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Friday, February 22, 2013

Moles smell in stereo

We humans are well aware that our two eyes and ears gather slightly different inputs that are combined into ‘stereo’ vision or hearing. We also have two nostrils, close together as they are. Can we also smell in stereo? If we’re like moles, we can.

Kenneth Catania of Vanderbilt University used blind eastern American moles (Scalopus aquaticus) as his models. Although these moles have eyes, their eyelids are fused shut, a reasonable adaptation for an animal that spends its days swimming through dirt. Instead, the animal navigates its subterranean home by using its nose.

Catania placed each mole in a special testing chamber. Upon entering the chamber, the mole was presented with a semicircle of 15 wells, only one of which contained a tasty tidbit (a segment of earthworm). The moles were videotaped and monitored to detect sniffing behavior. In some trials, one of the nostrils was blocked.

With both nostrils clear, the moles were 100% accurate in bearing straight for the food-containing well. However, with a nostril blocked, they first headed for one of the empty wells 70% of the time. More specifically, the moles were drifting toward the open nostril. That is, when their left nostrils were blocked, they veered right and vice versa. In all cases, the moles eventually found the food.

In a final set of experiments, tubes were inserted into the moles nostrils such that airflow direction could be switched. When these tubes were crossed, scent would flow into the left nostril from the right side of the head and vice versa. This completely confused the moles and them to miss the food entirely. You can see some of these experiments below. Keep in mind that these creatures are blind and are literally following their noses.

Taken together, these data indicate that the moles are using their two nostrils to gather separate clues about the location of their prey. If this were not the case, the moles should have followed the same path toward the food regardless of whether they were using one nostril or two (though perhaps a bit more slowly with only one nostril). They aren’t merely getting less input with only one nostril, they’re also losing directional information. This is no different than the change we experience if one of our eyes or ears is blocked.  

So does this finding apply to all mammals? That has yet to be seen, though it’s certainly possible. Moles were a good model for this study because their prey search isn’t confounded by visual clues as it would have been if Catania had used dogs or rats. If humans have this ability we probably don’t notice it as we rely so little on smell for orientation.

Catania, K. (2013). Stereo and serial sniffing guide navigation to an odour source in a mammal Nature Communications, 4 DOI: 10.1038/ncomms2444.