Where last we left our heroes of the Tunisian salt pans, we found that desert ants (Cataglyphis fortis) represent direction by using the sun as a compass in combination with a mental clock to correct for the changing position of the sun. Ants represent distance by counting its steps in 3D space, in a process called path integration. Once they get near home, they switch strategies and find the nest by visual clues.

Some animals also use olfactory (scent-related) cues to aid in navigation. Seabirds can detect subtle changes in plankton-derived olfactory cues over hundreds of kilometers, and pigeons seem to use olfactory cues for homing. It is known that some insects follow their noses (so to speak) to find food or a suitable mate. But Kathrin Steck, Marcus Knaden, and Bill S. Hansson of the Max-Planck Institute for Chemical Ecology wondered if the desert ants of Tunisia might use olfactory cues for homing.

Figure 1: Our old friend, the Tunisian desert ant.

First, they established that despite the visually featureless landscape, the salt pans of Tunisia do have scent-related features. Various soil composition, breaks in the salt, and dead plants all contribute different odors to the landscape. Do ants use the olfactory information in finding the entrance of the nest?

They isolated four odor-producing compounds from various objects found in the vicinity of the ants’ nest. They verified that the odors were not particularly repulsive or attractive to the ants. The ants were then trained to forage for food in an 8 meter long trough; the end of the trough with the food was always placed downwind from the nest entrance. The experimenters applied one of the odors to the floor at the nest entrance, and re-applied the odor every 20 minutes, to ensure that the scent remained stable over time.

Would the ants learn to associate the specific odor with their nests? They tested them in troughs that contained (1) the same odor, (2) a different odor, (3) a mixture of four odors including the trained one, and (4) an odorless control.

It turned out that the ants had learned to associate the specific odor they had been trained on with the nest entrance. They did the best under condition 1. And, they were able to pick out the trained odor from among the collection of four odors in the third condition. They were slightly less successful under condition 3 than condition 1, but way more successful than under conditions 2 or 4.

Figure 2: Trained odor on the top, then the four odors condition, then the control condition on bottom. Each one is statistically significantly different from the others. The closer to the zero-point, the closer to home they ended up.

So at this point, the humble ants have demonstrated that they count their steps in 3D space while tracking the position of the sun and keeping mental time in order to path integrate, that they take visual viewpoint-specific “snapshots” of the nest for the last bit of the journey home, and that all the while they monitor the olfactory cues of the environment. So they have a sort-of “olfactory snapshot” of the nest, as well.

Amazing. But we’re not done yet. Oh no. First we put ants on stilts. Then we cut off their legs to create stumps. We blindfolded them and trapped them under boxes. But we have not yet antennectomized the ants. Until now. A paper published in the journal Animal Behavior last week describes the procedure.

What’s an antennectomy, you ask? I’m glad you did. I’ll tell you. Ants have two antennae. If you cut one (or both) of them off, you have performed an antennectomy. What are antennae used for? Smelling.

So, think of it as a nose job for an ant.

Its a good thing ants don’t care much about their appearance. So much harder to perform a nose job if you’ve got two independent organs. “Doc, my one antenna is LONGER and THICKER than the other, and it’s HUMILIATING!” And it’s not like they can get an insurance write-off with a deviated septum or something. Also, 10 points to the authors for getting the word “antennectomized” into a paper.

But I digress.

So they trained the ants, as before, to forage in an open trough. One of four different odors was placed in each corner of an imaginary square adjacent to the nest entrance. Since ants do not antennate the ground while walking, we can be sure that the ants are using their antennae for smell and not for taste.

Figure 3: Schematic of the training trough, with the four odors indicated in the imaginary square, adjacent to the nest entrance (a 1 cm hole in the wall).

The food was always aligned downwind from the nest, and the nest was never located at a spot of the highest concentration of an odor; the location was gradient-specific, not odor-specific. Would the ants be able to use an odor gradient (as opposed to a single odor, as in the previous study) to find the nest? Do they perceive a sort of olfactory landscape? Are unilaterally antennectomized ants (ants with one antenna removed) at any disadvantage compared to intact ants?

There are so many conditions here that you’ll need the bar graph for this one. Lucky for us, they included one in the manuscript.

Figure 4: Results! The little antenna represents the antennectomized ants. The other conditions represent intact ants. Left three conditions represent odor gradients; right two conditions are point-source odors (as in the previous study).

Okay, so let’s pick it apart, going left to right.

Condition 1: Intact ants on the training array do really well.

Condition 2: Intact ants with a right-left reversed testing array perform significantly worse than in training. This suggests that ants don’t just detect the overall array, but are sensitive to the position and strength of the odors within the array. That is, they track which odors are coming from the right and which from the left.

Condition 3: Unliterally antennectomized ants traveled five times longer than intact ants in condition 1, and did significantly worse.

Condition 4: A repeat of the previous study. Intact ants trained to associate the nest with a single point source odor do well. No significant difference between this condition and condition 1.

Condition 5: Unliterally antennectomized ants, when trained to associate the nest with a single point source odor do well. No significant difference between these ants and intact ants, when it comes to point source odors.

What does it all mean? Ants which have one antenna removed aren’t completely deprived of using odor cues, as condition 5 indicates. However, they are unable to use odor gradients in an array to locate the nest, as in condition 3. This suggests that ants smell in stereo.

What does it mean to smell in stereo? Well, humans  smell in stereo thanks to our two nostrils, but this is hard to perceive. But humans also see in stereo. Each of your eyes detects a slightly different image, and the visual cortex uses the slight difference between the two retinal images in order to construct a three-dimensional viewpoint. Close one of your eyes, and you’ll realize that you have no depth perception. Humans also hear in stereo, thanks to our two ears. If you covered one of your ears, you would find it very hard to determine the location of a sound. Thanks to the fact that sound waves reach each of your ears at slightly different times, your brain is able to figure out where in space a sound is coming from.

This is, incidentally, what filmmakers capitalize upon to make 3D movies. The projected image is two-dimensional, obviously. But actually, the projected image is composed of two slightly different versions of the same image. In old school 3D movies, the two images were colorized differently; one lens of the glasses filtered out red light, and the other lens filtered out blue light. The resulting images were projected onto your retina, and your brain put the two images together to construct a 3D image. New-school 3D movies work the same way, but are based on the light’s polarity instead of color.

Figure 5: President and Mrs. Commander-in-Chief, sporting their 3D glasses.

In much the same way, the two antennae of the desert ant (or your two nostrils) each have a slightly different olfactory “viewpoint” of the same olfactory “scene.” If you remove the input to one of your eyes, you don’t completely lose vision, but you lose the ability to perceive three-dimensional depth. Likewise, if you antennectomize the desert ant, it doesn’t completely lose olfaction, but it loses the ability to organize olfactory cues in three dimensions.

References:
Steck, K., Hansson, B., & Knaden, M. (2009). Smells like home: Desert ants, Cataglyphis fortis, use olfactory landmarks to pinpoint the nest Frontiers in Zoology, 6 (1). DOI: 10.1186/1742-9994-6-5

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