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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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.

The straw that broke the camel’s back

One of the most interesting things in neuroscience is that we find again and again that the different nervous systems come up with the same solutions to related problems.  Take the ability to make a decision – something that is about as basic and fundamental as you get, while needing to be applied to all sorts of situations.  In monkeys deciding whether to look at or away from an object, if you track neurons in one area of the brain (LIP), you see that the activity fluctuates up and down until it gets to some threshold and the decision is made.

This principle extends beyond simple decisions to include what may seem to be more complex decisions such as the decision to fight or (at a later time) flee.  Although it may not be the first example to leap to mind, cricket fighting can give us plenty of insight into how this decision might be represented in the brain.  Cricket fights have been a popular past time in China for over a millenia (though imho beetle fighting is much more entertaining).  Crickets make for great subjects for scientific study: they’re small, don’t take a lot of resources, don’t complain too much, and have highly stereotyped behaviors which make quantifiable analysis simple.  When two male crickets meet, they will often fence with their antennae (pictured above), and as fights become more intense will move to engaging with their mandibles and eventually some pretty intense wrestling-like grappling.  The winner will then sing the loser off to prove his might.

All cricket fights are required to start with antennal fencing.  If their antennae are removed, the poor little guys will not fight.  They still recognize each other – they can still court – but there is no fighting.  Of course, not every cricket will want to fight every other cricket.  They have some sense of hierarchy, so a highly dominant cricket will be more likely to run off a highly submissive cricket.  And if a cricket is placed in a tiny little home, as soon as 2 minutes later they will be more likely to get aggressive in order to defend their home.  There’s something very anthropomorphic and sweet about that, I think.

Aggressiveness is represented in the brain through the neuromodulator octopamine, and this can have surprising side effects.  See, octopamine is the insect equivalent of neuroadrenaline and it is released by physical movements.  The fans of Chinese Cricket Fighting will already know this; it has long been suggested that if your cricket isn’t being aggressive enough for your taste you should just chuck the guy in the air.  And what do you know?  He’ll be more likely to put up a fight now.  Even better is it to make him fly for a while in a wind tunnel.  So we see that the representation of aggression can have surprising side effects.

The flip side of fight is flight, and a cricket needs to know when in a fight to switch to flight.  One can begin to determine how a cricket knows when to flee by mangling the cricket.  Sorry guys, that’s science for you.  You can blacken their eyes so they cannot see, lame their mandibles so that they cannot bight, and clip their claws so that they cannot tear, then mamed crickets to fight and see how they do.

Blinded crickets who fight crickets with mamed mandibles will win 98% of the time.  That’s quite a lot!  These blinded animals will not feel much of a physical blow from their opponents, and will not be receiving any visual social input either.  Remove either of these conditions – make a nonblinded cricket fight a lamed one, or a blinded cricket fight a healthy one – and the healthy one will probably win.  So how do these crickets know when to flee?  By the steady accumulation of visual and physical input.  Once enough of this input is received – possibly represented in the form of some hormone or peptide – it’s time to fly for the cricket!

It wouldn’t be surprising if something like this happened in humans, too.  We already have a proverb for it after all: the straw that broke the camel’s back.  Crickets will continue to fight after a serious injury, only to retreat seconds later for no apparent reason.  So too are humans known to accept plenty of punishment and grit out, only to have something small cause them to cry and give up when their threshold is reached.  This is one of the the fundamental lessons of decision neuroscience so far: discrete decisions are made when information has accumulated up to some threshold.  It’s just not always easy to tell what our thresholds are.

References

Stevenson PA, & Rillich J (2012). The decision to fight or flee – insights into underlying mechanism in crickets. Frontiers in neuroscience, 6 PMID: 22936896

Photo from