Fire ant nests may be distinguished by a unique mound-shaped structure which is oblong in shape. No visible entrance or exit may be seen on the top of the mound, but when viewed closely, ants may be seen entering and exiting the mound from openings that are located on the sides.
The structure of a fire ant mound takes advantage of a unique principal that allows the ants to, "optimize the colony's ability to thermoregulate."
The Merriam-Webster dictionary defines thermoregulation as: "the maintenance or regulation of temperature; specifically: the maintenance of a particular temperature of the living body." This principal is important to a fire ant colony because the nest temperature determines just how fast the brood grows. As long as the temperature remains fairly warm, the colony will flourish, but if the temperature exceeds 32 degrees Celcius (89.6 Degrees), temperature can have the totally opposite affect causing more ants to die and fewer to be born.
The mound of the fire ant nest is important to this process because it allows heat to be absorbed into the mound at a more rapid rate whether the mound is shaded or in direct sunlight. Since the surface area of a fire ant mound is greater than the overall volume ratio of the mound, the resulting heat transfer which occurs is more efficient, and this is vitally important to the ant colony's survival during the winter season.
Shortly after sunrise on any given day, fire ants move their entire brood up to the side of the mound that is being heated the most. This side is typically the side that faces the southern hemisphere all day long. As temperatures drop throughout the day, the brood moves deeper into the nest where the temperatures are maintained around 16 - 24 degrees Celsius (60.8 - 75 degrees) all year round.
There's a correlation between the size of the ant mound in relationship to the total number of ants that are thriving in the colony at any given time. Depending on the season of the year, an ant mound may be smaller or larger as colony members are born and die.
Fire ant mounds contain an elaborate network of tunnels that extend between 5 centimeters (cm), or 1.9 inches, to 10 cm (3.9 inches) below the surface, and these tunnels are intertwined within the root system of grassy surface. At 10-80cm below the surface the ants construct a catacomb of special chambers. These chambers are similar to the rooms you may find in a typical household. There are chambers for food storage, a pupae nursery, the queen and egg cleaning to name just a few. Since ants like to be tidy and don't visit the surface too much in the winter, they'll often create more than one trash pile chamber that is referred to as a midden. These are the areas where they'll store their waste products and dead until they're able to carry everything to the surface for disposal when the weather warms.
All of these unique chambers are connected by vertical shafts to facilitate the rapid movement from one chamber to the next. The total depth of a mound can be as much as 1-1.5 meters (3-5 feet) below the surface of the ground. By taking a look at a plaster cast of a harvester ant colony's dominion, you'll have a better understanding of what any particular fire ant nest may look like underneath the ground.
The size and structure of a fire ant nest may not appear to be so overwhelming or unique at a first glance, but when you take into account the fact that there may be as many as 100,000 to 500,000 ants occupying the nest, the size and structure are nothing less than intriguing to a curious mind.