On a recent stroll around a local bird refuge, I was struck by the appearance of a large ants’ nest, covered with a thatch of pine needles. The type of ants who construct these nests are called “mound builders,” and this particular mound was made by red wood ants. What do they do to survive the cold, I wondered?
There's not much scientific information about red wood ant winter activity in the United States. However, we do have some information on the red wood ant’s European cousin, which lives in a similar habitat.
In the fall, much of the food gathered by the wood ant workers is fed to the youngest of their caste, those that specialize in food storage. With their abdomens swollen with extra food, the young workers move with the queens into the deep chambers of the nest below ground, where they wait out the long winter months in a dormant state. The older workers also move to the lower chambers, but continue to forage for food at times when the outside temperature rises sufficiently.
The ants are able to create their own microenvironment by clustering together, which keeps the temperature in the nest above freezing. Another way ants control their environment is by building well-insulated nests. A covering of pine needles creates air pockets that store heat. And facing the nest towards the south increases the amount of sun exposure.
When the first sunny days of early spring appear, the European red wood ants wake up from their dormant state and migrate out onto the mound’s surface, where they will stand almost motionless in the sunlight. This awakening apparently activates the young workers into metabolizing the fats and proteins stored in their bodies, which they then regurgitate and feed to the queens as late winter egg production begins.
When the first sunny days of early spring appear this year, I'm planning to visit the red wood ant nest again to see if they have emerged above ground to stand almost motionless, absorbing the sun's heat - as I will be doing.
"Field Notes" is produced by the Montana Natural History Center.