Ants are social insects with colonies that control their environment through collective building activities. Unlike underground species, the South American grass-cutting ant builds a thatched nest on the surface of the ground. A thatched nest is a mound structure formed from plant fragments and debris. This structure has a single central chamber where the ants cultivate fungi to feed their young (the brood). The ants must effectively regulate temperature and humidity within the thatched nest to provide ideal conditions for the fungus and brood. This is possible with the help of a thatched nest structure and the ants’ building behavior.
In the long term, the thatched nest provides a good amount of insulation. This is because the thatch material, compared to soil and the environment, reacts less quickly to changes in temperature. This limits the amount of heat that flows through the structure. Limited heat flow prevents the nest from overheating during the day and prevents major heat loss at night. The ants even place a five to ten centimeter layer of regurgitated grass fragments (called mulch) on the ground to limit the heat exchange between the fungus and the underlying soil. The nest’s insulation effectively entraps the internal heat generated by the ants and the fungi, enabling the fungus garden to remain five degrees Celsius above the average soil temperatures in all seasons. This is essential for favorable growth of the fungus and the brood, which prefer an optimal temperature of 24.1 degrees Celsius and high humidity.
In the short term, the South American grass-cutting ants display building behaviors to maintain the internal nest climate. As the nest reaches a temperature that might be harmful to the fungus or the brood, the ants create numerous openings in the thatched structure to allow the cooler air in from the outside to reduce the heat. However, the ants also show a response to any humid air leaving the nest by depositing material to close and seal openings. The colony constantly makes trade-offs between these two actions to locally control temperature and humidity shifts. Furthermore, these ants were found to deconstruct clusters or piles of material and relocate the items to allow for thatch turnover. This constant shifting of the thatch improves the insulation by moving the humid organic material (which has less insulation) from the inside of the nest to the outside, where it can dry more quickly. It also loosens the structure to provide better nest ventilation.
The nest of the South American grass-cutting ant is able to thrive due to the temperature and humidity control provided by its thatched structure and the active participation of the ants. The adaptations of this species have ultimately enabled it to extend its distribution range more in southern temperate regions compared to other subterranean species.
This summary was contributed by Leon Wang and Jack Mevorah.
“Mound-building behavior in Acromyrmex has also been discussed as an adaptation to low environmental temperatures. The symbiotic fungus cultivated by leaf-cutting ants, which represents the sole food source for the developing larvae, requires high humidity and temperatures between 25 and 30° C for proper growth … Mounds built by ants in the northern hemisphere are often permeated with galleries, that allow them to move the brood to areas with appropriate temperatures …Conversely, mounds of Acromyrmex leaf-cutting ants, built with plant fragments and debris, are arranged by workers so as to form a thatch with a single central chamber for the fungus i.e. where the fungus and brood are not located within the thatch structure, but enclosed by the thatch material. This restricts the occurrence of fungus relocation as a thermoregulatory response, so that the achievement of a proper temperature for colony growth largely depends on how the thatch affects the heat exchanges between the fungus garden and the environment.” (Bollazzi and Roces 2010a:2-3)
“Taken together, results indicate that a temperature surplus inside the A. heyeri thatched nests was largely governed by the thermal properties of the thatch material, which prevented losses of accumulated heat into the cool air at night, and avoided overheating during the day. This would not be achieved if A. heyeri workers built the nest mounds using the readily-available surrounding soil, since it has a greater thermal diffusivity than the grass fragments…During the day, the incoming solar radiation directly reached the soil and the exposed thatched nests, but due to its low thermal diffusivity, the thatch material may limit the heat flow into the fungus garden. In fact, the measured thermal diffusivity of the A. heyeri thatch, which ranged from 0.22 x 10-6 to 0.28 x 10-6 (m2*s-1), was similar to that of materials considered as insulators, such as some woods… Due to the insulation properties of the thatch material, a thatched fungus gains heat at a lower rate than a subterranean one and the environment, i.e., the thatch precludes nest overheating by limiting the maximal daily temperatures that can be reached… Interestingly, these observations of A. heyeri nests indicate that fungus gardens are not directly placed on the soil in the single nest chamber, but on a 5-10 cm thick layer composed of exhausted grass fragments (mulch). This organic material is also expected to have lower thermal diffusivity than the surrounding soil, and should therefore contribute to limit the heat exchanges between the fungus garden and the underlying soil… a fungus garden inside a thatched nest achieves a temperature surplus of ca. 5° C regarding the surrounding soil, which will positively influence rates of both fungal and brood growth.” (Bollazzi and Roces 2010a:10-13)
“Our results showed that A. heyeri workers always created openings in the nest thatch, but the number was higher, the higher the internal nest temperature. As soon as the relative humidity of the surrounding air decreased, workers were observed to close a number of previously built openings. The fact that workers closed nest openings during the desiccation phase, despite this high temperature, indicates they trade off a thermoregulatory response, i.e., the opening of apertures on the thatch at high temperatures, for maintenance of internal nest humidity… In the leaf cutting-ants genus Acromyrmex, the building of a thatched nest seems to have allowed some Acromyrmex species to extend their distribution range more in the southern temperate regions, compared with those inhabiting subterranean nests…Besides their occurrence in temperate regions, thatch-builder Acromyrmex species have also colonized some South American semi-arid and hot regions.” (Bollazzi and Roces 2010b:10-11)
“The present results showed that in the grass-cutting ant A. heyeri, humid air leaving the nest thatch through a specific spot triggered a strong building behavior, namely the import and deposition of nest material on it… First, material turnover would improve the thatch’s insulating properties. On the mound surface, thatch material being turned over would get dry by the incident solar radiation, and dry organic material has a lower thermal diffusivity than the humid one. Second, material turnover and relocation aimed at the maintenance of a loosen thatch structure would improve nest ventilation…We argue that in the context of thatch construction for the control of nest climate in Acromyrmex, building responses are aimed at a continuous redistribution and rearrangement of the thatch material…In fact, our results showed that when the outflow of humid air was stopped at the experimental spot, the newly built pile began to be deconstructed, and the rate of material removal was higher with increasing initial pile size. We therefore conclude that as soon as an A. heyeri worker finds a cluster of material occurring on the nest thatch, it will remove items from the cluster and relocate them in the context of turnover of the thatch structure. Removal may continue until humidity losses eventually occur at the location, which are expected to trigger the opposite response, material deposition on the spot. Such deposition would result in the building of a pile that may initially diminish the initial stimulus (the air stream leaving the spot), and finally suppress it.” (Bollazzi and Roces 2010c:271-272)