Imagine living in a building where whenever it becomes too warm, the way to change the temperature is to rip a hole in one of the outer walls. Too cold? Easy, just patch up the hole you made earlier. Now imagine that the hundreds of other people living in the building were doing the exact same thing. It would be a recipe for a disaster! Yet somehow, ants make it work.
The nests of most ant species are in a constant state of change. To maintain comfortable temperatures within the nest, workers often add or remove material from the nest’s outer layers. Yet somehow, it doesn’t come crashing down on them. What’s even more impressive to me is their ability to manage the different temperature requirements for adults and brood. But that’s not all- on top of that, some species also cultivate microorganisms within their nests! Their temperature and humidity requirements are different from both brood and adult ants- yet somehow, colonies manage to collectively juggle all of these needs.
The nests of different species look vastly different, and employ a wide range of different techniques to keep everyone inside comfortable. Similarly, there’s a lot of variation in the roles that microorganisms play within these colonies. Here I’ll share examples of two ant species that nurture and exploit microorganisms within their nests.
Microbes & Temperature!
Some species of wood ants form gigantic nests in mounds on the forest floor using leaf litter. However, sterile leaves emit virtually no heat. How are the ants staying warm? Is the mound absorbing sunlight? Trapping the heat generated by the moving colony inside? Though these methods definitely are at play, the key heat source within these mounds is actually microbes living on the plant matter the ants bring inside!
This huge mound is a Formica polyctena (wood ant) nest! [1]
The center of the nest is where the most microbial activity takes place. However, the microbes need to consume oxygen, which has difficulty traveling through dense materials. One study suggests that these ants practice a form of ventilation by loosening the nest’s material to allow for more airflow, creating favorable conditions for the microbes. They observed that near the center of the nest, the material is finer and less tightly packed.
The heat generated by the microbes is highly dependent on both temperature and the water content of the materials they’re living on. In this way, heating up the nest a little bit can spur the microbes to produce even more heat. Some hypothesize that the initial heating used to kickstart the microbial activity comes from the dark ants absorbing sunlight outside of the nest, then bringing that heat back inside.
It seems that the ants are also cultivating the microbial growth to some extent. Not only do different nutrients cause the growth of distinct types of microbes in different parts of the nest, but the mound’s microbial makeup differs from the soil surrounding it. The nutrients and types of leaf litter the ants gather may be contributing to the growth of the microorganisms, and therefore the overall heating of the nest.
Fungus & Food!
A leafcutter runs back to its nest carrying a piece of a leaf in its jaws. [2]
You may have seen columns of leafcutter ants, carrying pieces of leaves across the forest floor. However, these species don’t actually eat the leaves they harvest. Instead, they farm fungus within their nests! The leaves they collect are actually going towards feeding their crop. However, maintaining a living food source creates some problems within the nest. First, the fungus consumes oxygen, which the ants also need. Second, the fungus is sensitive to humidity and temperature. So, not only do the ants have to figure out how to calibrate nest temperature to optimize their own activity and brood development, but they need to figure out the best conditions for the fungus!
This tricky optimization problem influences the thermoregulatory aspects of their nest design. The fungus doesn’t do well in warm temperatures- anything above 86ºF harms the ants’ food supply. However, temperatures below 77º are detrimental to the colony’s function. In an environment whose temperature regularly dips below 62º, the ants need to insulate their nest with plant matter that traps heat in. However, those materials will restrict the flow of gasses throughout the nest. The result is that a standard well-insulated nest can produce dangerously high carbon dioxide levels for the leafcutter ants.
Turrets on an Atta vollenweideri (a type of leafcutter ant) nest. [3]
Their solution? Ventilation! Multiple species of leafcutter ants dig two types of ventilation tunnels in their underground nests: inflow and outflow. Both types of tunnels open in turrets on the surface, which are generally not used by worker ants as entrances. Although these turrets are believed to primarily facilitate airflow within the nest (reducing carbon dioxide levels, particularly around the fungal chambers), the cold outside air they introduce influences the internal temperature of the nest. Some posit that turrets with many small openings (instead of one large one) could be used for thermoregulation, as ants can quickly adjust (opening and closing) holes in the turrets, increasing or decreasing the amount of airflow through ventilation tunnels.
Conclusion
Although the physical structure of these nests are very different (one being underground and the other on the forest floor), both species have methods for moving around nest materials to maintain optimal temperatures inside. This is only a glimpse into the different ways ants interact with microorganisms, as well as the different ways they ensure that each nest accommodates the climate needs of all its inhabitants!
Further reading
Temperature Regulation in Nests:
Kadochová, Štěpánka, and Jan Frouz. "Thermoregulation strategies in ants in comparison to other social insects, with a focus on red wood ants (Formica rufa group)." F1000Research 2 (2013).
Microbes:
Frouz, J. "The effect of nest moisture on daily temperature regime in the nests of Formica polyctena wood ants." Insectes sociaux 47.3 (2000): 229-235.
Coenen-Stass, Dieter, Bernd Schaarschmidt, and Ingolf Lamprecht. "Temperature distribution and calorimetric determination of heat production in the nest of the wood ant, Formica polyctena (Hymenoptera, Formicidae)." Ecology 61.2 (1980): 238-244.
Tunnels:
Kleineidam, Christoph, Roman Ernst, and Flavio Roces. "Wind-induced ventilation of the giant nests of the leaf-cutting ant Atta vollenweideri." Naturwissenschaften 88.7 (2001): 301-305.
Halboth, Florian, and Flavio Roces. "The construction of ventilation turrets in Atta vollenweideri leaf-cutting ants: Carbon dioxide levels in the nest tunnels, but not airflow or air humidity, influence turret structure." Plos One 12.11 (2017): e0188162.
Media Credits
[1] Photo by Michal Kukla. https://www.flickr.com/photos/99872024@N03/sets/72157635197510456/[2] Photo by Filo Gèn, Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Leafcutter_ant.gif
[3] Photo by Florian Halboth and Flavio Roces. https://journals.plos.org/plosone/article/figure?id=10.1371/journal.pone.0188162.g001.
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