To build or not to build: circulating dry air organizes collective building for climate control in the leaf-cutting ant Acromyrmex ambiguus

To investigate to what extent climatic variables are used by ant workers to decide whether and where nest building should be started, the effect of humidity content of the circulating air on building behaviour was studied in the leaf-cutting ant Acromyrmex ambiguus. In the laboratory, a colony was confronted with an experimentally induced circulation of either dry or wet air through its nest chambers, and the plugging of nest tunnels with leaf fragments was recorded as indicative building response. Results showed that circulating dry air triggered building responses. Tunnel plugs were constructed mostly in the inflows of dry air and to a much lesser extent in the outflows of dry air, indicating that the direction of the air movement inside tunnels spatially guided the workers' building behaviour. However, circulating humid air, or a passive loss of humidity from the nest, did not suffice to trigger tunnel plugging. Taken together, results showed that A. ambiguus workers use the direction of the airflow as an environmental cue for spatially guiding their building behaviour, yet workers only responded if the humidity content of the circulating air was low. These building responses seem to result from a compromise between the need of air renewal and the avoidance of unsuitable climatic conditions. To our knowledge, our results are the first in showing that ant workers respond to a specific climatic variable to decide whether and where a nest structure, related to the control of nest climate, should be built.