When thinking of an ant hill, many probably think of the classic wood ant hill (Formica family). They come in many forms and variations and every nest is unique. One of the most common ant hills is the one consisting of pine-needles and other material from the woodlands.
What does an ant hill look like?
How the structure looks like differs in-between species and areas. The ant hill is a clever solution that makes it possible for the ants to control the temperature and climate of the nest. They move the brood to the part of the ant hill that suits them the best at a given time.
The ant hill is not simply a random structure made of dirt and needles. This solid construction is very sophisticated and is what makes the climate control of the colony possible. With the help of twigs, pine-needles, pebbles, sand, dirt, leaves and other material from the surroundings the ant hill can be a true masterpiece of home-construction. The surface is often a solid crust making the nest waterproof and at the same time generating heat. Some species build their ant hills from pebbles and minerals while others use vegetation. In most ant species small stones or pebbles can be found in the structure and surface of the mound. The reason behind this is probably that the stones generate heat and thereby make the chambers of the colony warmer. Resin from trees and plants is also found in the ant hills. We don’t know why exactly, but a good guess is that the resin is antibacterial and kills off fungus. (1)
The inside of an ant hill
The ant hill consists of two parts: the one above the ground and the one under it. Both parts have a great network of tunnels connecting many chambers of varying size. It is very common for the ants to place the pupae in the top of the ant hill, taking advantage of the sun shining on it. The ant home is built to be as effective as possible, filling as many needs as it can. When the Nordic winter is almost over the Formica ant queens start laying eggs again. The temperature inside can still be around 26-28°C (78-82°F) even though the ant mound is covered in snow. (1)
With the seasons changing the different areas of the ant hills are used to create the optimal climate for the brood and queens. When it’s warm they are taken to the top of the ant hill. When there is a drought or simply too cold outside they are taken down to the warmth and moist of the lower chambers.
The surface of the ant hill might look like a boring crust of pine-needles, but it is actually filled with small entrances all over. The ants open and close them like we use doors. But instead of doors with hinges they use stones and other woodland material to shut out weather and enemies.
Which ant species build ant hills?
The most common ant hill builders are the Nordic family of Formica ants. At least if we’re talking about the classic pine-needle ant hill. Many other ants have similar techniques but they do not build as classic a mound as the Formicas.
How big can ant hills get?
There are of course both tiny and huge ant hills, and everything in-between. The ones building the largest ant hills are the Formicas. Their mounds (Formica aquilonia) can reach up to 2,5 meters (8.2 ft). And one wood ant’s nest (Formica polyctena) can reach an impressive circumference of 20 meters (65 ft). (1) That’s huge!
How old can an ant hill become?
Many ant hills are believed to be several hundreds of years old. But of course it depends on the species and what sort of environment they are in. The regular woodland ant (Formica) have a clever system of multiple queens (polygyne), making it easier for their colonies to survive longer. When one queen dies another can take her place. The monogyne colonies (Lasius niger for example) don’t outlive their queen. Even though she can get to 30 years of age the colony dies with her.
Where do ants build ant hills?
Ants like to use their surroundings to their advantage. This is why many ant hills are built strategically next to trees or on top of (or under) stones, logs or stubs. This solid structure helps protects the ants from enemies and the forces of nature. In turn this leads to the ant colony becoming more successful than its neighbors. The ant hills that survive the longest is probably the ones with a combination of a great nest site as well as a good environment (rich in material and food). It is very common for ant hills (in the northern hemisphere) to be built facing south, since that’s where the sun will shine the most.
Network of ant hills
Many ant species build more than one nest. The extra ant nests are called satellite nests and they can carry queens or only brood and workers. The satellite nests provide a way for the ants to expand their territory. No other ants (or insects) will be able to settle in the area controlled by the mother colony. The workers from the colonies meet on the ground when foraging but do not harm each other. They belong to the same colony even though their nests might be far away from each other. (2)
Other types of ant hills
Many ants build mound nests, but most of them do not use the same technique and materials as the Formicas. Species such as Lasius or Myrmica lets the excavated nest dirt form the mound on top. They also like to settle in tufts of grass since it provides much of the same protection and functions of the wood ant nest. The climate can be somewhat controlled while heat is generated on top of it.
1. Per Douwes, Johan Abenius, Björn Cederberg, Urban Wahlstedt (2012) Nationalnyckeln “Steklar: Myror-getingar. Hymenoptera: Formicidae-Vespidae” p. 40-41 (Swedish)
2. Bert Holldobler & Edward O. Wilson (1990) “The Ants” p. 173