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.

Usage tip: depravity vs. deprivation

Short and sweet, inspired by a recent article in my local newspaper:

Depravity refers to the acts or characteristics or acts of someone who is depraved--i.e., morally corrupt, evil, toxic. Deprivation refers to a state of being denied or refused access to something--the state of being deprived. 

Incorrect:

• Lots of people complain about daylight saving time because it leads to sleep depravity.
• It is hard for people who haven't experienced it to understand the acts of deprivation committed by war criminals.

Correct:

• New parents, suffering from sleep deprivation, are more likely to fall asleep while driving.
• Upon seeing the video footage of the massacre, I was left speechless by the depravity of the acts it showed.

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.