Through The Sandglass

Georgia Tech Crab Lab: guest post on the art of sidewinding

Over the life of this blog, I have written a number of times on the remarkable and innovative work carried out at Georgia Tech’s Crab Lab, investigations into how critters get around in loose sand and how this can be applied to robotics (see, for example, here, here, and here). Crab Lab is headed up by Dan Goldman, and I was recently delighted (and flattered) that he contacted me about the “Sand” book. I asked if he and his colleagues would be willing to contribute a guest post – and here it is. Courtesy of Henry Astley, a postdoc “who loves all things snakes” and collaborator Joe Mendelson: the state of the art on the physics of sidewinding.

 

In spite of their barren reputation, deserts around the world teem with life, including a wide array of animals from beetles to camels. While much has been written about their adaptations to deal with two of the most notable characteristics of deserts, extreme temperatures and scarce water, far less attention has been paid to their interactions with the other distinguishing characteristic of many deserts: sand.

The ability to move from place to place is crucial for animals to find mates and resources, regulate their body processes, avoid predators, and colonize new environments. But sand makes locomotion difficult, whether moving on it or through it. Particularly problematic is that sand will behave as a solid under certain loads, but will yield and flow like a fluid under others, and very small differences in foot placement and movement can be the difference between moving and becoming hopelessly stuck. Animals deal with this challenging substrate in a variety of ways, whether by anatomical changes or selecting the best movement patterns.

Perhaps the strangest movement pattern of desert animals is the famous “sidewinding” locomotion, seen in certain snakes of sandy deserts around the world. While prominent herpetologist Clifford H. Pope wrote in 1955 “A study of [sidewinding] is recommended to anyone who likes to be confused,”, the underlying motion is quite clear when examined in detail. The snake lifts its head and moves it forward, placing it on the ground, then repeats this motion in a propagating wave down the body – see Science paper supplementary movies.

This produces a characteristic trackway consisting of a series of parallel lines, with the imprint of each scale on the belly clearly visible, showing that the snake does not slip (which would smear the tracks and erase the fine scale imprints). Ultimately, this seemingly complex motion can be reduced to a pair of waves producing vertical and horizontal body undulation, +- 90 degrees out of phase, which propagate together down the body. This simple model may be a “neuromechanical template”, a simple model of a motion which captures all the essential features, potentially serving a simple “target” for the animals to control their locomotion (see Sidewinding with minimal slip: Snake and robot ascent of sandy slopes). This two-wave template of sidewinding also produces sidewinding locomotion when applied to a snake robot, allowing the robot to move on sand effectively.

 

 Screen-grab from Science supplementary movies

This provides a great experimental tool, because the physics of sand have yet to be reduced to simple systems of equations (as has been the case in fluids for almost 200 years), making computer modeling of results difficult and time-consuming. However, the snake robot provides a “physical model” for movement in sand, allowing us to test hypothesized biological mechanisms. Further observations of biological snakes have revealed the modifications of the two-wave template responsible for effectively sidewinding up inclines and turning (see the recent PNAS paper), which in turn have further improved the effectiveness of the robot.

In spite of these insights, the serpents of the sand still hold many mysteries. Why do some snakes sidewind, while others don’t? Why can some species move effectively on sand, while others fail? How do sidewinders deal with obstacles? Can we reconstruct the evolution of this remarkable mode of locomotion from the tracks it has left, or has this history been lost in the sands of time? 

[My sincere thanks to all at Crab Lab for their work and this post. Photographs by Henry Astley]

On the Edge of the Sahara: a guest post by Ann Lingard

At the edge of the Sahara, I stood for a while and watched a dune move. It pushed out its toes, testing the flat ground in front of it; a small trickle of sand cascaded down the lee side and staked a claim, a first footstep of a few millimetres, across the stony flat.

We were in south-east Morocco, and had spent much of that morning crossing the reg, a gravel plain, and were now into the erg, a broad sea of dune-waves. It was a windy day, the sky hazy with particles and the ground beneath our feet blurred by a spray of sand. The particles whipped round our calves as we tramped up each windward slope. From each crest – the sharply-carved edges so familiar from tv documentaries and photos – there would be a view of more dunes; and then we would stride down the lee side where the negative pressure, created as the wind skimmed above our heads, sucked the disturbed sand upwards into our faces.

Having long ago read Michael’s book Sand, and having skimmed the relevant sections in The Desert (his new book arrived two days before our flight), I was delighted to experience for myself the physics and lexicon of dune-formation (saltation, reptation – lovely words): to watch the bounding sand-grains knocking into each other, leaping in the air, and co-operating to form small avalanches; to see the different patterns of aeolian ripples - and to draw comparisons with water-formed ripples on my home patch, the Solway shore.

Less delightful was the determined ingress of sand grains into clothes, socks, boots, eyes, hair and later, food (but no ‘sand in the sandwiches’ here – much tastier ‘sand in the salad’ and tagine instead).

And then, after about four hours’ walking, we came to an area of flat ground on which the dunes were scattered, not contiguous, encroaching individually or in small groups. Formerly this had been the bed of a lake, on the flood-plain of the River Draa, but after the dam was built upstream near the Atlas mountains and the river’s path moved for irrigation purposes, the lake inevitably dried out, leaving a stone-scattered and gently-contoured plain. Now, the sand was taking advantage of this empty space and was piling up and moving in, millimetre by millimetre.

However, it wasn’t only the dunes that were intriguing, for whenever we came to areas of reg, the flat and stony desert, there were scatterings of red or grey clay potsherds – handles, fragments of bowls, even a couple of large, almost-whole pots. Some pieces were engraved with simple patterns, and a few had remnants of green glaze. Had these fragments been washed down from dwellings in the Atlas or the Draa Valley? How old were they? Sadly, other than a comment that they were ‘perhaps a hundred years old’ it wasn’t possible to get a definitive answer from our Berber guide.

The camels, carrying three days’ water, the food, cooker and utensils, and our tents and baggage, moved faster than we did, and one of the camel-drivers had staked a sack on top of a ridge to show exactly where that night’s camp was being pitched. The camp-site, on flat ground between the tall dunes of Zahar, felt remote, yet without the use of GPS or compass, our guide had led us to this spot.

The next morning, an hour after we had set off, a lightly-laden camel and his driver suddenly appeared from between the dunes and crossed our path, following an unseen (to us) route. Elsewhere we saw camel footsteps imprinted and dried into what had once been mud. Humans and their animals were moving purposefully through this landscape, clearly using natural signs and age-old knowledge. Lacking this knowledge, and with only the footprints of other members of our party to follow, we would soon have been lost.

But not all the footprints were caused by human-related passage: parallel scrapes of the clawed feet of laboriously-moving beetles; a lizard’s skittering footsteps straddling the straight line of its tail; small mammalian footprints – gerbils, scampering and churning the sand around their burrows; and the focussed, straight-line padding of a fox: apart from these signs, no-one would have known that animals were living in this landscape.

After four days trekking through erg and reg, wadis and hamada, and feeling the chill of February nights that were deeply silent except for the occasional camel grumbling, and seeing into the depths of the Milky Way, I came away with a sense of wonder at what I had seen – but also knowing that, apart from saltation and reptation, I really didn’t understand a thing.

  

Sunrise at the well of Tilhatine

 

[Ann is an old friend of mine and of the Solway Firth, a sheep-raiser and weaver – and a wonderful writer. Enjoy Solway Shore Stories, her blog, Solway Shore-walker, and her extraordinary and compelling novel, The Embalmer’s Book of Recipes, now out in its second edition. See also Ann’s website. Since she has recently been trekking in the Moroccan desert, I requested a guest post – and here it is, so many thanks, Ann.]

Ecosystem engineers and keystone species

 

In large areas of Australia there are probably several hundred tons of termites in every square kilometre.

From The Desert, Lands of Lost Borders, Chapter 6:

The most ubiquitous (and irritating) vegetation in the Australian outback is spinifex, strictly Triodia. This coarse, tough grass grows in landscape-smothering tussocks, and its spiky leaf tips contain small shards of silica that have a habit of embedding themselves in the skin of passing animals, including humans.

Spinifex performs an important function in terms of dune stabilization and is a key participant in the fire ecology of the desert, but it is essentially inedible for animals and would smother the land and clog the ecosystem if left unchecked. In other climates, plant debris is cleared by wood-decaying fungi, but the desert is too dry for them. However, crucially, termites eat spinifex and there are a lot of them. Spinifex may be an archetypal feature of the landscapes of the outback, but so are termite mounds. They come in a bewildering array of shapes and sizes, each one extending far below the surface for water supply and providing a complex climate-controlled home to a community of hundreds of thousands of individuals. The termites consume the spinifex (along with a vast variety of other organic matter) and keep it under control, but they cannot digest it. For that, through a remarkable example of symbiosis, they require the specialized microbes in their gut that convert the cellulose to acetate, a kind of vinegar that then feeds the termites. Termite mounds provide safe havens for a variety of other creatures (some lizards lay their eggs in them) and the process of their construction moderates the desert soils, influencing water infiltration and evaporation, changing the structure and permeability. This, in turn, promotes plant growth and diversity, the entire vertebrate and invertebrate burrowing ecology and the food chain as a whole. Termite mounds in the Sahara and the Sahel are referred to as ‘houses of the devil’, but without this ‘keystone species’ arid lands would be very different — it has been estimated that most or all of the biomass produced in the Chihuahuan Desert is consumed by termites.

As a recent press release from Princeton University observed, “Termites might not top the list of humanity's favorite insects”, but it went on to highlight our on-going and emerging understanding of the critical role that they play in the arid ecosystems of the world’s arid lands:

new research suggests that their large dirt mounds are crucial to stopping the spread of deserts into semi-arid ecosystems and agricultural lands. The results not only suggest that termite mounds could make these areas more resilient to climate change than previously thought, but could also inspire a change in how scientists determine the possible effects of climate change on ecosystems.

In the parched grasslands and savannas, or drylands, of Africa, South America and Asia, termite mounds store nutrients and moisture, and — via internal tunnels — allow water to better penetrate the soil. As a result, vegetation flourishes on and near termite mounds in ecosystems that are otherwise highly vulnerable to "desertification," or the environment's collapse into desert.

Princeton University researchers report in the journal Science that termites slow the spread of deserts into drylands by providing a moist refuge for vegetation on and around their mounds. They report that drylands with termite mounds can survive on significantly less rain than those without termite mounds. The research was inspired by fungus-growing termites of the genus Odontotermes, but the theoretical results apply to all types of termites that increase resource availability on and/or around their nests.

This research is fascinating, but it would not be entirely surprising to farmers in the Sahel who are resurrecting – very successfully – the traditional methods of water management for new trees and for new crops. Digging a planting pit through the hardened surface and adding organic matter creates not only a water-conserving environment for plant growth, but attracts termites that process the organic material for use by the plants and aerate the soil through their tunnelling. And those tunnels are extraordinary. The internal structure of a termite mound (a complex ecosystem in its own right) has been dramatically demonstrated by the work of Scott Turner at The State University of New York College of Environmental Science and Forestry. Through taking plaster casts, he reveals the architectural skills of the ecosystem engineers as strange and compelling sculpture: 

 

The tunnelling continues far below the surface (termite mound materials that bring minerals from the subsurface have been used for gold prospecting in Australia) and for many metres beyond the mound. The scale of this landscape management activity is staggering: look carefully at this photo of just a small area of the Australian desert, and you will see hundreds of termite mounds. 

No, termites may not be our favourite insects, but our planet would be a different – and, arguably, worse – place without them.

 

[Image, “Crater termites of the worker and soldier castes attending to damage of their nest, Schanskop, Pretoria”, GNU Free Documentation License, Author JMK]