Myrmecia aren't known as bull ants for nothing. `They are very ferocious, they have good eyes and a very painful sting,' explains Jochen Zeil from the Australian National University. However, despite their aggressive nature, two of these closely related species have achieved an arrangement that allows them to live in close proximity: Myrmecia croslandi forage by day while Myrmecia pyriformis only forage after sunset. Zeil explains that this partitioning of time could be explained by a number of factors such as competition and avoiding predators. However, Zeil points out that another factor could be responsible for the ants' lifestyles. `It could be that the night-active animals avoid the heat of the day and the day-active animals avoid the cool of the night,' he says. Intrigued by this possibility, Zeil and his colleagues Piyankarie Jayatilaka, Ajay Narendra and Paul Cooper decided to monitor the ants' activity patterns over a year and to measure their thermal tolerances to find out whether the insects' thermal tolerances influenced their lifestyle choices (p. 2730).
Finding M. croslandi and M. pyriformis nests on the university campus, Jayatilaka, Narendra and Samuel Reid monitored the ants as they set off foraging in the surrounding trees. Over a year, the team saw that the day-active M. croslandi were active at temperatures ranging from 10 to 35°C and their foraging pattern varied across the year. On hot summer days, the insects emerged from the nest at about 8 am and returned to the nest for a midday siesta before heading out a second time and finally turning in at 5 pm. However, in the cooler spring and autumn the ants stayed out foraging all day and as the temperatures dropped further in winter the ants closed the nest and hibernated. M. croslandi were extremely temperature sensitive and Zeil says, `There was a clear correlation with temperature at sunrise and the onset of foraging.'
However, the nocturnal M. pyriformis's behaviour was much less variable and they seemed completely unaffected by temperature as the year progressed. `They always came out about 20 minutes after sunset and they were active throughout the winter,' says Zeil, who adds, `Their foraging is determined by the light level at sunset time.'
Next, the team decided to find out whether the insects' foraging patterns were influenced by their thermal tolerance, so they measured the temperature at which the insects lose the ability to move. Filming M. croslandi as the temperature rose gradually, Jayatilaka, Cooper and Zeil were surprised to see that the ants suddenly began rushing around at about 35°C before collapsing later at 48.5°C. The ants seemed to be trying to escape the heat and Zeil says, `The increase in walking speed when they encounter an uncomfortable temperature is a much better predictor of the temperatures they stop being active outdoors.' After allowing the ants to recover from the high temperature, the team reversed the experiment and found that the ants were incapacitated at 10.4°C.
The team also repeated the same experiments with the nocturnal M. pyriformis, and the ants functioned well from 41.6°C down to 8.2°C.
`It is very clear to us that the physiology of these animals does not limit them to their activity schedules,' says Zeil: M. pyriformis could handle the heat of the day and M. croslandi could cope with cooler nights. The team suspects that the ants are restricted by their visual system instead, with the bright light of day sending M. pyriformis back to the nest and dim night conditions keeping M. croslandi out of M. pyriformis's way.