ANTS exhibit a characteristic "emigration" behaviour if their nests are disturbed. They initially panic, running in all directions, then settle down and immediately begin scouting for a new home.
Part of this behaviour relates to whether a new nest site is big enough for their needs. Ants will only decamp to a new location if it seems suitable, and they will delay leaving until the right quality of nest has been located.
Researchers at the University of Bath believe they have for the first time identified how the insects decide whether a nest site is the right size. The discovery comes from research towards a PhD by an Irish graduate from Trinity College, Mr Eamonn Mallon, of the Ant Lab in the Department of Biology and Biochemistry at Bath.
"The great thing about ants is how they do what they do, how they work as a colony," Mr Mallon explained. He marvels at all aspects of their individual and collective behaviour, how they choose the best nesting site, how they orchestrate their move, even simple things like "how they pick up a cornflake".
His study has focused on the ant species, Leptothorax albipennis, and the Ant Lab has about 20 colonies. They live in the smallest of cracks and crannies and a typical nest might measure 35mm by 25mm with headroom of less than a millimetre.
This makes them ideal subjects for observation, he said because they don't tend to clamber over one another and can be observed from above as though in two dimensions. Individual behaviour is studied by placing four paint dots on the ants in varying combinations of six colours and then videoing their travels through the nest and while foraging. A large number can be monitored in this way and separately identified, Mr Mallon explained.
He and his supervisor, Prof Nigel Franks, have co-authored a paper on nest site selection, published in the Proceedings of the Royal Society.
Mr Mallon explained how he observed a female worker make repeated visits to a site prior to the colony's move to a new nest. "The ant went in and she left and went back to the old nest." She then made visit after visit. "Each time she went back to the new nest she spent less time there than the time before. I wondered what it was she was doing."
He discovered that during the ant's first visit she laid a scent trail that only she could detect, leaving behind the same length of trail no matter how big the prospective nest site. "When she goes back in she returns looking for the trail to see how many times she crosses it."
The ant was not counting intersections with its original trail but the insect was likely to be able to respond to the rate of intersection. In simple terms, the more often she crosses her own trail, the smaller the total nest size.
Unknown to itself, the ant is using a form of geometrical probability described in 1777 by mathematician Georges Comte de Buffon and known as "Buffon's Needle". It involves dropping a needle onto a sheet of lined paper and determining the probability of the needle crossing one of the lines.
Surprisingly, this probability is associated with the value of pi, Mr Mallon explained. "It is a bit of a mathematical trick." The total area of the sheet of paper is related to the number of times the needle crosses a line by the value of pi.
Something similar is going on for the ants as they search for their initial trail through a prospective nesting site. "The intersection rate is inversely proportional to the size of the nest," Mr Mallon said. Fewer intersections would mean a larger nest.
He tested this by covering half the original trail area with a plastic sheet. The returning ant was far less likely to encounter a trail marker and the colony was regularly tricked into moving to an unacceptably small site, which they would invariably abandon.
The emigration procedure "is all very orderly", he said. The ant who identified the site will encourage another to follow, leading it to the site and demonstrating its location. More workers will be shown the site and eventually the move will begin.
Workers will pick up eggs and young and carry them to the new nest. They will also pick up other workers like so much baggage, dropping them at the new location. These carried workers will not attempt to leave the new nest, however, because they won't have learned the path back to the original nest.