Control of nest water losses through building behavior in leaf-cutting ants (Acromyrmex heyeri)

Abstract

Like many ants inhabiting temperate regions, leaf-cutting ant colonies of a number of Acromyrmex species build thatched nests that achieve more stable temperatures than those of the environment. Workers are expected to counteract short-term variations in nest microclimate by modifying the thatch structure, for instance by reducing thatch thickness or by permeating the thatch with openings so as to favor heat exchanges with the outside. Such thermoregulatory responses may compromise the control of nest humidity because of the concomitant water losses resulting from the escape of humid air to the environment. We asked whether water losses through the thatch trigger deposition of nest material as a regulatory response for humidity control. In a laboratory colony of A. heyeri, we defined four equidistant spots on the nest thatch through which losses of air with different humidities were experimentally simulated. The deposition of building material on the spot was recorded as indicative building response. Results showed that material deposition occurred significantly more frequently with higher humidity content of the air leaving the spot. Dry air leaving the nest hardly triggered material deposition. The observation of a continuous turnover of thatch material indicates that the building of a nest thatch is a permanent process, and that short-term responses for the control of nest climate, as the deposition of building material to suppress an outflow of humid air, are guided by specific cues such as the humidity content of the air stream.