Gravel from P.salinus nest.

Many Pogonomyrmex species cover the surface of their nest mounds with gravel (the mound structure beneath this consists usually of sand/sandy soil). This gravel is often collected by the ants from the area surrounding the nest - it is also brought up from underground, during nest-interior excavation. We have observed P.salinus obtaining gravel using both of the above-mentioned methods.

These gravel 'nest caps' probably benefit the ants in more than one way. Pogo nests are very often located in windy locations, and the gravel cap has been observed to help keep the underlying sand from blowing away (though it is not always effective [1]). In areas lacking suitable gravel, Pogonomyrmex maricopa has been known to substitute other materials (such as stem fragments), which are cemented together with calcium carbonate (the ants probably use salivary secretions to dissolve the calcium carbonate, and form a cement-like substance) [2].

P. salinus nest with gravel covering.

Thermoregulation seems to be another function of the gravel covering. It has been shown that the fully developed nest mounds of many Pogo species act as effective solar collectors - catching the sun's rays in the morning, and heating the upper reaches of the nest structure [3]. The gravel coating probably enhances this effect by heating up quickly, and retaining the heat for a longer period of time.

[IMAGE (top): gravel from a P. salinus nest in central Washington state, scale in centimeters/millimeters]
[IMAGE (left): P. salinus nest mound with gravel covering, in central Washington state © Ts. Richter]
[scroll down for additional notes/references]

ADDITIONAL NOTES/REFERENCES:

[1]
·Cole, A.C. 1932. The Rebuilding of Mounds of the Ant, Pogonomyrmex occidentalis, CRESS. Ohio Journal of Science, v32, n3 (May, 1932) 245-246
·Cole, A.C. 1968. Pogonomyrmex Harvester Ants: A Study of the Genus in North America. Knoxville, University of Tennessee Press

[2]
·Whitford, W.G. 2003. The Functional Significance of Cemented Nest Caps of the Harvester Ant, Pogonomyrmex maricopa. Journal of Arid Environments (2003) 53: 281-284

[3]
·Cole, B. J. 1994. Nest Architecture in the Western Harvester Ant, Pogonomyrmex occidentalis (Cresson) Ins.Soc. 41:401-410 (1994)
·Hölldobler, B. & E.O. Wilson.1990. The Ants. Cambridge, MA, Belknap/Harvard Press