Ant Mound Mystery

The weather was warm and sunny yesterday, so I decided to see what the ants were up to. Unless immobilized by cold, the Allegheny Mound Ants, Formica exsectoides, are always doing something of interest. The first mound I visited had lost the sunlight due to late afternoon shading, so there weren’t a lot of ants moving around. This makes it easy to get your face down to the mound for a close examination.

Allegheny Mound Ants are into everything and leave no part of the surrounding area unexplored. The few ants I could see were all staying on the mound, so I felt it safe to kneel down and get comfortable without fear of being swarmed by foraging ants.

The mounds are typically covered by bits of vegetation and other types of debris. I assume this helps protect the mound from erosion and also helps to regulate the mound temperature. Each mound tends to have its own preferred type of cover and in some cases the ants will bring in plant material from a considerable distance. On this mound I found masses of elongated pellets mixed in with the plant debris.

At first I thought these might be cocoons that had failed to hatch. On closer examination, I found that the pellets were composed of material much like sawdust. What would be making little sawdust pellets?

While I was watching, an ant picked up one of the pellets, moved it about a half inch and carefully fitted it into a new location. The size and shape of the pellets makes me believe that they are vole scat. Voles eat various plant material and can produce fecal matter with a high volume of fiber. Most herbivores a quite inefficient in the ability to digest fiber and expel a lot of partially digested plant material in their droppings. It’s possible that the ants have found vole droppings to be a source of food and the scat is brought back to the mound where the usable product is extracted by the ants. The left over fiber pellets are then discarded atop the mound and utilized in the same manner as other bits of vegetation.

After relocating the pellet, the ant repositioned a couple pieces of vegetation in the same manner. It’s also possible that foraging ants select the scat for the sole purpose of covering the mound. I checked a dozen nearby mounds and found none with these pellets, so the behavior seems to be unique to this mound of ants. It may be that the drought is responsible for the voles producing a drier scat this summer. I’ve never before seen pellets like these on a mound, but I’ll certainly be watching for them in the future.