How a Red Wood Ant Mound Works

Nov 2

3 min read

Updated: 1 day ago

From the outside, a red wood ant mound looks like a simple pile of needles. In reality it is a heated, ventilated and waterproof building that the ants maintain every day. Here is how a red wood ant mound works.

What is a mound made of?

The dome is built from conifer needles, small twigs, bud scales and sometimes small stones and pieces of resin. It is often built against an old tree stump. The dome is only the top of the nest: below ground, the nest continues for up to 2 metres deep and wide. Large mounds can reach 2 metres in height and 5 metres across.

A solar collector

Red wood ant workers busy on the needle surface of their mound — Workers on the surface of the mound.

Red wood ants usually build their mounds in sunny spots, such as forest edges and clearings. The dome works like a solar collector: it catches the sunlight, while the loose layer of needles insulates very well. That is why you rarely find a healthy mound deep in a dark forest.

Central heating

From spring to autumn, the ants keep the inside of their nest warm and stable, often 25 to 30 degrees. That warmth speeds up the development of the brood. The heat comes from three sources: the sun, the body heat of the ants themselves, and the heat released when microbes break down the nest material.

Sunbathing ants

In early spring, you can see clusters of workers sitting on top of the mound in the sun. They warm up, go back inside and release that heat in the nest. A study from 2025 measured this and found that these sunbathing ants really do help to warm the nest in spring, although their effect is smaller than that of the other heat sources.

Air conditioning

On hot days, a mound can easily get too warm. The workers then open ventilation openings to let warm air out, and close them again when it gets cold or starts to rain. The tunnels are built in such a way that rainwater does not get in.

Always under construction

A mound is never finished. The ants keep moving the top layers around, which keeps the material dry and prevents mould. Damage is usually repaired within a short time.

A medicine cabinet

Red wood ants carry pieces of conifer resin into their nest, up to 20 kg in a large mound. The resin protects the colony against bacteria and fungi. Read more about this in formic acid: how red wood ants spray.

Winter

In winter, the workers and queens move deep underground into protected chambers. There they rest in a cold, slow state until the sun warms the mound again in spring.

A home for other animals too

A wood ant mound is home to many other animals. Researchers have found dozens of other species living in the nests of wood ants, including the shining guest ant, a tiny ant that lives only in the nests of red wood ants.

Leave mounds alone

A mound is the result of years of work. Never poke it with a stick or take material from it, and enjoy it from a distance. Want to know more? Read our red wood ant facts or find out why red wood ants are good for the forest. In an ant setup they build a small mound of pine needles too. Read more in our Formica rufa care guide or have a look at our Formica rufa colonies.

Sources

WSL / Waldwissen. Red wood ants in Switzerland.
Planet Wissen. Rote Waldameise.
Chanas, P. and Frouz, J., 2025. Sunning clusters of ants contribute significantly, but weakly to spring heating in the nests of the red wood ant, Formica polyctena. European Journal of Environmental Sciences.
Chapuisat, M. et al., 2007. Wood ants use resin to protect themselves against pathogens. Proceedings of the Royal Society B.
Wikipedia. Formica lugubris.