Coordination without Communication

Stan Franklin
Institute for Intelligent Systems and
Department of Mathematical Sciences
University of Memphis

Abstract: Here we examine situations in which coordinated behaviors occur without prior planning via communication. Such situations are both common and effective in multi-agent systems, be they biological or computational. Such coordination results from stigmergic sampling of the environment and responding to it. We conclude that stigmergic coordination without communication should be considered as a control architecture when designing multi-agent systems.

Keywords: communication, conjoined twins, control architecture, coordination, ethology, motor control, multi-agent systems, planning, robotics, social insects, stigmergy

"Hey, Dad. Would you please hold this banner in place while I tape it to the wall?" asked my eight year old daughter, Sunny, early one Mother's Day morning. I did. And she did. It was a quite unremarkable example of coordination of actions following a mutual plan arrived at via communication. Without giving it any thought, I'd assumed that coordinated actions always resulted from such planning. The coordination of hunting lionesses reported by Donald Griffin (1984, p. 86) provides an example of my making such an assumption. Here's a paraphrase of his account from my Artificial Minds (1995, p. 59).

A group of five lionesses approached two groups of wildebeest separated by a road. Two lionesses mounted ant hills so as to be clearly visible but pose no threat. A third crept along a ditch paralleling the road until she was between the two bands of wildebeest. Then a fourth lioness charged out of the woods adjacent to the leftmost band, driving them across the road toward the other band. The lioness in the ditch easily killed one wildebeest as it jumped the ditch. All five then began to feed.

I go on to say (1995, p. 59), "It's hard for me to believe that the lionesses described by Griffin met as a committee to devise their plan. On the other hand, it's equally hard for me to believe that this scenario occurred unconsciously and without some sort of communication." While I still doubt that such coordinated actions could have occurred unconsciously, I'm no longer at all sure about the need for communication.

The story of the Hensel twins (Life, April 1996, p. 44; Time, March 25, 1996, p. 60) sowed the seeds of doubt. Abigail and Brittany, kindergartners, share a body. Their separate heads rise from a single pair of shoulders topping a body with the usual number of arms and legs. Inside, things are more complex, with two hearts pumping a single circulatory system, three lungs, two stomachs, and separate spines jointed at the pelvis. Their nervous systems are disjoint. A touch on the right side is felt only by Abby. Each controls one arm and one leg. And therein lies the rub. At fifteen months, they learned to walk. Now they swim, bike, and tie shoe laces, all requiring considerable coordination of actions. This coordination is apparently achieved without communication.

"But," I've been told, "they do communicate through the environment. Each is aware of the other's actions via vision, proprioception, etc. That's communication." Is it? True, all communications occur when one agent acts on the environment and the other senses the results of that action. But would all such acting and sensing comprise communications? I think not. It's far fetched to call my following the tracks of a bobcat in the snow communication between the bobcat and me. Communication, in the sense of the word I intend, requires the sending and receiving of signals. Is the bobcat signaling when he leaves tracks in the snow? I think not. This situation is quite analogous to me following a river downstream while canoeing. How about the tigress marking the boundary of her territory with urine? I think so. And the difference? Intention, in the folk psychology sense, not the philosophical sense. The tigress intends her signal to be received and to be meaningful. But what of animals who send sexual signals via pheromones? Surely not all such are sent intentionally. It seems that I've painted myself so far into a corner as to have to postulate unconscious intentionality brought on by evolution. Such problems are to be expected when we arrive at the fuzzy edge of any non-mathematical concept. Still, I see no evidence of Abby communicating with Britty while tying shoes.

Is such coordination without communication possible only under the most unusual circumstances of the Hensel twins? Let me give you another example that should have alerted me to the possibility of coordination without communication years ago. My older children during their teenage years were quite fond of a maze game called Labyrinthspiel. Picture a roughly one foot square wooden box some four inches tall. Mounted in the upper interior of the box were two nested trays. The inner tray was a maze containing some sixty holes strategically placed as pitfalls for the unwary traveler. The traveler was a small steel ball, just able to fall through one of the holes into the bottom of the box. The outer tray, mostly rim, was mounted at its center on a metal rod running north south. A knob, attached to the rod, could be rotated, tilting both trays in the east west direction. The inner tray was independently mounted on a second metal rod perpendicular to the first. Rotating its knob tilted only the inner tray in the north south direction. With the ball in the starting position, the player attempted to guide the ball through the maze, avoiding all the holes. This required well coordinated manipulation of the two knobs using the principle that balls roll down hill. It wasn't easy.

Over time, two of the children, Phil and Mimi, completely mastered Labyrinthspiel, being able to reliably guide the ball back and forth through the maze, missing all the pitfalls. But such mastery led to boredom. So, they invented two-person Labyrinthspiel, Mimi controlling one knob and Phil the other. To my amazement, they were soon able to coordinate their movements so as to successfully negotiate the entire maze.

It seems to me that this sort of coordination is impossible to achieve in real time via any sort of signaling. It can only result from each agent frequently sampling the environment, in this case watching the ball and the attitude of the maze, and reacting to it with its goal in mind. Reacting, in this case, should be taken as reacting not only to the ball's location, but to its velocity (including direction)1 as well. Coordination emerges incidentally. I also suspect that there are no internal signals from the right hand's controller to the left's when a single human is playing. Probably sampling and reacting suffices.

This suspicion is at least partly shared by my neurophysiologist friend Lloyd Partridge who specializes in motor control. Lloyd doubts that central nervous system coordination plays much of a role in such activities as playing Labyrinthspiel that require coordination of the actions of the two hands (personal conversation).

As another example, consider several adult mongoose attempting to guard their nest and young from a marauding cobra (as seen on a TV documentary). One mongoose moves just within the cobra's striking distance. Just as the cobra prepares to strike a second moves in, distracting him as the first retreats. Again, just before the cobra strikes, a third advances and distracts him. This script continues until the cobra wearies and departs. Again, coordination occurs though signaling seems neither needed nor possible.

Is such coordination without communication possible only by biological agents? Let's look at another scenario. Genghis, produced by Rodney Brooks and his students at MIT(1989), is a six-legged, insect like robot that walks. Each leg is attached to the body at a joint with two degrees of freedom. The movement of each joint is controlled by two servo motors. When carefully programmed so that the six legs coordinate well, Genghis walks. Later, Pattie Maes and Brooks (1990) deprogrammed Genghis, and augmented his sensors. They fitted Genghis with touch sensors on the under front and under back of his abdomen which sent a "pain" message whenever his belly touched the ground. They also attached a trailing wheel which sent a "pleasure" message when the wheel rolled forward. With this reinforcement and punishment in place, Genghis learned to walk with an insect's tripod gate. This coordination of the six legs was achieved with no communications between them.

Can this be so? Genghis is built on Brooks' subsumption architecture (1989) with twelve leg swing behaviors, six forward and six back. Each of these behaviors senses its environment and acts on it directly in pursuit of its own agenda. That is, each behavior, together with its senses and effectors, is an autonomous agent (Franklin and Graesser, 1996). Each behavior acts so as to increase it "pleasure" and to minimize its "pain." Though each behavior samples the environment and can be aware of which legs are on the ground and which not, no behavior communicates with any other. Yet they learn to coordinate their actions and walking emerges.

Another robotic example of coordination without communication was modeled on the behavior of ants (Deneubourg et al, 1990). Noting that dead ants were carefully removed from an ant colony's nesting area, an entomologist strewed the area with several thousand dead ants. On the following day he observed numerous small piles of dead ants. The second day after strewing brought a smaller number of large piles. By the third day there was only one (or sometimes two) piles. He conjectured that a probabilistic explanation of this behavior as follows: Faced with a dead ant, an ant picked it up with probability inversely proportionate to the number of other dead ants in the vicinity. While carrying a dead ant, an ant put it down with probability directly proportional to the density of dead ants in the vicinity. Thus the coordinated behavior of piling dead ants in a single pile resulted from a simple local rule controlling the behavior of a single ant. The piling behavior is accounted for without postulating communication between the ants.

Hoping to model this complex global behavior emerging from simple local rules, Beckerts, Holland and Deneubourg (1994) produced small, puck-piling robots. Picture a fifteen to twenty foot in diameter circular area inhabited by two or three shoe box size robots and very many hockey-puck sized wooden discs, uniformly distributed. Each robot is equipped with a scoop in front. When turned on, a robot moves forward until it accumulated three discs in its scoop, at which time it backs up, turns in a random direction, and again moves straight ahead. Hitting a wall also produces this backing and turning behavior, as does sensing (infrared) an impending collision with another robot. This simple local behavior results in global behavior modeling that of the ants, going through the stages of many small piles of discs, fewer larger piles, and eventually a single pile. All this again with no communication between the robots. The robots essentially ignore each other except for avoiding collisions. A single robot will go through the same stages of piling up discs.

Observing the nest-building behavior of termites led entomologist P. P. Grassé (1959) to define the notion of stigmergy, referring to actions of one agent being influenced by the effects of prior actions of other agents. Here's a description (Beckerts, Holland and Deneubourg, 1994, page 181) of the kind of collective, coordinated behavior that led to this concept.

When they start to build a nest, termites modify their local environment by making little mud balls and placing them on the substrate; each mud ball is impregnated with a minute quantity of a particular pheromone. Termites deposit their mud balls probabilisticly, initially a random. However, the probability of depositing a mud ball at a given location increases with the sensed presence of other mud balls and the sensed concentration of pheromone. The first few random placements increase the other termites probability of putting their loads at the same place. By this blind and random game little columns are formed; the pheromone drifting across from neighboring columns causes the tops of the columns to be built with a bias towards the neighbouring columns, and eventually the tops meet to form arches2, the basic building units.

Clearly we've just seen a description of coordinated behavior. But what of communication? Here we must face the issue dodged above. Is the impregnating of mud balls with pheromone signaling with intent to communicate? It's certainly signaling, even to being arbitrary in that it signals other termites and not a passing ant or bee. And, one might well postulate an unconscious intent to communicate evolved into the termite. But is it communication in the sense of our opening example of taping the mother's day banner to the wall? I think not. In that case the intent of the communication was to plan the coordinated action. I also wrongly attributed to the lionesses communication with the intent of planning their coordinated behavior. In the case of the termites no plan of coordinated action results from their communication, rather the carrying out of an evolved-in plan. The example of the termites requires that I refine my already narrow notion of communication, as used here, to mean communication with the intent of planning coordinated behavior. With this clarification, nest-building termites exhibit coordination without communication.

Nest-building behavior of termites is the prototypical example of stigmergy. Which of our other examples of coordination without communication could be classified as stigmergic behavior? Holland and Beckers (1996) distinguish between cue based stigmergy and sign based stigmergy, the behavior of the termites being an example of the second and that of the cemetery filling ants of the first. Pheromones serve as a sign to termites while ant corpses cue ants. The puck-piling robots modeled after the ants also exhibit cue base stigmergy. I suspect that shoe tying by the Hence twins is also cue based stigmergic. The two-person Labyrinthspiel seems almost the epitome of cue based stigmergy. How about the lionesses preying on the wildebeest? It seems plausible that the lioness circling into the woods was cued by the lioness creeping forward in the ditch, and that those on the termite mounds were cued by the first two. A timeline of the actions comprising this scenario would be helpful in judging how plausible.

I've conjectured above that a single human tying a shoelace is coordination without communication. Now I'd classify this behavior as a case of cue based stigmergy. But what of that human learning to tie shoes? I'd suspect that conscious planning of the sequence of actions happens. I'm tempted to call this central nervous system coordination internal communication between the brain structures controlling the two hands.

So far we've explored a rather diverse collection of examples of coordination without communication. So what? What are the conclusions? What can we learn from these scenarios? One obvious observation is that coordination, with or without communication, is a property of multiple agent systems. Another, originally less obvious to me, is that coordination without communication is both common and useful in such multi-agent systems. Yet another is that repeated and frequent sampling of the environment, and responding thereto, is the underlying mechanism of such coordination. The act of responding can be a cognitive one involving prediction, avoidance of pain, etc. This sampling and responding would include both sign based and cue based stigmergy. A final conclusion would have us consider stigmergic coordination without communication as a control architecture when designing multi-agent systems, be they robots, software agents, artificial life agents, or whatever.

Another conjecture I'm tempted to make is that common long-term goals on the part of the agents are required, although short-term goals can differ as exemplified by the Hence twins tying there shoes. But here's a shot at a counterexample. Let's alter the puck piling scenario described above in two ways. First, each disc will leave some sort of a trail as it moves. And second, an additional class of robots will be present who avoid discs and other robots and remove any disc trails they come across. Coordinated behavior sans communication will occur, but the long term goals of the two classes of robots will be different. One creates a puck cemetery, the other produces a clean floor.

Do note that the issue of common goals is not always a concern of the agents in a multi-agent system. In many of the systems we've described each agent is driven only by its local goals. In the case of the puck-piling robots, each robot is only interested in avoiding other robots and backing and turning after encountering an obstacle. The piling up of discs is purely a side effect. An outside observer would describe their global behavior as puck-piling; the individual robot wouldn't. But to a designer of multi-agent systems, this issue is immaterial. It's common in computer science to base a system's behavior on side effects.

This leads us back to the issue of stigmergic coordination without communication as a control architecture for multi-agent systems. Why should a system designer consider such an architecture? To possibly save computing resources. Communication is expensive, requiring additional architecture, and more intelligence. Planning is expensive in these same ways, and computationally as well. If a system can accomplish its tasks without communication and planning, so much the better. Sampling the environment and responding to it is required in any case. Evolution seems to have come to this same conclusion in many instances, as we've seen.

Agre and Chapman offer a substitute for planning, participatory improvisation (1987, p. 2). "Improvisation, like Planning, involves ideas about what might happen in the future. Improvisation differs from Planning in that each moment's action results, effectively, from a fresh reasoning-through of that moment's situation." (They capitalize "Planning" to denote the formal making of plans. On the other hand, their use of "reasoning-through" should be read as consideration, not as indicating some inference in a formal logic.) While improvisation is well beyond the reach of some of the systems we've discussed, say the puck-piling robots, it fits well in others, say two-person Labyrinthspiel.

Let me conclude by suggesting an experiment that could shed light on one of the conjectures made here. The conjecture is that central nervous system coordination (internal communication) plays little role in such activities as tying one's shoes or playing Labyrinthspiel. An experiment which compared learning times for one-person Labyrinthspiel with learning times for two-person Labyrinthspiel (neither player having previously played) should provide evidence for or against.

Acknowledgments: I'm grateful to Lloyd Partridge, Michele and Kaveh Safa, and to participants in the Cognitive Science Seminar at the Institute for Intelligent Systems of the University of Memphis for helpful discussions of these ideas.


Agre, Philip E., and Chapman, David. (1987). "From Reaction to Participation." MIT Artificial Intelligence Laboratory.

Beckers, R., Holland, O. E. and Deneubourg J_L (1994). "From Local Actions to Global Tasks: Stigmergy in Collective Robotics," in R. Brooks and P. Maes eds. Artificial Life IV, Cambridge, Mass.: MIT Press, 181-9.

Brooks, Rodney A. (1989). "A Robot That Walks: Emergent Behaviors from a Carefully Evolved Network." Neural Computation, 1: 253-62

Deneubourg, J. L., Goss, S., Franks, N. R., Sendova-Franks, A., Detrain, C., and Chretien, L. (1990). "The Dynamics of Collective Sorting: Robot-like Ants and Ant-like Robots." In Meyer, J-A, and Wilson, S., eds, Simulation of Adaptive Behaviour: from animals to animats, Cambridge, Mass.: MIT Press, 356-65.

Franklin, Stan (1995). Artificial Minds. Cambridge, Mass.: MIT Press.

Franklin, Stan and Graesser, Art (1996). "Is it an Agent, or just a Program?: A Taxonomy for Autonomous Agents," Proceeding of the 3rd International Workshop on Agent Theories, Architectures, and Languages, Springer Verlag

Grassé P. P. (1959). "La reconstruction du nid et les coordinations inter-individuelles chez Bellicositermes natalensis et Cubitermes sp. La theorie de la stigmergie: Essai d'interpretation des termites constructeurs. Ins. Soc., 6, 41-83.

Griffin, Donald R. (1984). Animal Thinking. Cambridge, Mass.: Harvard University Press.

Holland, O. E. and Beckers, R. (1996). "The Varieties of Stigmergy: Indirect Interaction as a Control Strategy for Collective Robotics." Robotica. Forthcoming.

Maes, Pattie, and Brooks, Rodney A. (1990). "Learning to Coordinate Behaviors." In Proceedings of the Eight National Conference on Artificial Intelligence. Menlo Park, Calif.: AAAI. 796-802


1Though "velocity" is commonly used as a synonym for speed, in physics it's a vector including both speed and direction.

2I seem to recall that the movement from post and lintel architecture to true arches was considered a hallmark of human civilization rising to a higher level.

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