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TERMITE TUNNELLING AND THE DIVISION OF LABOUR

In some social insect species, the amount of work performed by each member can vary greatly. Is the same true of termites when it comes to tunnel building?


“Of necessity, he who pursues a very specialised task will do it best.” This concept of the division of labour, noted by a student of Socrates in the 4th century BC, is what has made human society, and social insect colonies such as those of ants, bees, and termites, so successful; creating efficiencies out of their large workforces. Although social insects survive by working together, studies have shown that within the colony there are key individuals who work harder than the rest, and these individuals have the ability to influence the others on a large scale.

In his work on ants, entomologist James Traniello observed that within each insect caste, key individuals perform a disproportionate amount of the colony labour. In other words, not everyone pulls their weight! He went on to propose three types of key individuals: catalysts, performers, and organisers. Performers he defined as individuals who were highly active and accounted for a disproportionate amount of the labour on any task. Catalysts stimulated others to be more active through their actions, and organisers served to commence the work process and maintain colony cohesion throughout the task. Each of these individuals has the ability to make a significant impact on the output of the colony.1 But can this framework be applied to termites?

 

The role of the individual

When we look at a social group and consider how a key individual can influence the overall outcome of the task, it is obvious in a predator/prey situation how one individual can have a greater impact on the group outcome – think of an individual releasing an alarm pheromone. However, when we look at termite tunnel construction, the role of a key individual is less obvious.

It is well known that termites tunnel along pre-existing vectors and that tunnel direction reflects a consensus of the different orientation vectors of the colony. Coptotermes species excavate tunnels as a group of workers, removing the soil in rotation. This alternation of workloads should reduce the influence of any single worker in the construction process. Working together as a group should reduce any individual errors in tunnel direction.

This is often referred to as the ‘many wrongs principle’, by which biologists explain the migration of animals. While each individual makes incorrect decisions about where to go, when the group pools their estimates, they cancel out each other’s errors, and mutually arrive at the correct destination. Within the termite colony, the orientation vectors of individuals may drift, but they will not all err in the same wrong direction or by the same magnitude. So, if enough workers labour together, the resulting tunnel is likely to correspond to the original vector. However, if key individuals dominate the excavation process, then this neutralising effect is reduced.

 

Do some workers dominate tunnelling decisions?

 

How termites get to work

The first step in a termite worker joining a tunnel working party is for it to enter the tunnel from the introduction chamber. Bardunias et al examined this behaviour.2 The researchers hypothesised that all termites that entered the introduction chamber would partake in the tunnel construction. However, only about 15% of the termites that entered this chamber actually began tunnelling.

Of those workers that did enter the tunnel, many did not contribute equally to the tunnelling process. In all trials, some workers worked much harder than the rest. In fact, some termites that entered the tunnel did no work at all! It was concluded that nearly 85% of the workers contributed less than 15% of the tunnelling effort. Small groups of workers were highly productive (completing at least 50% of the total work on the tunnel) whilst representing only 21% of all the termites that entered the tunnel. This number reduced to only 3.3% of the total number of workers present, if one also includes all the termites in the introduction chamber that never entered the tunnel.

Looking at the time the termite workers spent ‘on the job’, they observed how some termites excavated steadily over time, while others worked in short bursts of high activity before leaving the tunnel. High excavation rates were reported when key individuals performed a series of uninterrupted excavations but dropped significantly when a larger group of termites worked together, alternating the digging process between the group. Referring back to Traniello’s framework, how do we categorise these high performing, hardworking termites: as catalysts, performers or organisers?

 

Types of termite workers

As they carried out most of the digging, it is fair to call them ‘performers’ as they performed the tunnelling task at a high rate relative to the other worker termites. A more thought-provoking possibility is that the highly active workers are acting as ‘organisers’. The researchers noted that the most high performing termites dominated the direction and shape of the tunnel following a breakdown in the usual group rotation system. Equally, they may be acting as ‘catalysts’, leading others to improved effort.

Since termites follow the course of the tunnel to its end to dig, and because small irregularities in the tunnel wall can affect the location of excavation behaviour, this means these key workers can change the course of tunnels away from the originally intended direction or cause branching. This conflicts with the ‘many wrongs principle’, as key individuals can cause other workers to veer off track completely. This may not be a bad thing, as it may allow the termites to cover a larger search area. This ability of key individuals to alter tunnel direction, or create branching, may also enable tunnels to respond to environmental stimuli, such as moisture.

Because excavation is focused on the ends of lengthening tunnels, the width of tunnels at the end and the number of tunnel ends may be more important in determining the size of the workforce in tunnels rather than the total number of available termites outside the tunnel. Surplus termites would have no access to tunnel ends and would be relegated to excavating along the tunnel walls after the initial construction. This also likely explains why many workers are found in the introduction chamber, but do not enter the tunnel to work – if they physically cannot enter the tunnel due to the small tunnel size, then they have little choice but to wait in the introduction chamber.

Understanding the division of labour within insect colonies sheds light on evolutionary aspects of termite behaviour and improves our knowledge of the mechanics of tunnel excavation.

 

Steve Broadbent, Regional Director, Ensystex Australasia

 

1 Robson, S.K., Traniello, J.F.A., 1999. Key individuals and the organisation of labour in ants, pp. 239–259. In: Detrain, C., Deneubourg, J.L., Pasteels, J.M. (Eds.), Information Processing in Social Insects. Birkh user, Basel.

2 Bardunias, P., Su, N.Y., Yang, R-L., 2010. Behavioral variation among tunnelers in the Formosan subterranean termite. Journal of Asia-Pacific Entomology 13, 45–49.

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