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The Evolution of Coy vs. Fast Females and Faithful vs. Philander Males:
A Computer Tutorial and Simulation 


Michael E. Mills, Ph.D.
Psychology Department
Loyola Marymount University
Los Angeles, CA 90045

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(c) Copyright 1991 Michael E. Mills, Ph.D.


Press the "Pg Dn" (page down) key to continue.





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In a sexually reproducing species,  individuals typically must invest
a lot of hard work to reproduce successfully.   Unlike asexual
reproducers (cloners), sexual reproducers have to (a) find a potential
mate, (b) court him/her, (c) engage in sexual intercourse, and (d)
bring up baby.   Each of these activities exact a cost in terms of
time and energy. 

For example, imagine a purely monogamous society composed of totally
honest individuals.   In this hypothetical society, females are "coy:" 
they demand a prolonged courtship period before copulating 
to assess the male's 
willingness to commit and provide resources.  The males are "faithful:" 
they do not desert the female after copulation, but instead help with
the arduous task of raising the offspring.  In his book 
The Selfish Gene, Dawkins (1976) uses  "payoff"
and "cost" point values for these various
reproductive activities to assess the resulting fitness outcomes.
The various point values:
  
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        +15 points reward to each parent for each child 
                      successfully raised
        -10 points cost to each parent to raise a child 
         -3 points cost to each in wasted time in prolonged 
                   courtship

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How many points does a monogamous couple get for raising a child?
By adding up the points above, each parent gets a +2 point "reward"
for each child raised.   In general, this sounds pretty good -- everyone,
male and female, benefits.  This population of all coy females and
all faithful males results in what Dawkins calls the "domestic
bliss" population: everyone is content with his/her payoff.

However, imagine that a genetic mutation occurs resulting in a female 
that will copulate without a prolonged courtship -- a "fast" female.
What will her payoff be?  Remember, all of the males in the current population
are faithful males: honest courters, noble in 
their intentions, excellent providers and fathers, and with no
intentions of deserting the family.  Since all males will be faithful,
the fast female loses nothing by not insisting on a long courtship.
She doesn't have to pay courtship costs (-3 points), so her 
payoff is +5  --  higher than that of a coy female (+2). 
The gene(s) that produces "fast" females will begin to spread in
the population. The males are happy about this too 
since they also no longer have to pay courtship costs,  
and their payoff now is +5.

After many generations, the population of fast females starts to outnumber
the population of coy females; but all the males remain faithful.
However, now a mutation occurs in a male.  Instead of being "faithful"
this mutated male is a "philanderer."  A philanderer is a male that,
instead of helping his mate raise the offpring, deserts her. 
He leaves to go out philandering:  
chasing other females to obtain additional reproductive opportunities.

Now, since the majority of the population of females are "fast" females, the
philandering male doesn't have to pay two costs: (a) the cost 
of courting (-3 points) or (b) his share of the cost of raising 
baby (-10 points).
Accordingly, his net payoff is huge: he leaves a trail of children 
behind him, and he gets +15 points for each.   For example, say 
he leaves 5 children,
a rather conservative number.  His total points accumulated will
be +75.  This is huge compare to the payoff to a faithful male who
also has 5 children, gets +2 points for each, and thus has an 
accumulated point value of only +15. 
Since the philandering male's payoff per child
is so high, the gene(s) that produce philandering males
explode through the population.   Faithful males soon become rare. 
But those lying, good for nothing philandering males are everywhere.   
From the perspective of the females, now a good man is very hard to find. 

With so many philandering males running around, 
fast females, who were once in an advantageous position when most males
were faithful, are now losing miserably. 
She must pay the entire -20 point childrearing cost herself.
Her net payoff is actually negative: -5 points.  

But the few remaining coy females, who were not doing well before, now find 
themselves in a better position.  Since coy females demand a prolonged
courtship, they never mate with a philandering male (since he is
unwilling to wait that long to copulate).  Rather, the coy females 
hold out for the few remaining faithful males (who are still willing to wait
through a courtship period before copulating).    For coy females,
their net payoff is either zero if they never find a faithful male,
or +2 points if they do.   Clearly, zero or +2 is better than the -5 points
each fast female is paying per child.   So, soon the pendulum starts
to swing back in favor of coy females over fast females.  
Following this shift, the advantage
also slowly goes back to faithful males over philandering males. 

So we see an oscillation in the relative proportions of the various
types.  What is the best strategy?  That depends on the current
relative proportions of the various types.  Clearly if 
you are male, and the majority of the population consists of 
fast females and faithful males, it would be to your
advantage to be a philanderer.  And so on...  

But eventually the oscillation will come to some type of equilibrium, 
called an "evolutionary stable strategy," or an ESS.  An ESS is a strategy 
that, on average, and over the long run, cannot be bettered by an 
alternate strategy.  It turns out that, given the costs 
and reward point values for each of the outcomes given above,  
the resulting ESS 
will be a population consisting of about 5/8ths faithful males, 
and 5/6ths coy females.  

In addition, these ESS proportions can also work in terms of how much time 
an individual spends playing each role.   For example, an equally effective
ESS would be for a male to spend 5/8ths of his time being faithful,
and for a female to spend 5/6ths of her time being coy.  

To see this ESS actually evolve, we will set up a small
computer simulated universe, and define the rules of its operation. 
We will observe the resulting proportions of fast, coy, faithful and 
philandering types.  

Here are the rules.

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1.  Males and females are free to move about in any direction.
Males and females not of reproductive age are grey.  Reproductive
males are blue, reproductive females are red.

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2.  When a reproductive male and reproductive female encounter
one another, they spend a second or two courting. 
Mating is indicated by the pair blinking on
the screen and the sound of a beep on the computer speaker.

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3.  Offspring are placed as close to the parents as possible, in 
the first available empty space.  Offspring do not become 
reproductive for a few seconds,
and until they become reproductive they are grey.

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4.  After mating, and childrearing (if any), the individuals split apart.

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5. Individuals die after about 10 seconds of life.  Their death is
indicated by their disappearance from the screen.

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6.  Mating rules and resulting points will be pretty much as
were described above.  
We will be able to identify each type on the screen by its color.
Coy females will be red; fast females will be light red;
faithful males will be blue, and philandering males will be light blue.

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7.  We will start out with a population composed mostly of
coy females and faithful males: about 40% coy, 40% faithful, 
10% fast, 10% philanderer. 

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We should see the relative proportions coy females and faithful males decline,
while the few initial numbers of fast females and philandering males increase. 

However, since there is some
randomness built into the simulation (e.g., in direction of movement, etc.),
each run of the simulation will give a slightly different final result.

Let's see what happens.   Press the ESC key to start.

