Carbon dioxide sensing in the social context: Leaf-cutting ants prefer elevated CO₂ levels to tend their brood

Social insects show temperature and humidity preferences inside their nests to successfully rear brood. In underground nests, ants also encounter rising CO₂ concentrations with increasing depth. It is an open question whether they use CO₂ as a cue to decide where to place and tend the brood. Leaf-cutting ants do show CO₂ preferences for the culturing of their symbiotic fungus. We evaluated their CO₂ choices for brood placement in laboratory experiments. Workers of Acromyrmex lundii in the process of relocating brood were offered a binary choice consisting of two interconnected chambers with different CO₂ concentrations. Values ranged from atmospheric to high concentrations of 4% CO₂. The CO₂ preferences shown by workers for themselves and for brood placement were assessed by quantifying the number of workers and relocated brood in each chamber. Ants showed clear CO₂ preferences for brood placement. They avoided atmospheric levels, 1% and 4% CO₂, and showed a preference for levels of 3%. This is the first report of CO₂ preferences for the maintenance of brood in social insects. The observed preferences for brood location were independent of the workers' own CO₂ preferences, since they showed no clear-cut pattern. Workers' CO₂ preferences for brood maintenance were slightly higher than those reported for fungus culturing, although brood is reared in the same chambers as the fungus in leaf-cutting ant nests. Workers' choices for brood placement in natural nests are likely the result of competing preferences for other environmental factors more crucial for brood survival, aside from those for CO₂.