Researchers have gained insight into the mysteries of termite mound-building behaviour in Coptotermes

Why some termites build mounds and others don’t is a bit of a mystery. Working out the factors that may have driven the evolution of mound building is an ongoing body of work.

All mound building termite species are in the Termitidae (the higher termites), except for several Coptotermes species in Australia (Rhinotermitidae).

Of the eight Australian Coptotermes species, only three build mounds: C. acinaciformis (north), C. brunneus and C. lacteus (main picture, above). The other species typically build their nests in trees. Coptotermes acinaciformis is particularly interesting as it has two clades (most likely two different species) – one which builds mounds in the tropical north and one which does not build mounds in the more temperate, southern areas of its distribution.

The mounds built by the three mound-building Coptotermes species have a very similar structure: an outer wall of compacted clay, an inner network of carton galleries made of digested wood and a well-defined royal chamber.

Coptotermes acinaciformis only forms mounds in the tropical north

The three species developed their mound-building behaviour independently of one another. The three mound-building Coptotermes species have completely separate distributions: C. acinaciformis (north) in the tropical north, C. brunneus in mid-coastal WA and C. lacteus in the southeast of Australia. Despite their very different environments, is there some common factor that provided the evolutionary stimulus to drive the need for mound building?

As part of his PhD, Dr Tim Lee, supported by a group of researchers including Professor Nathan Lo and Associate Professor Theodore Evans, recently completed a study investigating whether specific environmental conditions – abiotic factors such as rainfall, temperature and soil type – could be correlated with mound-building behaviour.

The distributions of the eight Coptotermes species were more influenced by rainfall than by temperature or soil type, with the different species often responding optimally to different rainfall patterns. The exception was Coptotermes frenchi, for which temperature was the biggest limiting factor. The authors suggest that greater tolerance to moisture variation could well explain its very wide distribution across temperate and arid environments (and C. frenchi may be three species).

However, none of the abiotic factors appeared to influence mound building. There was no evidence of any common abiotic factor in the ranges of the mound- building species and there was no common point of difference when comparing the abiotic factors experienced in the ranges of the mound building versus the non-mound-building species.

The authors proposed two beneficial factors that may promote the evolution of mound building: nesting site availability (or lack thereof) and defence against predators. Eucalyptus trunk girth is generally narrower in the tropical north, so for tree-nesting species to maintain (or increase) colony size in the north, and still be protected from predation, a mound is required. Note that the three mound-building species are all evolved from tree-nesting species, generally found in the south: C. acinaciformis north from C. acinaciformis south, C. lacteus from C. frenchi, and C. brunneus from C. michaelsoni.

The authors recommend future investigations to focus on predator-prey relationships, other biological factors such as the number of soldiers and abiotic factors not considered here (such as terrain), to try and clarify the drivers for the evolution on mound-building behaviour. They note that the predators may be now extinct, such as the giant echidna, Zaglossus hacketti.

T.Lee et al. (2019). Ecological diversification of the Australian Coptotermes termites and the evolution of mound building. Journal of Biogeography. 44(6):1405–1417