Excavation and aggregation as organizing factors in de novo construction by mound-building termites

Abstract

Termites construct complex mounds that are orders of magnitude larger than any individual and fulfil a variety of functional roles. Yet the processes through which these mounds are built, and by which the insects organize their efforts, remain poorly understood. The traditional understanding focuses on stigmergy, a form of indirect communication in which actions that change the environment provide cues that influence future work. Termite construction has long been thought to be organized via a putative 'cement pheromone': a chemical added to deposited soil that stimulates further deposition in the same area, thus creating a positive feedback loop whereby coherent structures are built up. To investigate the detailed mechanisms and behaviours through which termites self-organize the early stages of mound construction, we tracked the motion and behaviour of major workers from two species in experimental arenas. Rather than a construction process focused on accumulation of depositions, as models based on cement pheromone would suggest, our results indicated that the primary organizing mechanisms were based on excavation. Digging activity was focused on a small number of excavation sites, which in turn provided templates for soil deposition. This behaviour was mediated by a mechanism of aggregation, with termites being more likely to join in the work at an excavation site as the number of termites presently working at that site increased. Statistical analyses showed that this aggregation mechanism was a response to active digging, distinct from and unrelated to putative chemical cues that stimulate deposition. Agent-based simulations quantitatively supported the interpretation that the early stage of de novo construction is primarily organized by excavation and aggregation activity rather than by stigmergic deposition.

Surface curvature guides early construction activity in mound-building termites

“…Density plots and averages of activity as a function of curvature for all orientations are shown in SI Figure A.6. While excavation and deposition cannot be separately quantified with our method, visual inspection showed that deposition occurred primarily in the immediate vicinity of excavation sites (SI Figures A.3-A.5), consistent with previous studies[10].Figure 2quantifies the correlation matrices between surface geometric factors (curvature, inclination and height) and both termite early locations and construction activity.…”

“…To investigate how these factors influence termite movement, we manually marked termite locations in each video at every 10 seconds for the first 5 minutes of each recording (for a total of 40 frames per replica). This time range was chosen in order to capture initial exploration activity, during a period before excavations began, since active excavation influences activity of other termites [10]. Since termites sometimes do not start moving for some time after being placed on the surface, we excluded termites that had not yet moved to a distance of at least 30 pixels from their positions in the first tracked frame.…”

“…In the classic formulation, a putative cement pheromone is hypothesized to be added to soil pellets deposited by workers; other workers encountering this pheromone respond by depositing additional soil, leading to a positive feedback loop that results in the accumulation of soil at focal locations [3,6]. This emphasis on cement pheromone has been challenged by a growing number of studies in termites [7][8][9][10] and ants [11]; it is also noteworthy that no cement pheromone has yet been identified. Instead, other cues have been identified as playing a role in organization of insect construction activity.…”

Homeostasis as a fundamental principle for a coherent theory of brains

“…There is another dimension of homeostasis at work here, engaging cognitive interactions of termites with soil. We have identified at least five such cognitive interactions, involving various aspects of mound maintenance, construction and repair [45,62,63]. One is the classic concept of stigmergy [57], which we prefer to call focal building, because it draws soil into foci of building.…”

Fast, Accurate, Small-Scale 3D Scene Capture Using a Low-Cost Depth Sensor

“…Adapting such approaches to the case of construction by termites has likewise begun to provide new understanding [5], but faces distinct challenges that follow from the problem domain. In particular, tracking termite building activity involves tracking the movement of soil pellets created and manipulated by the termites.…”