We know how termites build their nests, but what triggers the building of a termite mound in the first place?
How and why different termite species build mounds has been the subject of previous articles in this magazine. Prevailing climatic conditions and the need for air ow and temperature regulation appear to play a role, however, this does not deal with how an individual nest starts building a mound.
There has commonly been the belief that there could be some (as yet unidentified) ‘cement pheromone’ involved in the construction process that stimulates worker termites to deposit excavated soil. Whilst computer models support this theory, there was no evidence for the existence of such a pheromone. More recent research has suggested that the excavation and the initiation of mound building can be explained with non-pheromone-based behaviours.
Looking at two Macrotermes species in Africa, which produce different shaped mounds – M. michaelseni produce tall, narrow mounds, whilst M. natalensis produce short, broad mounds – researchers carried out a series of laboratory experiments in which they used video tracking to map the behaviour of individual termites. They collected soil from active building sites at the termites’ mounds and then washed it in acetone before rinsing with water to remove any organic compounds. The soil, along with 25 major workers, were placed in each experimental area and the excavation behaviour was recorded.
The researchers determined that the excavation process involved two stages. The first stage was considered disorganised with termites exploring the area and a few digging out individual soil pellets. The second phase was more organised, with co-ordinated excavation occurring at a few common sites.
A key observation was that the initiation of a new excavation site was a relatively rare event. Of the nearly 400 termites tracked in the experiment, only 34 individuals initiated excavation sites. This observation is consistent with work on other species and indeed there is some evidence that the chief excavators are older workers. However, nearly 50% of the termites became involved in excavation, often for extended periods of time. Excavated soil was placed either side of the excavation point – only 3.4% was moved beyond the vicinity of the excavation site.
The video analysis indicated that there were two factors that influenced whether an individual termite joined an excavation site: firstly, its individual propensity to become involved in excavation and secondly, the number of other termites actively excavating at the site. The more termites at the site, the more likely additional termites would join.
Whilst not excluding the involvement of pheromones, the research suggested that aggregation of termites at excavation sites rather than a potential ‘cement pheromone’ at deposition sites, is a key mechanism driving excavation and mound initiation.
Green, B et al (2017). Excavation and aggregation as organizing factors in de novo construction by mound-building termites. Proceedings of the Royal Society B – Biological Sciences.
Yang, R-L and Su, N-Y (2009). Individual Task Load in Tunnel Excavation by the Formosan Subterranean Termite (Isoptera: Rhinotermitidae). Annals of the Entomological Society of America, 102(5): 906-910.
More information on termite nests and termite reproduction.