Natures wonders: army ant raids (Introduction)

by David Turell , Wednesday, May 26, 2021, 20:24 (1275 days ago) @ David Turell

Each individual has its simple response. Numbers of individuals active dictates colony response:

https://phys.org/news/2021-05-army-ants-iconic-mass-raids.html

"Army ants form some of the largest insect societies on the planet....They live in very large colonies and consume large amounts of arthropods. And because they eat so much of the other animals around them, they are nomadic and must keep moving in order to not run out of food. Due to their nomadic nature and mass consumption of food, they have a huge impact on arthropod populations throughout tropical rainforests floors.

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"The raids are a coordinated hunting swarm of thousands and, in some species, millions of ants. The ants spontaneously stream out of their nest, moving across the forest floor in columns to hunt for food. The raids are one of the most iconic collective behaviors in the animal kingdom. Scientists have studied their ecology and observed their complex behavior extensively. And while we know how these raids happen, we know nothing of how they evolved.

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"In a small nest of 25 ants they used 5 sets of colors and painted each ant with a unique set of colors. The researchers placed a single fire ant pupa (the prey) in the foraging arena outside of the nest. The nest sends out a scout to look for food. Once the scout finds the food, she lays a pheromone trail back to home. Inside the nest she releases, what researchers believe to be, a recruitment pheromone that attracts the ants to her. They spill out of the nest and follow her trail to the food in a group raid.

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"'Our experiments show that in larger colonies, the ants become more synchronized in their leaving of the nest to scout. In other words, when an ant leaves, the chances that more ants will follow her are higher in large colonies. While we cannot directly say much about the actual mechanism underlying this observation, we know from other complex systems that an increase in synchronicity is a result of stronger positive feedbacks between individuals," said Gal. "In the army ant size limit, this will result in what we know as a mass raid."

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"The team concluded that expansions in colony size in the ancestors of army ants are sufficient to have caused the transition from group raiding to mass raiding behavior.

"'Probably the most common pattern is that collective behavior evolves via natural selection acting on and tweaking the interaction rules that the individual animals follow," said Kronauer. "But our study is a nice example of a different mechanism: scaling effects associated with group size can give you dramatically different outcomes in terms of collective behavior, even though the individual rules don't change much." (my bold)

"Gal agreed, "Of course, it has been long known that changing group size can have a dramatic effect on emergent collective behavior. This has been shown both theoretically and experimentally. We have now shown that this effect can also be harnessed by evolution, and that collective behavior can be adapted over evolutionary timescales without actually modifying the behavior of individuals."

"As far as is known, the coordinated behavior of clonal raider ants is one of the most complex social behaviors that can be induced or studied in the lab. The authors are currently working on a detailed study of how individual ants behave during the course of the raid, and how the structure of the raid responds to variation in environment and colony composition.

"'We suspect that the ants specialize to some extent on specific tasks," said Chandra. "There's probably some very interesting division of labor going on, and there's also clearly complex communication—the ants use several different pheromones to talk to each other and to organize the raid. And there are several decisions the colony must make in the course of the raid. It's an incredibly rich behavior and there are many questions we could ask in the future and we're laying the groundwork for that.'"

Comment: In ant bridges and rafts individual ant responses are generally the same cooperative response with little individual differences. I suspect the same will bg=e found here.