When my very good friend Deb Sparrow sent me this photograph of a harvester ant collecting a piece of a blue corn chip that had fallen from her lunch, I immediately took it at face value. A harvester ant carrying a corn chip seems like an obvious combination. After some research, however, I am beginning to wonder whether the ant was also interested in another, not so obvious, aspect of the corn chip.
Recent research has shown that certain ants collect salt (sodium chloride). Michael Kaspari, Stephen P. Yanoviak, and Robert Dudley presented different species of ants baits with varying concentrations of salt (NaCl) versus two concentrations of the standard bait attractant, sucrose solution in water.
The scientists found the ants responded to the salt solutions depending on their distance from the ocean. Ants near the ocean presumably would have more salt available, carried to the land by wind and storms. In general, the further from the shore, the more the salt solution attracted ants and the steeper the dose response.
One interesting exception, however, were ants that generally are thought of as carnivorous, for example fire ants of the genus Solenopsis. The idea is that carnivorous ants get their salt through their prey, whereas vegetarian species, like the harvester ant above, would need to supplement their diet. The researchers also found that ants of the genus Pheidole did not respond to salt. The diets of many Pheidole are not well-studied, but at least a portion are also seed harvesters.
Guess I'll be paying more attention to ants and salt from now on. I have to say that here in Arizona our water, and thus irrigated soil, is full of salts. In fact one of our main rivers, the Salt River, is named for the natural salt springs that feed it. Maybe our natural abundance of salt is one of the reasons why we have such a variety of ants.
What do you think of ants and salt?
On the biogeography of salt limitation: A study of ant communities by
Michael Kaspari, Stephen P. Yanoviak, and Robert Dudley.
Proc Natl Acad Sci U S A. 2008 November 18; 105(46): 17848–17851.
Ants More Attracted to Salt Than to Sugar by James Parker for The Daily Californian
Inland Ants Crave Salt, and Hurricanes May Help by John Roach for National Geographic News
Edit: The Circus of the Spineless is now up at Greg Laden's Blog