phase change."

In the third experiment, Anstey et al applied serotonin directly to the locusts' nerves (again, they also used a control group that received just saline). They also injected a third group of locusts with a serotonin agonist (i.e., a chemical that increases the activity of serotonin--the opposite of an antagonist). The treated locusts became much more friendly, but the control locusts remained aloof. In other words, just increasing serotonin levels (without actual stimuli) can make solitarious locusts more gregarious.

In their final experiment, the researchers determined whether increasing the ability of the locusts to produce serotonin would cause them to become more gregarious after only a small amount of stimulation. Typically, a solitarious locust has to hang out with other locusts for a couple of hours before putting on its party shoes. However, when solitarious locusts were injected with a serotonin precursor, 30 minutes of exposure was enough to get them dancing.

There's some hope that these results might lead to new possibilities for locust control. Individually (i.e., in the solitarious phase), locusts aren't too much of a problem--no more so than, say, grasshoppers, really. It's only when they start to swarm that they become economically disastrous. If a way could be found to prevent locusts from become gregarious, even when crowded, then locust swarms could be controlled. (Too late for Rameses, of course.) Such possibilities are still in the future--currently, there is no locust-specific serotonin antagonist that can be applied appropriately--but it does give some hope.

Until then, there is one thing we can definitely conclude: keep the locusts away from the Prozac!

Anstey, Michael L., Stephen M. Rogers, Swidbert R. Ott, Malcolm Burrows, and Stephen J. Simpson, 2009. "Serotonin mediates behavioral gregarization underlying swarm formation in desert locusts." Science 323: 627-630. doi 10.1126/science.1165939

Stevenson, P.A., 2009. "The key to Pandora's box." Science 323: 594-595. doi 10.1126/science.1169280

(Yes, I know this isn't quite 1,000 words. But it's pretty darned close!)

You would think...

...that in 200 years, the anti-science crowd would have come up with some new objections to evolution. Apparently not: Check out this quote from Origin.

Long before the reader has arrived at this part of my work, a crowd of difficulties will have occurred to him...These difficulties and objections may be classed under the following heads:--First, why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms?...

Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some other animal with widely-different habits and structure? Can we believe that natural selection could produce, on the one hand, an organ of trifling importance, such as the tail of a giraffe...and, on the other hand, an organ so wonderful as the eye?

Thirdly, can instincts be acquired and modified through natural selection? What shall we say to the instinct which leads the bee to make cells, and which has practically anticipated the discoveries of profound mathematicians?

These questions, of course, have many parallels in the standard litany of "problems" with the theory of evolution often spouted by creationists and intelligent design proponents. Is this yet another example of Darwin's apparent prescience? Or is it more accurate to say that Darwin's statements echo those of modern-day denialists because they are building on the "work" of those who came before, who undoubtedly read Darwin? If the latter, it's really a shame they didn't read the whole book. Even if they'd read a few pages further on, they would have come across this beauty:

When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science.

Perhaps it might be better said that Vox populi, vox veritas "cannot be trusted in science." But the sentiment still holds: Just because most people think it's so, doesn't make it so. The fact that so many people argue that we should teach the Bible as science because "most Americans believe in God" speaks to a fundamental lack of understanding of the way science is done. But I'm not the first to make that statement, nor will I be the last.

Self-catalytic RNA enzymes

GumbyTheCat recently posted about a study in which researchers created self-replicated RNA enzymes. There was a bit of discussion in the comments about what those enzymes are all about and whether they are, indeed, "true" enzymes (i.e., proteins).

As I have access to Science magazine online, I was able to read the actual article (Gumby's post was based only on the abstract, I think), and so I can now answer the question of what, exactly, an RNA enzyme is, and how the research group got it to replicate itself.

An RNA enzyme, it turns out, is not a true enzyme. That is, it isn't a protein made up of amino acids. It's actually a strand of RNA. The particular RNA enzymes this group made look kind of like a capital T with one side of the crossbar a lot longer than the other. Like all RNA, they're made up of nucleotides (a nucleotide is a molecule consisting of a sugar molecule--ribose, in the case of RNA--a phosphate group, and a nitrogenous base). (I am forced to conclude that the "RNA" in "RNA enzyme" is an adjectival form, rather than a description of what the enzyme catalyzes.)

To understand how the enzyme works, you first need to know a bit about bonding in nucleic acids (DNA and RNA). What follows is a brief discussion; details can be found in any introductory biology textbook.

A single strand of a nucleic acid is a polymer (a really big molecule made up of a lot of similar, smaller subunits called monomers). As mentioned above, the monomers in nucleic acids are nucleotides. When nucleotides join together to form a nucleic acid, the sugars and phosphates bond together to form a "backbone." The nitrogenous bases stick off one side of the backbone. There are five nitrogenous bases that can form nucleotides: thymine, adenine, uracil, guanine, and cytosine. They are abbreviated T, A, U, G, and C, respectively. A, T, G, and C are found in DNA; RNA contains A, U, G, and C. So, a single strand of RNA looks kind of like half a ladder; the rungs are A, U, C, and G molecules. A double-stranded nucleotide (such as DNA) looks like a full ladder; the base in each "rung" is bonded to another base on a rung on the other side of the ladder. The bonded bases form a full rung. (Of course, a DNA molecule really looks like a twisted ladder, but the physics of why it twists isn't important for our purposes here.)

These bases aren't just any random molecules, though. As it turns out, their molecular structures force them to bond together in specific ways: A can bind only with T or U, and G can bind only with C (and vice versa, in each case). In a double-stranded nucleotide, therefore, each rung is made up of either a C-G pair or an A-T (or A-U if it's RNA) pair. You can probably see the beauty of this arrangement: it means that if you have one half of a double strand of RNA or DNA, you can construct the other half.

As I mentioned before, the RNA enzymes in this study look like lopsided Ts. The stem of the T is actually a double strand of RNA: part of the RNA molecule has bonded to itself. (A similar structure is found in some kinds of RNA that take part in transcription and translation in eukaryotic cells.) The crossbars of the T are single strands of RNA.

Each enzyme forms from two smaller pieces of RNA: a straight piece (called "B") and a piece that looks like a regular (i.e., not lopsided) T (called "A"). The straight piece binds to one of the crossbars of the T-shaped piece to form the lopsided T (which the researchers refer to as "E", for enzyme).

Each enzyme (and each sub-enzyme piece) actually exists in two "mirror-image" forms (i.e., E and E', A and A', and B and B'). The mirror-image forms can bind to each other because of the way the bases pair. However, A doesn't bind to A', or B to B'. Instead, A binds to B', and B bonds to A'. The A-B' combination forms E; the A'-B combination forms E'. [EDIT: the previous sentences should read "Instead, A binds to B, and B' binds to A'. The A-B combination forms E; the A'-B' combination forms E'."] The drawing below shows my lame attempt to summarize.

Essentially, when the researchers put some E into a mixture of A, B, A', and B', the A' and B' pieces bonded to the E to form molecules of E'. Once there was some E' in the mixture, the A and B molecules could bond to it to form new E molecules, and Presto! self-replicating RNA.

Of course, it wasn't really that simple. And actually, the not-simple part is kind of cool: The original E that the researchers used wasn't very efficient at catalyzing its own formation. So, basically, the researchers evolved it. They generated new A and B with mutations--variations in the sequences of bases on the backbone--and selected the ones that formed E that could replicate itself most quickly.

Because they have groovy tools (such as polymerase chain reaction machines) and computer to do the analysis, they were able to try a whole lot of different combinations in order to find the set of A and B that produced the most efficient E.

All in all, a really groovy little study!

Lincoln, Tracey A., and Gerald F. Joyce, 2009. "Self-sustained replication of an RNA enzyme." Sciencexpress. published online 8 January 2009; 10.1126/science.1167856.