Self-regulated systems without central control – the secret of the ants
There are surprising parallels between artificial and natural systems. Systems are sets of components working together as part of a mechanism, and they often rely on central components (e.g., CPU, brain) to receive information and mobilize components to perform tasks. Natural systems, however, are frequently composed of multiple individual organisms, like ants colonies, for example. In these cases, a staggering number of interactions would be necessary for a central component to gather information regarding the whole system and mobilize individuals accordingly. That should be a significant limiting factor to the functioning of the system as a whole. Despite that, such natural systems still manage to thrive. The mechanisms behind this unlikely success have been the centre of attention of scientists for decades. A recent study on the behaviour of ants has shed some light on this mystery.
Ants are a highly diverse group of insects that live in colonies with complex social structures ranging in size from a few dozen to millions of individuals, depending on the species and environment in which they live. The queens and males are responsible for all reproductive labour. Meanwhile, the other colony members divide the remaining activities related to keeping the colony alive according to characteristics like caste, age, or social hierarchy. The combined effect of the activities of the workers must meet the colony requirements as a whole.
The foragers are the group of ants responsible for gathering food within a colony. They collect food outside the colony’s nest and distribute it to the other ants. This distribution is often through mouth-to-mouth interactions, in which they pass regurgitated food to other individuals. All the food that goes into the colony passes through the foragers. Thus, the food requirements of the colony rely on the combined effect of the behaviours of these individuals reacting to their local environment.
Fig 1. Workers of the species Polyrhachis dives sharing food mouth-to-mouth. Photo by Andreas.
Even though there is seemingly no way that foragers could assess the colony’s requirements as a whole at any point, when introduced to a source of food, the amount of food in the colony follows a logistic distribution through time. This observation shows a coordinated regulation of the foragers’ activities in response to the amount of food accumulated in the colony. This regulation happens through adjustments in the behaviour of individual foragers, like the number of active foragers or the rate at which each forager leaves the nest to gather food. However, the local origins of this global regulation remained poorly understood until recently.
Fig 2. A representation of the way food is accumulated in an ant colony, from starved to saturate. The grey and red lines show the corresponding changes in the foraging activity of the colony and individual ants, respectively. Adapted from Greenwald et al. (2021).
Using microscopic measurements of real-time individual crop loads and food-flows in single ant-to-ant interactions, a group of scientists led by Dr. Efrat Greenwald decided to investigate this topic. They unveiled how the self-regulation of a colony can emerge from the activities of single ants that are unable to assess the system requirements as a whole. To manage that, they provided fluorescently labelled food to a starved colony of individually tagged Camponotus sanctus.
Fig 3. Workers of the genus Camponotus. Photo by Rakesh Kumar Dogra.
The foragers have a cyclic behaviour, composed of two phases: outside, gathering food; and inside the nest, distributing it. On each mouth-to-mouth interaction, the crop load of the recipient ant determines the volume of food transferred, which is on average 14% of the space left to fill her crop. On average, this volume represents the colony satiation level, decreasing as the colony approaches saturation. As the food accumulates in the colony, foragers transfer less food per interaction, and available recipients (ants that are not satiated) become scarcer. Consequently, the foragers need to interact with more ants and travel deeper into the nest to find available recipients to unload, resulting in more time inside the nest. In this feedback, foragers increase colony satiation, which in turn inhibits foragers’ trip frequency. The colony satiation state, reflected by the average crop loads of the ants, regulates their interaction with foragers, ultimately defining the rate of food inflow to the colony.
If interpreted as data in the environment, “information can be obtained from a range of environmental stimuli and phenomena; not all of which are intended to ‘convey’ a message, but which can be informative when appropriately interpreted” (Madden 2020). In this context, the foragers access and respond to information regarding the entire colony through the principles of sampling theory, as the average crop load of the ants represents the colony state. In short, the mechanisms described by Dr. Efrat Greenwald et al. show how these ants evolved a system that allows single components (i.e., foragers) to access the global state (i.e., colony) despite the inexistence of central control.