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Social insects build highly
sophisticated nests on a scale that far exceeds the individual. Understanding
the mechanisms that enable these insects to work together to build structures
as complex as anthills is one of the major challenges of current research
into animal cognition. Combining techniques from non-linear sciences and
ethology, an international team bringing together the CNRS, the Université
Paul Sabatier in Toulouse, the Free University of Brussels, the Polytechnic
University of Catalonia, and the Santa Fe Institute has succeeded in deciphering
and modeling the mechanisms that enable ants to spatially organize the
way they gather the corpses of their dead. Their results show that the
morphogenesis of these structures is based on a mechanism of local activation
and long-range inhibition. This is the first example of patterns resulting
from such a mechanism to be shown in higher organisms. This work was published
in Proceedings of the National Academy of Sciences in its July
11, 2002 issue.
Insect societies make good models for the study of morphogenesis phenomena
in biology. Given the scale of such phenomena, experimental study of the
individual mechanisms involved in forming patterns can be approached much
more simply than in any other biological system.
Insects do not have any representation, plan, or explicit knowledge of
the overall structure that they help create. The structures produced (nests,
networks of trails, etc.), whose scale can reach several hundred times
the size of an individual, are not programmed explicitly at individual
level, but rather they result from chaining together a large number of
inter-individual interactions or interactions between the individuals
and their environment. In most cases, the information to which the individuals
have access is local and its scope or range is much smaller than that
of the resulting structures. The insect has no grasp of the overall structure
to which it contributes, much like a molecule within a chemical or biological
system.
By using a joint approach that closely combines experimentation and modeling,
Guy Theraulaz, from the “Laboratoire d’éthologie et cognition
animale” (Ethology and Animal Cognition Laboratory - CNRS - Université
Paul Sabatier in Toulouse), in collaboration with an international team,
has studied the spatial patterns resulting from a phenomenon that is very
common in ants: gathering their dead and forming cemeteries. This work
has made it possible to show the individual behavior patterns that govern
the way the dead bodies are arranged by Messor sancta ants. The
studies conducted concern the combined effects of different dead body
densities and different enclosure sizes, on the number and the spatial
distribution of the piles that are formed by the ants. In the experimental
situation, the ants arrange the dead bodies and form piles that are organized
uniformly in space. This property comes from the fact that the behavior
patterns of ants lead to local amplification, self-catalysis of deposit
(the larger the pile, the higher the probability of new deposit by an
ant), and to inhibition at longer range, the dead bodies deposited at
a pile no longer being available to initiate new piles.
This research has shown that this process has the main characteristics
of the reaction-diffusion models, namely competition between modes, symmetry
break, and emergence of a characteristic wavelength between the clusters
formed by the ants. The results of a mathematical model incorporating
the characteristics of the individual behavior of the ants are similar
to the experimental results. Thus, for identical initial conditions and
density, the spatio-temporal dynamics of clustering obtained in the model
and in the experiment are similar. Likewise, the distributions of the
inter-pile distances obtained in the model are similar to those obtained
experimentally. Furthermore, the predictions of the model have been verified
experimentally.
This work reveals, for the first time, the existence of morphogenesis
processes based on a mechanism of local activation and long-range inhibition
in biology of higher organisms. It shows that social insects can use this
type of instability to build their nests and to produce a whole set of
spatial patterns. These results also give a new perspective from which
to view animal cognition and, in particular, the individual cognitive
capacities necessary to produce structures as complex as a nest or a communications
network. These findings would also suggest that numerous spatial structures
produced by social insects involve procedures similar to those formulated
by Alan Turing, fifty years ago, to explain the morphogenesis underlying
the formation of patterns on the coats of tigers, zebras, giraffes, etc.
Reference
Theraulaz, G., Bonabeau, E., Nicolis, S., Solé, R.V., Fourcassié,
V., Blanco, S., Fournier, R., Joly, J.L., Fernandez, P., Grimal, A., Dalle,
P., & Deneubourg , J.L. 2002. Spatial Patterns in Ant Colonies. Proceedings
of The National Academy of Sciences USA July 11, 2002.
Photos available on request
Researcher
contact :
Guy Theraulaz
Laboratoire d'éthologie et cognition animale (CNRS-Université
Paul Sabatier, Toulouse)
Tel: +33 5 61 55 67 32
e-mail: theraula@cict.fr
CNRS Life Sciences
Department contact :
Françoise Tristani
Tel: +33 1 44 96 46 26
e-mail: francoise.tristani@cnrs-dir.fr
Press contact
:
Martine Hasler
Tel : +33 1 44 96 46 35
e-mail : martine.hasler@cnrs-dir.fr
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