---final-evo-moths.nlogo

NETLOGO

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WHAT IS IT?
-----------
This model demonstrates moths flying in circles around a light.  Each moth follows a set of simple rules.  None of the rules specify that the moth should seek and then circle a light.  Rather, the observed pattern arises out of the combination of the moth's random flight and the simple behavioral rules described below.

Scientists have proposed several explanations for why moths are attracted to and then circle lights. For example, scientists once believed that moths navigated through the sky by orienting themselves to the moon, and that the moths' attraction to nearby, earthly light sources (such as a street lamp) arose because they mistook the terrestrial lights for the moon.  However, while this explanation may seem reasonable, it is not supported by available scientific evidence.

This model proposes and tests an alternate explanation: that moths need to find each other to mate, and that a light source is as good as any other area to meet.


HOW IT WORKS
------------
Each moth has a like-light value that determines their basic behaviour.  If their personal like-light value is greater than the global like-light-bias, then when they detect a light source from a distance they tend to fly straight toward the light.  If the moth's like-light value is below like-light-bias then moths fly away directly away from perceived light.  If any moth is too close to a light (as determined by their global sensitivity), they will turn away by their turn-angle amount in order to avoid the light.

To determine when to turn away, the moths compare the light immediately ahead of them with the light at their current position.  If the ratio of 'light just ahead' to 'light here' is above a threshold value, then moths turns away from the light.  The threshold is determined by the moths' sensitivity to light.

If the moths do not detect any light, or if there simply are no lights in the space where the moths are flying, then the moths flutter about randomly.

Note that light energy is represented in this model as decreasing with the square of the distance from the light source.  This characteristic is known as a "one over r-squared relationship," and is comparable to the way electrical field strength decreases with the distance from an electrical charge and the way that gravitational field strength decreases with the distance from a massive body.

Reproduction is assumed to require close physical proximity.  Thus, whenever two moths are sufficiently close to each other (within two patches), they pick a random number less than 100.  If the number is less than fertility-rate, and there are fewer than 100 moths already in the population, a new moth is spawned in a random location.  The new moth has a turn-angle and like-light value very similar to one of its parents (which parent exactly is randomly chosen.)


HOW TO USE IT
-------------
Click the SETUP button to create NUMBER-LIGHTS with LUMINANCE and NUMBER-MOTHS.  Click the GO button to start the simulation.

NUMBER-MOTHS:  This slider determines how many lights will be created when the SETUP button is pressed.

NUMBER-LIGHTS:  This slider determines how many lights will be created when the SETUP button pressed.  Note that this value only affects the model at setup.

LUMINANCE:  This slider influences how bright the lights will be.  When a light is created, it is assigned a luminance of 20 plus a random value between 0 and LUMINANCE. Lights with a higher luminance can be sensed by moths from farther away.  Note that changing LUMINANCE while the model is running has no effect.

SENSITIVITY:  This slider determines how sensitive the moths are to light.  When SENSITIVITY is higher, moths are able to detect a given light source from a greater distance and will turn away from the light source at a greater distance.

FERTILITY RATE: This slider determines the odds of a single mating resulting in an offspring.

LIKE-LIGHT-BIAS: Each moth has a like-light value between -50 and 50.  If a moth detects light and it's like-light value is greater than LIKE-LIGHT-BIAS, it will turn towards the light.  Else, it will turn away from the light.  Thus, the higher the LIKE-LIGHT-BIAS, the more moths that will fly away from the light.

LIFE-EXPECTANCY: Each moth has an age that starts at 0 when they are created and is advanced by one each cycle.  Whenever a moth is older than LIFE-EXPECTANCY, it has a 10% chance of dying each turn.


THINGS TO NOTICE
----------------
When the model begins, notice that the graph says the average like-light is zero, meaning that as many moths are attracted to the light as are repelled by it.  What happens after the simulation has run for awhile?  Can you change the sliders to make the opposite happen?

Why, after a few generations, may the moths be attracted to the lights?


THINGS TO TRY
-------------
Set the FERTILITY-RATE to 50 and run the simulation a few times without any lights.  Does the population always survive?

With the simulation stopped, use the following values:
  NUMBER-LIGHTS: 1
  LUMINANCE: 2
  NUMBER-MOTHS: 10
  SENSITIVITY: 1.50
  FERTILITY-RATE: 50
  LIKE-LIGHT-BIAS: 0
  LIFE-EXPECTANCY: 500
Run the simulation 4 or 5 times.  Does the simulation have the same results each time?  Why do you think this is?

While keeping the rest of the variables stable, move the SENSITIVITY between the two extremes.  Does this change the results of the model?  Why do you think this is?

Raise the bias so that more moths tend to fly away from lights.  Can the model still result in mainly light-seeking moths?

It would be interesting to better understand the flight patterns of the moths in the model.  Add code to the model that allows you to track the movements of one or more moths (for example, by using the pen features).  Do you see a pattern?  Why might such a pattern appear and how can it be altered?


EXTENDING THE MODEL
-------------------
This model offers only one set of rules for generating moths' circular flight around a light source.  Can you think of different ways to define the rules?

Alternatively, can you imagine a way to model an earlier theory of moth behavior in which moths navigate straight lines by orienting themselves to the moon?  Do rules that allow moths to navigate according to their position relative to the moon lead to the observed circling behavior around light sources that are much, much closer than the far-away moon?

The model of reproduction here isn't very realistic.  It might be interesting to require that two moths are near each other for consecutive turns or something similar.

It seems possible that predators may be attracted to lights as well.  It might be interesting to add some kind of "predator chance" of being killed that is increased when the moth is in a brighter area.  Could the population still grow to like lights?


NETLOGO FEATURES
----------------
This model creates a field of light across the patches, using SCALE-COLOR to display the value, and the moths use UPHILL or DOWNHILL to traverse the light field. 

RELATED MODELS
--------------
Ants, Ant Lines, Fireflies, and Flocking.


CREDITS AND REFERENCES
----------------------
Adams, C.  (1989).  Why are moths attracted to bright lights?  Retrieved May 1, 2005, from http://www.straightdope.com/classics/a5_038.html

To refer to this model in academic publications, please use: Wilensky, U.  (2005).  NetLogo Moths model.  http://ccl.northwestern.edu/netlogo/models/Moths.  Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

In other publications, please use: Copyright 2005 Uri Wilensky.  All rights reserved.  See http://ccl.northwestern.edu/netlogo/models/Moths for terms of use.
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NetLogo 3.0
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<experiments>
  <experiment name="simple" repetitions="1" runMetricsEveryTick="true">
    <setup>setup</setup>
    <go>go</go>
    <timeLimit ticks="600"/>
    <metric>like-count</metric>
    <enumeratedValueSet variable="number-moths">
      <value value="10"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="number-lights">
      <value value="1"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="sensitivity">
      <value value="1.25"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="life-expectancy">
      <value value="500"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="fertility-rate">
      <value value="50"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="luminance">
      <value value="2"/>
    </enumeratedValueSet>
    <steppedValueSet variable="like-light-bias" first="-50" step="5" last="50"/>
  </experiment>
  <experiment name="experiment rep" repetitions="5" runMetricsEveryTick="true">
    <setup>setup</setup>
    <go>go</go>
    <timeLimit ticks="600"/>
    <metric>like-count</metric>
    <enumeratedValueSet variable="number-moths">
      <value value="10"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="number-lights">
      <value value="1"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="sensitivity">
      <value value="1.25"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="life-expectancy">
      <value value="500"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="fertility-rate">
      <value value="50"/>
    </enumeratedValueSet>
    <enumeratedValueSet variable="luminance">
      <value value="2"/>
    </enumeratedValueSet>
    <steppedValueSet variable="like-light-bias" first="-50" step="5" last="50"/>
  </experiment>
</experiments>
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