Hexbyte Glen Cove Learning about system stability from ants thumbnail

Hexbyte Glen Cove Learning about system stability from ants

Hexbyte Glen Cove

The army ant Eciton burchellii. Credit: James Herndon

A new type of collective behavior in ants has been revealed by an international team of scientists, headed by biologist Professor Iain Couzin, co-director of the Cluster of Excellence “Centre for the Advanced Study of Collective Behavior” at the University of Konstanz and director at the co-located Max Planck Institute of Animal Behavior, and Matthew Lutz, a postdoctoral researcher in Couzin’s lab. Their research shows how ants use self-organized architectural structures called ‘scaffolds’ to ensure traffic flow on sloped surfaces. Scaffold formation results from individual sensing and decision-making, yet it allows the colony as a whole to adjust dynamically to unpredictable environmental challenges.

Who does not know this situation? You are stuck in a traffic jam due to road construction, or you wanted to take the train instead of your car, but it did not show up. A problem common to many complex artificial systems, be they traffic infrastructures or other technological systems, is a lack of robustness in the face of disturbances. These systems are often rigid and inflexible, and centralized or hierarchical control structures make them vulnerable to single-point perturbations. Biological systems, on the other hand, often employ forms of distributed control and can be astonishingly robust to environmental challenges.

To learn more about the underlying stability and resilience in natural systems, Konstanz-based biologist Iain Couzin and his lab, together with international colleagues from Australia and the US, investigated how coordinate traffic during foraging. Their study, published in PNAS, describes how ants of the species Eciton burchellii organize into living architectural structures termed “scaffolds” on sloped surfaces, to avoid traffic disruption and conserve resources. The researchers propose a mechanism for formation in which each ant adjusts its behavior based on its own experience, without a need for group-level communication. This simple but effective mechanism of proportional system control from the animal world may inspire designs for artificial systems, from autonomous vehicles to future forms of resilient infrastructure that respond to changing conditions.

Voracious predators and gifted architects

For their study, the scientists traveled to Panama, where the species under investigation—the army ant Eciton burchellii—inhabits the tropical forest of Barro Colorado Island. Eciton army ants are social insects, living in large colonies with hundreds of thousands of workers. During the day, they hunt for prey in massive swarm raids that can sweep out an area of four tennis courts in a single day. Among the many evolutionary adaptations that rank these ants among the top invertebrate predators in the tropical forest is their remarkable ability to self-organize into living architecture. For the benefit of the colony, individual ants join forces to temporarily modify the environment and ensure the flow of traffic during the colony’s hunts.

The PNAS study describes one type of architecture these ants construct—called ‘scaffolds’ by the authors—for the first time in detail. Under natural conditions, scaffolds form when E. burchellii trails cross inclined surfaces, such as branches or rocks, and individual ants stop and cling to the surface, remaining fixed in place. The authors discovered that, in doing so, the ants provide additional grip for other ants, which continue along their path, marching over the immobile conspecifics.

Scaffolds were shown to be highly adaptable, growing to different shapes and sizes depending on the context—from just a few dispersed ants arranged like a climbing wall, to dense aggregations of ants forming a protruding shelf. “Scaffolds form rapidly in response to disruption, preventing ants from slipping and falling along the foraging trail. This is especially important when you are transporting valuable resources like prey through dense traffic, and your trail leads through an unpredictable rainforest environment with all kinds of slopes and obstacles,” Couzin describes.

Group-level coordination without communication

To experimentally investigate collective scaffolding behavior, the authors designed an apparatus that allowed for the introduction of defined slopes into the raiding trails of wild ant colonies in the field. With repeated experiments on slopes of different angles, the researchers found the ants to reliably organize into scaffolds when crossing surfaces inclined more than 40 degrees. The steeper the slope, the more ants initially slipped or fell from the platform, and in response, the larger the scaffolds grew. Once a scaffold had formed—a few minutes after ants began crossing—the number of slipping and falling ants returned to a low level, thanks to the support structure.

In accordance with their field observations and supported by theoretical modeling, the scientists suggest a surprisingly simple mechanism for scaffold formation: When an animal slips on a sloped surface and then regains its footing, it has a certain probabilistic tendency to claw the ground and remain in place. In doing so, it either starts or joins a scaffold. The more animals show the behavior, the less slippery the tilted surface becomes, as the scaffold grows. The structure eventually stops growing, because the trailing ants can use the existing scaffold to cross unhindered. “In a way it surprised even us how simple the mechanism is. If you observe these collective phenomena for the first time, you intuitively think that there has to be some sort of communication among the ants. However, in this particular case, there is no need for it. Each individual adjusts its behavior based on its own experience as it crosses,” Couzin explains.

Taking inspiration from nature

As human technological and social systems increase in complexity, it is crucial to find and implement mechanisms that robustly and rapidly correct for errors, increasing stability. The example of scaffolding in Eciton army ants offers one such mechanism. Due to its simplicity—only requiring information about the state of individual elements instead of complex group-level communication—this may serve as a blueprint for robust yet flexible engineered systems with similar distributed forms of control. Lutz, an architect whose fascination with self-organized patterns in biology led to his Ph.D. and postdoctoral research on collective behavior and self-assembly in Couzin’s lab, concludes: “Because the mechanism is quite simple in terms of sensing and communication, it may be useful for applications at many scales, across disciplines. These range from swarm robotics, where restrictions on sensing and communication can be limiting factors, to the design of self-healing materials, bio-fabrication techniques, and new models of responsive infrastructure.”

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Hexbyte Glen Cove The solar system follows the galactic standard—but it is a rare breed thumbnail

Hexbyte Glen Cove The solar system follows the galactic standard—but it is a rare breed

Hexbyte Glen Cove

Illustration showing an artist’s interpretation of what the TRAPPIST-1 solar system could look like. The seven planets of TRAPPIST-1 are all Earth-sized and terrestrial, and could potentially harbor liquid water, depending on their compositions. Credit: NASA/JPL-Caltech

Researchers at the Niels Bohr Institute, University of Copenhagen, have investigated more than 1000 planetary systems orbiting stars in our own galaxy, the Milky Way, and have discovered a series of connections between planetary orbits, number of planets, occurrence and the distance to their stars. It turns out that our own solar system in some ways is very rare, and in others very ordinary.

It is rare to have eight planets, but the study shows that the solar system follows exactly the same, very basic rules for the formation of planets around a star that they all do. The question about what exactly makes it so special that it harbors life is still a good question. The study is now published in MNRAS

Eccentric planet orbits are the key to determining the number of planets

There is a very clear correlation between the eccentricity of the orbits and the number of planets in any given solar system. When the planets form, they begin in circular orbits in a cloud of gas and dust. But they are still relatively small in size, up to sizes comparable to the moon. On a slightly longer they interact via gravitation and acquire more and more eccentric or elliptic orbits. This means they start colliding because elliptical orbits cross one another—and so the planets grow in size due to the collisions. If the end result of the collisions is that all the pieces become just one or a few planets, then they stay in elliptical orbits. But if they end up becoming many planets, the gravitational pull between them makes them lose energy—and so they form more and more circular orbits.

The researchers have found a very clear correlation between the number of planets and how circular the orbits are. “Actually, this is not really a surprise,” professor Uffe Gråe Jørgensen explains. “But our solar system is unique in the sense that no other solar systems with as many planets as ours are known. So perhaps it could be expected that our solar system doesn’t fit into the correlation. But it does—as a matter of fact, it is right on.”

The only solar systems that don’t fit into this rule are systems with only one planet. In some cases, the reason is that in these single-planet systems, the planet is orbiting the star in very close proximity, but in others, the reason is that the systems may actually hold more planets that initially assumed. “In these cases, we believe that the deviation from the rule can help us reveal more planets that were hidden up until now,” Nanna Bach-Møller, first author of the scientific article, explains. If we are able to see the extent of eccentricity of the planet , then we know how many other planets must be in the system—and vice versa, if we have the number of planets, we now know their orbits. “This would be a very important tool for detecting like our own solar system, because many exoplanets similar to the planets in our solar system would be difficult to detect directly, if we don’t know where to look for them.”

The Earth is among the lucky 1%

No matter which method is used in the search for exoplanets, one reaches the same result. So, there is basic, universal physics at play. The researchers can use this to say: How many systems possess the same eccentricity as our solar system? – which we can then use to assess how many systems have the same number of planets as our solar system. The answer is that there are only 1% of all solar systems with the same number of planets as our solar system or more. If there are approximately 100 billion stars in the Milky Way, this is, however, still no less than one billion solar systems. There are approximately 10 billion Earth-like planets in the , i.e. in a distance from their star allowing for the existence of liquid water. But there is a huge difference between being in the habitable zone and being habitable or having developed a technological civilization, Uffe Gråe Jørgensen stresses. “Something is the cause of the fact that there aren’t a huge amount of UFOs out there. When the conquest of the planets in a solar system has begun, it goes pretty quickly. We can see that in our own civilization. We have been to the moon and on Mars we have several robots already. But there aren’t a whole lot of UFOs from the billions of Earth-like exo-planets in the habitable zones of the stars, so life and technological civilizations in particular are probably still fairly scarce.”

The Earth is not particularly special—the number of planets in the system is what it is all about

What more does it take to harbor life than being an Earth-size planet in the habitable zone? What is really special here on Earth and in our solar system? Earth is not special—there are plenty of Earth-like planets out there. But perhaps it could be the number of planets and the nature of them. There are many large gas planets in our solar system, half of all of them. Could it be that the existence of the large gas planets are the cause of our existence here on Earth? A part of that debate entails the question of whether the large gas planets, Saturn and Jupiter, redirected water-bearing comets to Earth when the planet was a half-billion years old, enabling the forming of life here.

This is the first time a study has shown how unique it is for a solar system to be home to eight planets, but at the same time, shows that our solar system is not entirely unique. Our solar system follows the same physical rules for forming as any other , we just happen to be in the unusual end of the scale. And we are still left with the question of why, exactly, we are here to be able to wonder about it.

More information:
Nanna Bach-Møller et al. Orbital eccentricity–multiplicity correlation for planetary systems and comparison to the Solar system, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa3321

The solar system follows the galactic standard—but it is a rare breed (2020, November 30)

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