The ant smelloscope

Blast it, I hate it when the authors of a research article come up perhaps the best possible title.

“Do desert ants smell the scenery in stereo?”

I can’t top that title. All I can do is try to explain a little.

Cataglyphis fortis is a name that is not well-known to many, but to an ethologist like myself, they're kind of famous. This is a desert ant species that has taught us a phenomenal amount about how animals navigate in their environment. These ants, with their small brains, are doing sophisticated navigational things like memorizing landmarks, counting steps, and looking at the sky for polarized light cues.

Adding to that already impressive repertoire, Steck and colleagues are showing how these ants can use smell to find locations. Locating things by smell is tricky, because an odour molecule has no inherent directional information. You have to sample repeatedly, and follow rules of thumb like “more odour means closer to source.” And again, it’s more sophisticated in these ants than you might expect.

These ants, like many ants, have a nest. They run out from the nest great distances, relative to their body size, to find food and water. Remember, these are desert ants, so resources are thin on the ground. Then, they turn around and head back into the nest. If they don’t find the entrance where they think it should be (because some experimenter has moved things around, say), they perform a very regular searching behaviour trying to find it.

Steck and colleagues placed four fairly strong odour sources around the entrance to the ant’s nest, let the ants run away, then removed the entrance. The odour sources were 7 cm – reasonably far relative to the ant’s body, but still local cues that would be intermixing and intermingling. If the odour sources were the same as when the ant left the nest, the ant searched in a narrow range, as if it was thinking, “It’s got to be here somewhere...” But if the researchers messed with the odours while the ant was gone, the ant searched much more widely, as if it was thinking, “Where did I put that nest again? Am I having a senior moment?”

The interesting thing is that if they left all the four odours in place, but changed their position relative to each other (moving ones left of the entrance to the right of the entrance and vice versa), the ants searched much more widely, exhibiting less certainty about where the entrance was. It isn’t just the presence of the odours, but their positions on a very small scale, that the ant is noticing and remembering.

Interestingly, if you removed one of the ant’s two antennae, trained them to this four odour landmark, they also showed a lot less confidence in the entrance location, searching widely. But if you trained them to only one odour, the removal of the antennae didn’t make things worse.

The ant with one antennae is like a person who can’t see out of one eye at a 3-D movie. It seems that both are needed for the animal to sense and learn the smell of its surroundings, indicating that the ant is doing some fairly sophisticated comparisons between the signals coming in from each antennae.

Given the small size of the animal, trying to sort out the neural activity underlying all this sophisticated learning is a little tricky, but certainly not impossible. There’s also a huge background literature on insect olfaction generally, so working out some of the neurobiology could be a next logical step to take in the next half decade or so.

Reference

Steck, K., Knaden, M., & Hansson, B. (2010). Do desert ants smell the scenery in stereo? Animal Behaviour DOI: 10.1016/j.anbehav.2010.01.011