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.

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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.

On the trail of genetic gastronomy

If I told you that our diets were shaped by our environments, would you be surprised?  If I said that what we ate was shaped by what we liked, would you be surprised by that either?  I should think the answer is no to both questions.  But still, thinking as a neuroscientist the question becomes: what is the neurological basis for why we like the food that we do?  Why do some people enjoy cilantro and some think that it tastes of soap?

It’s clear that our diet has shaped our evolution – just look at the relatively recent emergence of lactose tolerance in European populations.  It is a small leap to assume that the recipes we use may shape how we taste as well – and thus how we experience the world.  A news article in Nature explores the science of taste and recipes:

The Silk Road offers a potential paradise for such genetic exploration. The route traverses massive mountain ranges such as the Pamir and the Tian Shan in central Asia and passes through pockets of the nomadic tribes who originally populated the region, as well as ethnically diverse groups descended from traders who settled en route, often near the roadside inns called caravanserais. These populations did not tend to share their genes, but they did share recipes. Cuisines are remarkably similar along much of the Silk Road — variations on tandoor breads, noodles with vegetable or mutton sauces, and dried or fresh fruit. This means that differences in food preferences between groups are likely to be down to variations in genes rather than in dietary cultures, making them even more appealing to the geneticists…

The scientists have already identified eight variants in known genes, including one for an ion channel involved in sensing spicy-hotness, which are associated with a taste for particular foods. And they have found that variants of the gene for the TAS1R2 protein, part of a sweetness receptor, are associated with a strong liking for vodka and white wine…

There may be bigger scientific stories hiding in the data. Gasparini says that the team is seeing an emerging association in Tajikistani populations between an olfactory receptor gene and both sensitivity to bitter tastes and a tendency to mistake smells. If the finding holds up, it will be the first demonstrated genetic link between smell and taste perception, and it could help to explain how signals from different senses combine to sculpt individual food preferences.