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Home > Resources > Research > buildingIssue 19, December 2000
I am always looking for an excuse to talk about my own research enthusiasm, animal building behaviour, and this time I think I can slip a couple of stories into On the Research Front. There is at present an interest among architects with a 'green' streak in devising more environmentally friendly and less wasteful human architecture. A change that is from much of the architecture that characterised the 20th century, where glass-curtained steel and concrete towers leaked heat in the winter but threatened to bake their occupants in the summer sun. Technology readily tackled these problems but at a cost; heaters and coolers were installed to be used according to season and big fans pushed all the conditioned air around. 'Green' architects are all apparently aware of certain animals that make a much more efficient job of air conditioning their living space, using the architecture itself to direct the air flow, and the heat differences in the building to drive it. These are of course mound building termites. Their widely known ventilation system was described by Luscher in the 1950's for the enclosed mounds of Macrotermes bellicosus. For the first time since Luscher's classic study, it has been investigated again and been found to be wrong.
Luscher, by studying the internal spaces of Macrotennes mounds, realised that air could circulate in a regular way within them and concluded that it was driven by the metabolic heat of the termites and their fungus gardens at the centre of the mound. Heated air rose up through this central living area into a large void at the top of the mound from which it was pushed into narrow channels in the walls of the mound. There, as it moved down through these channels, it lost heat and carbon dioxide to the exterior and gained oxygen. The conditioned air was then pushed into a cool basement below ground level before being drawn up into the living area once more by the heated air rising above it.
According to Korb & Linsenmair (2000) the air does indeed circulate in the mound but predominantly in the reverse direction and is not powered by the internal heat of the mound but by the sun. During the day the sun heats the outer wall of the mound, causing air in the superficial passages to rise and be forced down into the cooler living area. At night, the greater heat of the mound core compared to the wan may actually bring about the circulation Luscher claimed. In some ways this flexibility in the way the architecture directs the air flow is even more satisfying than Luscher's model, and crucially the present study confirms the feature that 'green' architects are most excited about; the apparently solid walls do 'breathe', allowing gas exchange and at the same time keeping hostile aspects of the outside world at bay.
My second animal building story is about parasite manipulation of host behaviour. Parasites exploit their hosts, feeding on their resources in order to multiply and move on to new hosts. Some parasites can manipulate the behaviour of their hosts in order, for example, to make transmission to another host more likely. The manipulation may be very simple; it can, for example, be argued that a common cold virus, by making you sneeze in a crowded bus, is manipulating your behaviour to aid its transmission.
Manipulating the host to build a safe place for the parasite to rest up is an altogether more sophisticated kind of control. The host in this case is a tropical spider and the parasite is the larva of a parasitic ichneumonid wasp. The larva fives on the outside of the spider, lapping its blood through a small wound. The spider meanwhile continues to make webs, catch insect prey and try to grow. It is, however, the wasp larva that is doing most of the growing and on reaching maturity has almost no further use for the spider. Almost, but not quite - some kind of chemical signal is now injected by the parasite through the wound into the spider's blood, whereupon as its final act the spider makes a unique miniature web from which the parasite larva can hang its cocoon in safety while it transforms itself into an adult wasp.
References Eberhard, William G. (2000). Spider manipulation of a wasp larva. Nature, 406, 255 - 256. Korb, Judith and Linsenmair, Karl E. (2000). Ventilation of termite mounds: new results require a new model. Behavioural Ecology, 11, 486 - 494.