Bull ants wait for twilight before they start foraging for food, instead of just relying on their body clocks or the temperature, according to a study from The Australian National University.

The study was led by Dr Ajay Narendra of The Vision Centre at the Research School of Biology at ANU. It shows that instead of just relying on their circadian rhythms to identify their foraging time or reacting to specific temperature changes, the bull ant, Myrmecia pyriformis, is actually keeping a close eye on ambient light levels.

"While bull ants have a circadian rhythm which is based on sunset and sunrise, our study has shown that the time that they go out foraging is based on a specific ambient light intensity," said Dr Narendra.

"Foraging onset occurs later when light intensities at sunset are brighter than normal, or earlier when light at sunset is darker than normal. We also found different activity rhythms in nests under clear skies and overcast conditions, but the defining characteristic is that it all happens during twilight."

Dr Narendra added that the bull ants were probably waiting for twilight to go out looking for food so that they would be less of a target to potential predators.

"If bull ants are active in the morning then they would likely be a target for Currawongs and Magpies, but if they go out too late at night then there isn't enough light around for them to get information of landmarks, which is what they normally use to navigate. So twilight, where limited light is present but where protection against predators is also available, seems to be an ideal time window to begin activity."

The team conducted the research by bathing Canberra ants' nests with diffused electric lighting and replicating the reducing light of the onset of twilight at a slightly different time of day. Their paper, 'The twilight zone: ambient light levels trigger activity in primitive ants', is published in Proceedings of The Royal Society B today.

The researchers say that the findings could lead to greater understanding about the cues animals use to begin or stop activity each day.

"The next challenge will be to find out how the ants navigate in this very dim-lit zone. We want to know how their visual systems are tuned to get the information that they need for foraging in a fairly dark environment," said Dr Narendra. The research was supported by the Australian Research Council through the The Vision Centre, an Australian Postdoctoral Fellowship and the Centre for Vision Sciences.