After having spent several days of nocturnal lifestyle studying the Bull ant, Myrmecia pyriformis in Canberra, I am quite pleased to report that one of our findings has been published. Our earlier discovery that these ants exhibit strong bursts of inbound and outbound activity in evening and morning twilight respectively in the summer, prompted us to ask whether the onset activity of these ants is restricted to the twilight period for the whole year and if so how do they determine the time of the day in such a dim-lit temporal niche.
By regular monitoring of activity schedules at different nests it became clear that temperature does not play a major role in this – a factor thought to play a major role in regulating ant foraging. Of course when surface temperature dropped to 6.5°C the ants decided to take a break (thankfully for us!), which indicated that this might be close to their critical minima – that’s another story.
The interesting bit here was that in overcast conditions (determined by measuring light levels) ants began their activity much earlier relative to sunset time and conversely under clear skies began activity much later. This indicated that light intensity might play a role in the timing of foraging and that the setting sun alone was not a cue to begin activity.
Inspired by Edward Hodgson’s observations in 1955 on leaf cutter ants3 we set about modifying light intensity during twilight. The challenge of was to make the twilight brighter ensuring no point light sources would be available for orientation. We did this by using a dome shaped diffuser and suspending it above the nest entrance. This also ensured that if the ants came close to the nest entrance the visual panorama was still available for them to navigate. Keeping the lights on for the first 60 minutes of the twilight did not result in ant activity. As soon as the lights were turned off, activity began within a few minutes. The necessary controls were carried out and replication at three nests provided very similar results.
Publications in Proceedings B is currently available for free download as they are celebrating their 350th anniversary. So go on and download this paper.
Narendra A, Reid SF & Hemmi JM. 2010. The twilight zone: light intensity triggers activity in primitive ants. Proceedings of the Royal Society, Biological Sciences. doi: 10.1098/rspb.2009.2324
Hodgson ES 1955. An ecological study of the behaviour of the leaf-cutting ant Atta cephalotes. Ecology 36: 293-304