Eciton burchellii is a species of nomadic army ant, native to tropical rainforest biomes throughout Central and South america. Every few weeks, these ants travel to a new temporary nesting site, called a bivouac. To gather food, army ants engage in massive swarm raids, in which large portions of the colony comb through forest floor leaf litter in search of arthropod prey, such as insects and spiders.
During these raids, army ants must travel over a wide span of uneven terrain, often carrying heavy food items. In order to accomplish this, army ants use a wide range of navigational strategies, working as a group to make a difficult journey easier. Researchers Matthew J. Lutz and colleagues have looked into one such strategy, in the hopes of better understanding how army ants, and other animals that form complex social organizations, work together to overcome environmental challenges.
This strategy is known as scaffolding, a behavior in which Eciton burchellii army ants stop and grip onto an inclined surface, forming a temporary bridge and remaining stationary while other ants pass (Fig. 1). These scaffolds, when observed in nature, vary widely in the amount of ants that participate, and in the proximity with which they cluster together.
In the Lutz study mentioned above, researchers tested what factors lead to observed differences in the quantity and closeness of army ants participating in scaffolding behaviors, using a rotating sandpaper platform, which could be set at 10 degree intervals (Lutz et al. 2021). This rotating platform was hinged between two stationary 3D printed walkways (Lutz et al. 2021). This structure was then placed in a way which would disrupt army ant traffic, with the rotating platform set to one of the angles being tested (Lutz et al. 2021). These experimental angles were at 10 degree intervals, between 20 and 90 degrees (Lutz et al. 2021). When ants began traveling over the first walkway, they were video recorded for 10 minutes, and this process was repeated 10-12 times for each angle tested (Lutz et al. 2021).
Using these recordings, researchers noted two important factors in determining how many ants participated in scaffolding behavior, and how closely they were clustered together. The first was the angle of the platform. The clustering and quantity of ants in a scaffold tended to increase as the angle of the platform increased (Lutz et al. 2021). Scaffolding behavior appeared to emerge starting with a 40 degree incline, as little or no scaffolding behavior was observed in the 20 degree and 30 degree groups (Lutz et al. 2021). Researchers also noted that scaffolds tended to increase in the quantity and clustering of participating army ants, when larger quantities of prey were being transported over an inclined surface (Lutz et al. 2021). Both findings indicate that scaffolding behavior is a response to environmental conditions, which attempts to mitigate the increases in ant slipping, falling, and prey item loss, which occurs when army ants travel over inclined surfaces (Lutz et al. 2021).
How do army ants manage to carry out such a complex group behavior? Many people would guess that scaffold formation would require detailed communication between ants. However, the data collected by Lutz and colleagues suggests otherwise (2021). It appears that scaffolding is actually an emergent property, caused by each individual ant’s responses to their own, immediate environment. When ants begin slipping while on an incline, they stop, carefully regrip the surface, then remain still (Lutz et al. 2021). As each ant does this, it reduces the chance of other ants slipping, since they can use these stationary ants to remain stable (Lutz et al. 2021). This process is all that is needed to form the self-regulated scaffolds observed. Ants will continue to slip, stopping and gripping the inclined surface, until the scaffold is large enough that ants are no longer at risk of slipping (Lutz et al. 2021). This also accounts for the increased size and cluster of scaffolds at higher angles, since more ants packed together in tighter scaffold formations are required before ants stop slipping down the steeper slope.
An understanding of army ant scaffolding as something coming out of the error correcting decisions of individual ants, has important implications for the future study of large group behavior in animals. While it is easy to assume that all instances of widespread cooperation between animals are organized through communication between these individuals, this research indicates that quite often, these phenomena can arise from the behavior of individuals interacting only with the information provided by their own senses.
Works Cited
Lutz MJ, Reid CR, Lustri CJ, Kao AB, Garnier S, Couzin ID. Individual error correction drives responsive self-assembly of army ant scaffolds. Proceedings of the National Academy of Sciences. 2021 Apr 27;118(17). https://www.pnas.org/content/118/17/e2013741118
Eciton burchellii is a species of nomadic army ant, native to tropical rainforest biomes throughout Central and South america. Every few weeks, these ants travel to a new temporary nesting site, called a bivouac. To gather food, army ants engage in massive swarm raids, in which large portions of the colony comb through forest floor leaf litter in search of arthropod prey, such as insects and spiders.
During these raids, army ants must travel over a wide span of uneven terrain, often carrying heavy food items. In order to accomplish this, army ants use a wide range of navigational strategies, working as a group to make a difficult journey easier. Researchers Matthew J. Lutz and colleagues have looked into one such strategy, in the hopes of better understanding how army ants, and other animals that form complex social organizations, work together to overcome environmental challenges.
This strategy is known as scaffolding, a behavior in which Eciton burchellii army ants stop and grip onto an inclined surface, forming a temporary bridge and remaining stationary while other ants pass (Fig. 1). These scaffolds, when observed in nature, vary widely in the amount of ants that participate, and in the proximity with which they cluster together.
In the Lutz study mentioned above, researchers tested what factors lead to observed differences in the quantity and closeness of army ants participating in scaffolding behaviors, using a rotating sandpaper platform, which could be set at 10 degree intervals (Lutz et al. 2021). This rotating platform was hinged between two stationary 3D printed walkways (Lutz et al. 2021). This structure was then placed in a way which would disrupt army ant traffic, with the rotating platform set to one of the angles being tested (Lutz et al. 2021). These experimental angles were at 10 degree intervals, between 20 and 90 degrees (Lutz et al. 2021). When ants began traveling over the first walkway, they were video recorded for 10 minutes, and this process was repeated 10-12 times for each angle tested (Lutz et al. 2021).
Using these recordings, researchers noted two important factors in determining how many ants participated in scaffolding behavior, and how closely they were clustered together. The first was the angle of the platform. The clustering and quantity of ants in a scaffold tended to increase as the angle of the platform increased (Lutz et al. 2021). Scaffolding behavior appeared to emerge starting with a 40 degree incline, as little or no scaffolding behavior was observed in the 20 degree and 30 degree groups (Lutz et al. 2021). Researchers also noted that scaffolds tended to increase in the quantity and clustering of participating army ants, when larger quantities of prey were being transported over an inclined surface (Lutz et al. 2021). Both findings indicate that scaffolding behavior is a response to environmental conditions, which attempts to mitigate the increases in ant slipping, falling, and prey item loss, which occurs when army ants travel over inclined surfaces (Lutz et al. 2021).
How do army ants manage to carry out such a complex group behavior? Many people would guess that scaffold formation would require detailed communication between ants. However, the data collected by Lutz and colleagues suggests otherwise (2021). It appears that scaffolding is actually an emergent property, caused by each individual ant’s responses to their own, immediate environment. When ants begin slipping while on an incline, they stop, carefully regrip the surface, then remain still (Lutz et al. 2021). As each ant does this, it reduces the chance of other ants slipping, since they can use these stationary ants to remain stable (Lutz et al. 2021). This process is all that is needed to form the self-regulated scaffolds observed. Ants will continue to slip, stopping and gripping the inclined surface, until the scaffold is large enough that ants are no longer at risk of slipping (Lutz et al. 2021). This also accounts for the increased size and cluster of scaffolds at higher angles, since more ants packed together in tighter scaffold formations are required before ants stop slipping down the steeper slope.
An understanding of army ant scaffolding as something coming out of the error correcting decisions of individual ants, has important implications for the future study of large group behavior in animals. While it is easy to assume that all instances of widespread cooperation between animals are organized through communication between these individuals, this research indicates that quite often, these phenomena can arise from the behavior of individuals interacting only with the information provided by their own senses.
Works Cited
Lutz MJ, Reid CR, Lustri CJ, Kao AB, Garnier S, Couzin ID. Individual error correction drives responsive self-assembly of army ant scaffolds. Proceedings of the National Academy of Sciences. 2021 Apr 27;118(17). https://www.pnas.org/content/118/17/e2013741118
