We have all seen this kind of situation.
The crack in the wall.
The ants coming into our kitchen or our bathroom,
searching for food,
and when they find it,
being able to attract all their little friends
so quickly so that they can collectively devour
whatever we dropped on the floor.
You have probably seen the vast trail systems
that they can create in your backyard or in public park somewhere.
Ants are really amazing in their ability
to collectively forge for food in a very efficient way.
We talked about this in a little bit in unit 1.
And we are going to revisit here in this unit
along with some amazing behaviors
of ant colonies.
So the basic idea of ant foraging,
as we talked about in unit 1,
is that ants come out of a nest
and move around randomly in different directions.
When an ant encounters a food source ,
it brings some food back to the nest.
but leaving a pheromone trail.
Where pheromone is a kind of chemical
excreted by the ant
that attracts other ants.
Ants encountering the trail are likely to follow it
in the abscene of reinforcement
the pheromone will dissipate
but as long as ants can continue
to follow the pheromone trail
and continue to find food,
then they will continue to reinforce it.
Here is a video of a famous experiment
that has been repeated many times.
in which ants are given the choice
of two different paths to food
a long path like this one.
and a shorter path.
Well the shorter path
ends up attracting more ants,
finally all of them
because the pheromone is able to
remain reinforced on the shorter side
more than it is on the longer side.
These ants going down this way
Their pheromone evaporates
often before other ants get to follow it.
So here we go.
You can see the ants very reliably
following this trail.
A few of them manage to go over here.
There is some randomness in what they do.
But for the most part,
they are following this shorter path
to the food.
You probably remember this model,
ants new.
It concise of ants nests
and three food sources
and it models ant foraging along the lines
of the algorithim I just described to you.
So if we do go,
we slow it down a bit.
The ants go out from the nest
if one of them encounters food,
they are moving around randomly,
if one of them encounters food
they take it back to the nest
laying a pheromone trail.
Let's run that again,
so we can see it more slowly
and other ants are attracted
to the pheromone trail.
They will follow it,
but if the trail is not reinforced by ants
like if the food source dries up
the trail evaporates eventually
to this trail evaporating.
But if there is still food along the trail,
the ants will find it and they will reinforce the pheromone trail.
So in this way the ants are able to build
the kind of model using their trails
to represent what they know collectively
about their food environment.
Using this model
we can do an experiment
to see what is actually the role
of the pheromone in helping the ants find the food.
So, to do that
we are going to do a simple experiment.
Where I'm going to turn off the pheromone.
To do that I'm going to
put the evaporation rate
at its highest level
which means that the pheronome will evaporate.
so fast the ants can't sense it.
So let's see what happens then
So here the ants have no pheromone to sense.
They are randomly finding food
They are not interacting with one another
and we will see how long it takes them.
I just speeded it up
This whole simulation stops when the food is gone.
ok and it took them 538 ticks to find the food
When there were five hundred ants.
I'll mark that down here.
We will say no pheromone, 538 ticks.
To find the food.
So know let's turn the pheromone back on again.
We'll put it down to two.
A very low evaporation rate.
We will see how that effects the behavior.
Of course because there is alot of randomness.
If I really wanted to get accurate numbers here
I'd have to this many times.
I'm actually going to let you do that in your own homework
and average the results.
I'll do this informal test here.
So we will see if there is a change in the time it takes
for all the food to be gone
and indeed it only took 415 ticks
so we will say with pheromone 415 ticks.
so this simplified model
Let's have some intuition of the role
of pheromone in helping ants
efficiently forage for food
Just to reiterate.
for ants that are foraging at any given time
the existing trails and the concentration of pheromone on those trails
encode the colonies collective information
about its food environment
moreover the information is able to adapt
to changes in environmental conditions
once the food is eaten for example
the trails will evaporate
and if new food is found then
new trails will be formed
this way the ants are able to
adaptively forage
even in a changing environment.
Now i want to briefly mention
another kind of self organizing behavior
in ant colonies and that's the notion
of task allocation..
Deborah Gordon is a biologist
at Standford University
who has done extensive work on many behaviors
in ant colonies.
But in particular we are going to talk about her work
on task allocation.
According to her,
talk allocation is the process that adjusts
the number of workers engaged in each task
in a way appropriate to the current situation
task allocation operates without central
or hierarchical control
as a self organizing activity
Gordon has studied task allocation in harvester ants
particular kind of ant
In this kind of ant
workers in a colony divide themselves
among a number of tasks
Nest maintenance, looking around cleaning up the nest.
here is a picture of a nest
Patrolling, foraging, and refuse sorting
and so on
So here are some representative pictures
of different kind of ants doing different kinds of tasks
but the question is how it at any given time
that ants decide tasks they should be doing.
and it turns out that ants actually switch tasks.
according to what is going on in their environment
so the number of workers pursuing each kind of task
adapts to changes in the environment
due to weather,
food availability,
predators
and so on.
For example Gordon showed that
the if the nest is distrubed
the number of nest maintenece workers
will increase
and the number of foragers will decrease
more ants are taking on the task of nest maintenaince.
when it is needed
and fewer are taking on the task of foraging
and she was able to do this experiment
by putting different toothpicks
at different places around the nests
and the ants really didn't like this evidently
and so the number of nests maintence
to remove the toothpicks increased.
She also found
that is the food supply is large
and high quality
the number of foragers will increase.
In short ants are able to choose tasks according to
what tasks need the most attention
and they also look at how many other ants
are already doing these tasks
How do they do this?
What is evident ants are not able
to get the big picture of what is going on
in their environment
They only interact locally
with other ants
and have to learn about what is going on globally
from limited interactions
similar to the other self organizing systems we've seen
so here is the question in a nutshull.
How does an individual ant decide which task to adopt
in response to nestwide environmental conditions
even though no ant directs
the decision of another ant
and each ant interacts
with a small number of other ants
Well here is what Gordon found
doing extensive experiments
She found that ants decided to adopt
a particular task such as foraging
or patrolling
or nest maintence
as a function of two things
1. What they encounter in the environment.
2. Their rate of interaction with ants
performing different tasks.
An Ant might encounter a toothpick
or some other debris on their nest.
and that would possibly make them switch
to do nest maintenece
but also they are influenced by whatever
ants they come in contact with
Gordon found that ants could actually tell
what job another ant is doing by
sensing chemical residue on its antennae.
of the other ant
here is a little video that Deborah Gordon's
research group took using a video scope
inside an ants nest.
and here you can see
ants coming out of the nests and into the nests
and actually touching each other
with their antennae
and that way they can sense
what job another ant has been doing.
if ants coming out of the nest sense.
many foragers coming back with food.
then they will be more likely to adopt the task
of being a forager.
So ants act as little statisticians.
they measure the rate of interaction
of ants they encounter doing different tasks
those rates are one of the things
they use to decide what tasks to adopt
it's not known how the brain
of ant is able to do this
but its seem from the data
that Gordon and her research collaborators
have taken is exactly what the ants are somehow doing.
So here is a little Puzzle
Gordon and her colleagues observed that
larger ant colonies are more deterministic
and more consistent in task allocation
than smaller ant colonies
Why do you think this is?
Pause the video for just a minute
and think about it.
Why would a larger aunt colony
be in sense better at optimally allocating tasks
in a consistent and deterministic way
than smaller colonies
Well the hypothesis
that Gordon and her collegues put forth
that ants in larger colonies
basically can get better statistics
on interaction rates
because they have more samples
each ant has on average more sample
of interactions with other ants
and so they get better statistics
so next time you see ants
in your kitchen, in your bathroom, or out in your garden.
or even crawling on your leg.
Just remember that
ants are very brilliant, little statisticians.
Think how great it would be
take advantage of their statistical abilities.