Short reportArrestant property of recently manipulated soil on Macrotermes michaelseni as determined through visual tracking and automatic labeling of individual termite behaviors
Introduction
Nest structures of termites arise from the aggregate labor of many individuals. Grasse (1959) proposed that labor is coordinated by “stigmergy”, an indirect mode of communication whereby the work product of a builder acts to guide subsequent workers. In his proposed framework, construction is driven by a positive feedback interaction between termites, mediated by “cement pheromone” used to stimulate additional construction at scent-labeled sites by other termites. Based on Grasse’s initial work (Grasse, 1959), and bolstered by a small number of empirical studies (Bruinsma, 1977, Jones, 1979, Jones, 1980), many have attempted to virtually replicate the construction process of termites in simulation models (Bonabeau et al., 1998, Ladley and Bullock, 2005, O’Toole et al., 1999, Theraulaz et al., 2012). These models assume that an olfactory component of the nest material acts as a releaser of deposition behavior.
To prove the existence of a chemical releaser for soil deposition, Bruinsma (1977) presented termites with an extract from nestmates’ salivary glands, the putative source of the scent label. Termites deposited in proximity to the extract, seemingly evidential of a pheromone label for material deposition. But in this same study, Bruinsma (1977) presented similar termites with extracts of freshly deposited nest material and week-old nest material, and both nest material extract and salivary extract were equally effective in eliciting deposition behavior. This is problematic because for a pheromone marker to be informative in construction, it must be ephemeral, like trail pheromone (Traniello and Robson, 1995, Affolter and Leuthold, 2000, Gessner and Leuthold, 2000), as portrayed in models of stigmergic nest construction (Bonabeau et al., 1998, Theraulaz et al., 2012). Without decay of the old labels, the system becomes saturated and the ratio of the deposition signal to the noise of the background will obscure the necessary information for stigmergic feedback at any site.
More recently, in their work on tunnel excavation in Coptotermes formosanus, suggested that termites are directed to dig or deposit by traffic flow patterns, Bardunias and Su (2009), and tactile interactions, Bardunias and Su, 2010a, Bardunias and Su., 2010b, and questioned the existence of cement pheromone. Instead of a signal that specifically triggers deposition behavior, the odor of freshly deposited soil (henceforth designated as “nest material”) may simply act to arrest termites (Dethier et al., 1960, Barrows, 2011); causing them to stay near the nest material, but otherwise exhibit their normal behavioral patterns. Deposition is then more likely to occur in the presence of nest material simply because termites prefer a scent akin to a colony odor (Thorne and Haverty, 1991). Chemicals that release aggregation behavior are known to exist in Blattoidea (Riveault et al., 1999), which includes termites. While in most cases these substances are not thought to be true pheromones (Bell et al., 2007), in the few that possess true aggregation pheromones, the substance is derived from salivary glands (Brossut et al., 1974).
In this study, we presented major workers of Macrotermes michaelseni (Sjöstedt) with a choice between a substrate of clean soil devoid of colony scent cues and freshly deposited nest material in order to determine if termites preferentially reside on either substrate. Additionally, we examined whether nest material specifically elicits deposition behavior, or if all behaviors are performed more often on nest material. This would suggest an arrestant property of fresh deposits that may have been confounded with cement pheromone properties in earlier studies utilizing arenas of clean soil.
Section snippets
Material and methods
We used major workers of M. michaelseni, collected from Omatjenne Research Station in Northern Namibia (−20.4°, 16.5°) in May 2012. These were sorted into clean Petri dishes, kept hydrated with moistened filter paper, and stored for less than 2 h. The nest material within the collection traps was collected in airtight containers, and a small sample was left in the sun to dry. Weighing the sample before and after drying allowed us to estimate the water content of the nest material. Experimental
Results and discussion
Upon introduction to the arenas, termites milled rapidly around the perimeter of the dish for the first 6 min (Fig. 1b). After this initial period, increasing numbers of termites engaged in behaviors other than wandering around the dish: termites spent the most time wandering (35.20% ± 17.88), excavating (34.82% ± 23.48), or resting (22.61% ± 27.43), and less time transporting (5.68% ± 7.52), depositing (1.33% ± 1.42), and interacting (0.35% ± 1.40). Most excavation sites were on or within 5 mm from the nest
Conclusion
Our findings suggest that nest material acts as an arrestant when compared to clean soil. This suggests a property of nest material that may have been confounded with cement pheromone in earlier studies based on experimental arenas composed of clean soil. On a substrate of clean soil, the only nest material present will be soil excavated and subsequently deposited by termites. Additional deposition at these sites may simply occur because termites carrying soil are arrested in proximity to the
Acknowledgments
We thank Olena Bodila with the Harvard Extension School for manually tracking termite paths, and Nan-Yao Su, University of Florida, for providing meeting space. This work was supported by the Wyss Institute for Biologically Inspired Engineering.
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