Stigmergy, the key principle of
complex self organization in Multicellular Organisms
Organization by stigmergy occurs when individual parts of the system communicate with one another indirectly by modifying and sensing their local environment. Termite mounds are a classic example, but cells do it and computers do it too.
The third of the four principles of multicellular systems is that much of the communication between cooperating entities (cells, social insects or computers) is indirect and distributed. The entities deposit long-lived cues in external structures -- connective tissue, termite mounds, or databases as the case may be. For example, chess pieces on a chess board structure the actions of chess players who interact with each other by changing the locations of the pieces. The external persistent information helps to organize the collective behavior of the cells, insects, computers or people. The term stigmergy was coined in the 1950’s [1] to put a name to these sorts of reciprocal relationships between social insects and the stigmergy structures that they build, e.g., termite mounds (see photo below), ant hills, beehives and even the pheromone-marked ant trails of nomadic social ant species such as army ants. More recently, the idea has been adopted by other disciplines including computer science.
The term stigmergy
is relatively new. But the phenomenon itself is ancient. The
cytoskeletons within individual cells are stigmergy structures that
help to organize
cellular functions. The very bodies of all multicellular
organisms are also stigmergy structures. Cells in a multicellular
organism create a growing body
whose
shape and boundaries are defined by a non-living
extracellular matrix created by the cells. The cells deposit all sorts
of cues, in
the form of messenger molecules, in or on this matrix. The shape of the
extracellular
matrix and the signaling molecules attached to it direct the movement,
differentiation,
and specialized function of the cells. So it is fair to think of the
bodies
of animals and plants as stigmergy structures akin to very complex
termite mounds.
The extracellular matrix of the earliest multicellular organisms was nothing more than a “slime” excreted by the cells. That slime formed a clump or thin film in which the cells lived and through which messenger molecules diffused from one cell to its neighbors. The more complex extracellular matrix structures of higher order animals and plants support more subtle and complex communication. Plants create rigid stigmergy structures made largely of cellulose. Animals create various sorts of connective tissue that gives structure to their organs and generally holds their bodies together. Mollusks (snails, clams, etc.) create shells, insects create chitinous exoskeletons, vertebrates create bone that is akin to coral (itself a stigmergy structure). Unlike coral, bone is constantly reshaped by the cells that create and maintain it to adapt to the changing stresses it encounters.
Brains, the neuron-based information processing systems of higher
multicellular
creatures, also depend upon stigmergy in the form of memories laid down
in long-term
physical changes to neurons and synapses. Memory modifies behavior and
behavior
modifies memory...that's the essence of stigmergy.
Computing relies on stigmergy structures in various sorts of memory as well --whether in the form of RAM, ROM, FLASH, disk file-systems or huge databases.
Stigmergy and "Self"
Stigmergy is intimately related to the somewhat slippery notion of "self." Whatever the philosophical niceties, self is clearly about the organism as a whole rather than just a collection of cells that share the same DNA. That is, self refers to a multicellular organism's body which includes both the cells of the organisms and the nonliving extracellular matrix that gives shape and structure to the organism. The extracellular matrix, which is the organism's stigmergy structure, persists for the life of the organism whereas most kinds of cells die and are replaced many times over during the lifespan of the body. In a very real sense, cells are part of the self only insofar as they participate in the self-organizational dance of stigmergy.
Communication via Signals versus Cues
Social insects, cooperating cells, cooperating neurons, and cooperating computers communicate both with signals and cues. The distinction is that signals are active communication events in real-time whereas cues are information embedded in the stigmergy structure to be read and reread many times. Both are specialized messages in the sense that they mean different things to different specialized receiving cells (or insects or computers). However cues are further specialized by their location -- that is, in addition to the information intrinsic to their molecular form or digital content, there is also information inherent in their location in the stigmergy structure. In chess terms, a pawn is just a pawn; the important information is which square on the board it occupies. Because cues have both a message content and a location, cues support more complex kinds of communication than do signals and hence tend to support more complex social organizations. For example, complex ant societies rely more on cues whereas simple ant societies rely more on signals[2].
As is the case with social insects, cells in multicellular organisms communicate both by signals (polymorphic messenger molecules moving indiscriminately through blood, lymph or other liquids) and by cues (polymorphic messenger molecules attached to the extracellular matrix). For example, bone, when stressed, provides cues to osteocytes and other bone cells for its own reshaping to better handle the forces placed upon it. And smooth muscle cells in the walls of blood vessels modulate their contractility according to cues from the extracellular matrix [3]. Not surprisingly, as with social insects, simple multicellular organisms communicate primarily by signals whereas complex multicellular organisms communicate more by cues.
Analogously, computing systems in complex human organization such as businesses rely on records (cues) deposited in databases (stigmergy structures), whereas loose organization, e.g., file-sharing, can work with real-time peer-to-peer messaging. Here again, multicellular computing recapitulates biology. Stigmergy is ever present in complex computing systems and many novel Internet stigmergy structures.
[1] See “Self-organization in social insects.” Bonabeau, E., Theraulaz, G., Deneubourg, J.L., Aron, S. & Camazine, S., Trends in Ecology and Evolution, vol 12, pp. 188-193, 1997.
[2] “Individual versus social complexity, with particular reference to ant colonies,” Anderson, C & McShea, D. W. Biol. Rev., vol 76, pp. 211-237, 2001. p. 228
[3] Extracellular matrix controls myosin light chain phosphorylation and cell contractility through modulation of cell shape and cytoskeletal prestress.” Polte, TR, Eichler, GS, Wang, N, & Ingber, DE. Am J Physiol Cell Physiol 286: C518-C528, 2004.
Contact: sburbeck at mindspring.com
Last revised 6/14/2012