The smell of rain: what is petrichor?

pet·ri·chor
ˈpeˌtrīkôr/
noun
  1. a pleasant smell that frequently accompanies the first rain after a long period of warm, dry weather.
    “other than the petrichor emanating from the rapidly drying grass, there was not a trace of evidence that it had rained at all”

After the goofy first 40 seconds, this video lists two things that make up the ‘smell of rain’: ozone and petrichor.

Petrichor is the decomposing plant matter that rain causes to erupt from the soil. This same substance is – supposedly – a signal to plants that the soil has been dry and will prevent seeds from sprouting.

But when I went looking for more scholarly information on petrichor I found it…practically non-existent. There were no articles that mentioned it in the 2000s. There was one article that mentioned it in the 1990s. There were a few articles that mentioned referred to a specific article from the 70s. In fact, the were only two original research articles that I could find investigating petrichor as a scientific concept, both by the same two authors: IJ Bear and RG Thomas. These are “Petrichor and Plant Growth” and “Genesis of Petrichor“. As far as I can tell, none of the research was followed up on or replicated though the idea has occasionally been taken up in other contexts.

Petrichor clearly exists as a smell, but it turns out that there is precious little knowledge of what it actually is.

[via Ed Yong]

The in-between of nature and nurture

There’s a debate that never seems to die down, and it’s one of nature versus nurture.  It’s a bit of a silly debate because the answer in every debate is (almost) always “both”, but it does seem to get a lot of play.  And it’s even sillier when you realize that one can ask the question about any behavior in our life, and we already know the answer.  Take, for example, what type of food you like.  There are certain foods that, innately, everyone likes, things that are required for survival: I imagine these are things like bacon and butter and otter pops.  But other foods, foods that are not as full of fat and grease and sugar, these things take some acclimation: brussel sprouts and pigs feet and rocky mountain oysters.  There’s always a genetic underpinning – for instance, there is a specific genetic mutation that determines whether we can taste certain bitter flavors – whose behavioral expression gets modified through the environment.  The question is though: when we learn to like these foods, what exactly are we learning?

To understand how this type of learning works, we can turn to the hawkmoth Manduca sexta.  As caterpillars they feed on tobacco and tomato plants, but when they become adult moths they flap about in the dark, feeding on the nectar from night-blooming flowers.  These plants exist in a symbiotic relationship with the hawkmoths – without the moths, the flowers would have trouble getting pollinated.  Perhaps that is why the flowers that the hawkmoths prefer to visit all release a very similar chemical bouquet; other flowers, even genetically related flowers, have different scents, and these flowers do not get pollinated by the hawkmoths.

Now if  you go in and stick an electrode into the antennal lobe, the area where odors are first received by the hawkmoth, what do you see?  Recording from the projection neurons, the neurons most responsible for sending this odor information back to other parts of the hawkmoth brain, you will see what appear to be two different types of responses.  The hawkmoth odor neurons will respond to the attractive odors – the odors that were taken from flowers that the hawkmoths prefer to pollinate – and these responses all look pretty similar.  They are sudden, strong neuronal responses.  But if you now spray the hawkmoth with odors that aren’t particularly attractive, you get much weaker responses that look very little like the responses to attractive odors.

There’s the nature part of your story: there are some odors that even a naive, laboratory-raised hawkmoth will love, and others that it won’t care about.  But that’s clearly not the end of the story.  Just like humans can take that first sip of beer and spit it out because it’s disgusting only to find themselves savoring a good porter years later, hawkmoths can learn that a new, unknown odor might signal something delicious.  And it’s quick: with just three tastes of an odor paired with some nectar, the hawkmoths learn to like the odor.  It’s not just anywhere that the moths learn to like the odor, either.  It could be that the odor becomes more attractive somewhere deep in the brain, where reward neurons respond when they see the responses corresponding to this specific odor.  What actually happens is that it is the olfactory projection neurons themselves that change how they respond, to look like the responses to other attractive odors.  Even though there are odors that are genetically programmed to be attractive to the hawkmoth, interaction with the environment can directly modify how an odor is sensed to make it more or less attractive: nature, and nurture.

Screen shot 2013-01-14 at 10.29.16 AM

References

Riffell, J., Lei, H., Abrell, L., & Hildebrand, J. (2012). Neural Basis of a Pollinator’s Buffet: Olfactory Specialization and Learning in Manduca sexta Science, 339 (6116), 200-204 DOI: 10.1126/science.1225483
Photo from…I realize this is a different species of Hawkmoth, but work with me here!  There’s only so many decent pictures of Hawkmoths under the Creative Commons license at flickr.