Why do harvester ants collect and deposit stones around their nest entrance? (Bob)


Dear Ant Experts,

We encountered a harvester ant mound during a recent hike for volunteer naturalists. We were discussing why the ants cover their mounds with small stones. One article I found says that they do this so the sun will warm the nest more quickly and radiate heat for the nest during the night. Do harvester ants bring stones out of a nest early in the day, place them in the sun for warming, then bring them back in for the night as a solar heat-source?

Also, do they "plug" the nest with a stone each night?


Dear Bob,

It is a really good question of why some ant species collect and deposit small stones around their nest entrances. Scientists have been addressing these questions, but it still remains unclear. We have contacted Robert Johnson who studies harvester ants and here is what he has to say:

"A number of harvester ants collect pebbles and bring them back to the nest, meaning that they intentionally retrieve these object - they are not excavated from inside the nest. The reason for this behavior is poorly understood, but there is a paper attached that discusses these ideas and why they might do it."

Chris R. Smith and Walter Tschinkel have published some papers, in which they investigated the question you asked and here is a link to the website with links to the pdf flies. The following publications might be of particular interest:

Smith, C. R., and W. R. Tschinkel. 2007. The adaptive nature of non-food collection for the Florida harvester ant, Pogonomyrmex badius. Ecol. Entomol. 32: 105-112.

Smith, C. R., and W. R. Tschinkel. 2005. Object depots in the genus Pogonomyrmex: exploring the "who", what, when, and where. J. Insect Behav. 18: 859-879.

Smith, C. R., and W. R. Tschinkel. 2007. The adaptive nature of non-food collection for the Florida harvester ant, Pogonomyrmex badius. Ecol. Entomol. 32: 105-112.

All the best,
Robert Johnson (guest expert), Steffi Kautz & the AntAsk Team

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