A version of this post was originally published on my Wordpress blog on March 15, 2010. Click the archives image to see the original post.
Most animals, at some point in their day-to-day lives, face the same problem. After they've gone out in search of food, they need to find their way home. But some of the places where these critters live lack any real visual landmarks - like the open ocean or wide expanses of desert. Instead of relying on vision, some animals have developed the ability to use olfactory (scent-related) cues to aid in navigation. Seabirds can detect subtle changes in the smell of plankton in the air over hundreds of kilometers, and pigeons seem to use olfactory cues for finding their way home. 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.
It turns out that despite the visually featureless landscape, the salt pans of Tunisia actually have scent-related features. Variations in soil composition, breaks in the salt, and dead plants, all contribute different odors to the landscape. Do ants use the olfactory information in finding their way back to the entrance of their nests?
The researchers isolated four odor-producing compounds from various objects found in the vicinity of the ants nest. The first order of business was to verify 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, with the end of the trough that had been baited with food always downwind from the nest's 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) no odor at all.
Indeed, the ants 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. In this figure, the trained odor condition is on the top, then the four odors condition, then the control condition is on the bottom. Each one is statistically significantly different from the others. The closer to the zero-point, the closer to home the ants wound up.
Pretty cool. 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.
What's an antennectomy, you ask? 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, of course.
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!"
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 (in the figures below, this is represented by the four different letters). 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.
The food was always aligned downwind from the nest as before, and the nest was never located at a spot of the highest concentration of an odor. Therefore the nest 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?
In this figure, the little antenna represents the antennectomized ants. The other conditions represent intact ants. The left three conditions represent odor gradients; and the right two conditions are odors at a single spot ("point-source odors" as in the previous study). Going from left to right:
Condition 1: Intact ants on the training array did really well.
Condition 2: Intact ants with a right-left reversed testing array performed 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: Antennectomized ants traveled five times longer than intact ants in condition 1, and did significantly worse.
Condition 4: This was basically a replication of the previous study. Intact ants trained to associate the nest with a single point source odor performed well. There was no significant difference between this condition and condition 1.
Condition 5: Antennectomized ants, when trained to associate the nest with a single point source odor performed well also! There wasn't a statistically significant difference between these ants and the intact ants, when it came to point-source odors.
Taken together, this means that ants that 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, and this is probably a little more intuitive. 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 scene in your mind. Close one of your eyes, and you'll realize that you have no real depth perception to speak of. 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. However, 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, rather than color.
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.
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
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If you pay attention and if the smell isn't yet filling the air, you'll notice that you can tell whence an odour is coming.