Army Ants: A Collective Intelligence
Franks, N. R. (1989). Army Ants: A Collective Intelligence: A neural network Seems an apt analogy as a colony of army ants navigates the tropical rain forest. American Scientist, 77(2), 138–145.
Individual army ants are very unsophisticated and remain so even in groups of 100, in which they will walk around in circles until they die. However, colonies of ants show very sophisticated behaviour and problem-solving capabilities. Thus studying army ants helps us understand how intelligence arises: it could be an emergent property of collective communication. Franks hypothesises that human consciousness could similarly be an epiphenomenon of parallel processing.
Background to army ants
- The complicated structure of a colony has evolved independently multiple times, though the best known species are Eciton burchelli and Eciton hamatum.
- Colonies tend to be large because of the necessity of many individuals to overwhelm larger arthropods.
- The size and impact of the colony means that the colony has to be nomadic, because they constantly deplete the local resources.
- Raids can be 20m wide and 200m long, at is inconceivable that any of the 200,000 workers have knowledge of the whole raid.
- When the colony isn’t migrating, there is a central bivouac (nest) from which raids proceed radially outward.
- Simulations have shown that the swarm’s structure can be achieved by simple self-organising interactions in which individual ants produce and react to pheromones.
- Eciton workers are not blind, but have very simple eyes. It is possible that the ants communicate their limited vision to other ants, resulting in collective vision.
Division of labour
- Worker ants belong to one of four physical varieties (‘castes’), determining their main role in raids.
- Submajors are the second-largest caste, and are primarily used for moving prey (they are 3% of the workforce, but comprise 26% of prey-porters).
- they have the higest leg-body ratio, which lets them walk quickly to the front of the raid.
- during colony migrations, the submajors carry other ants.
- submajors often initiate the transport of prey, before other workers join.
- enough workers join such that the speed of transport is always the same, to avoid congestion
- Groups of workers can carry disproportionately large loads (e.g three ants can carry something together when a single ant couldn’t carry a third of that weight).
- Majors are the largest caste, existing for defence of the colony. Their mandibles cannot be released once they bite; they hold until their death.
- Minims are tiny workers who stay in the nest and look after the larvae
- The remaining caste is the medium workers, who help with everything and make up the bulk of the force.
- The bivouacs are formed by a mesh of living workers, which houses the queen and brood.
- The ants can maintain the temperature despite 8˚C external variation.
- This thermoregulation may not be very sophisticated, it could just be that when the ants are hot they move away from the centre and ventilation improves.
- On certain days, raiding ants do not return home to the nest, they begin to form a new one. This precedes an emigration, in which the whole colony moves to the new nest.
- Emigrations are followed by many symbiotic organisms (myrmecophiles), including beetles and mites.
- Colonies only migrate when they are rearing larvae.
- The queen produces 100,000 eggs which need 15 days to grow. Over these 15 days, the colony raids every day and emigrates almost every night.
- Once the larvae pupate, the next 20 days are stationary. Statary raids proceed in spokes on about 14/20 days, each being separated by 123˚, which allows the surrounding area to replenish.
- After pupation, the 35-day cycle repeats (including a new batch of workers).
- At the start of the dry season, the colonies no longer produce workers, and instead produce about six queens and 4000 males.
- Once the queens hatch, the colony stages two raids in opposite directions. The queens then try to run to the front of the raids, and the two successful queens become the centres of new bivouacs. The other queens are left to die.
- The workers seem to either help or hinder some of the queens, which are probably chosen based on the strength of their pheromones.
- The male ants have wings, and look like wasps. However, the workers (who protect the queen) can effectively choose which males have access to the queen.
This article was suggested by a book by Melanie Mitchell, Complexity: A Guided Tour. Really fascinating introduction into complex systems.