we've 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've probably seen the vast trail
systems that they can create in your
backyard or in a public park
somewhere ants are really amazing in
their ability
to collectively forage for food in a
very efficient way
we talked about this a little bit in
unit one
and we're going to revisit it here in
this unit along with some other 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 absence
of reinforcement
the pheromone will dissipate but as long
as ants continue to
follow the pheromone trail and continue
to find food
then they'll continue to reinforce it
here's a video of a famous experiment
that's 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 if they go 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
managed to go over here
there is some randomness in what they do
but for the most part they're following
this
shorter path to the food
you probably remember this model ant's
new
it consists of a ant's nest
and three food sources and it models ant
foraging along the lines of
the algorithm i just described to you so
if we do go
let me slow it down a little bit the
ants go out from the nest
if one of them encounters food they're
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 you can see it
more slowly and other ants are attracted
to the pheromone trail
they'll follow it but if the trail is
not reinforced by ants like if the food
source dries up
the trail evaporates eventually
see this trail evaporating but if
there still is food along the trail then
the ants will find it and will reinforce
the pheromone trail
so in this way the ants are able to
build a 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 actually is the role of the
pheromone in helping the ants find the
food
so to do that i'm 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 pheromone will
evaporate so fast that the ants can't
sense it
so let's see what happens then so here
the ants have no pheromone to sense
they're just randomly finding food
they're not interacting with one another
and we'll see how long it takes them i
just speeded it up
this the whole simulation stops when all
the food is gone
okay and it took them 538 ticks
to find all the food when there were 500
ants
i'll mark that down here we'll say
no pheromone five
uh 538 ticks to find the food
so now let's turn the pheromone back on
again we'll put it down to
two a very low evaporation rate
and we'll see how that affects the
behavior of course because there's a lot
of randomness if i really wanted to get
accurate numbers here i'd have to do
this many times i'm actually going to
let you do that in your homework
and average the results but i'll just do
this informal test here
so we'll see if there's a change in the
amount of time it takes
for all the food to be gone
and indeed this time it only took 415
ticks
so we'll say with
pheromone 415
ticks so this simplified model lets us
have some intuition about 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 along those
trails
encode the colony's 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 so in 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
debra gordon is a biologist at stanford
university who has done extensive
work on on many behaviors in ant
colonies but in particular we're going
to talk about her work on task
allocation
according to her task 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
it's a self-organizing activity
gordon has studied task allocation in
harvester ants a 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's a picture of a nest
patrolling foraging refuse sorting
and so on so here are some
representative pictures of different
kinds of ants doing different kinds of
tasks
but the question is how is it at any
given time
that ants decide what task they should
be doing
and it turns out that ants actually
switch tasks according to what's 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 if the
nest is disturbed
the number of nest maintenance workers
will increase
and the number of forgers will decrease
more ants are taking on
the task of nest maintenance when it's
needed and
fewer are taking on the task of foraging
and she was able to do this experiment
by
putting toothpicks at different places
around the nest and the ants really
didn't like this evidently
and so the number of nest maintenance
workers to remove the toothpicks
increased she also found that if 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's evident is
that ants are not able to get the big
picture of what's going on
in their environment they only interact
locally with other ants
and have to learn about what's going on
globally from
limited interactions similar to the
other self-organizing systems that we've
seen
so here's the question in a nutshell how
does an individual ant decide which task
to adopt
in response to nest wide environmental
conditions
even though no ant directs the decision
of any other ant
and each ant interacts only with a small
number of other ants
well here's what gordon found by doing
extensive experiments
she found that ants decide to adopt a
particular task such as foraging
or patrolling or nest maintenance as a
function
of two things one what they encounter in
the environment
and two 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 maintenance but also
they're influenced by what other ants
they come into contact with
gordon found that ants can actually tell
what job another ant has been doing by
sensing
chemical residues on its antennae on the
antennae of the other ant
here's a little video that deborah
gordon's research group
took using a video scope inside an ant
nest
and here you can see ants coming
out of the nest and into the nest
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 since
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 statistician
they measure the rate of interaction of
the ants that they encounter doing
different tasks
those rates are one of the things they
use to decide what task to adopt
it's not known how the brain of an ant
is able to do this
but it seems from the data that gordon
and her research collaborators have
taken
that that is exactly what the ants are
somehow doing
so here's a little puzzle gordon and her
colleagues observed that
larger ant colonies are more
deterministic
and 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 ant colony be in some
sense
better at optimally allocating tasks in
a consistent and deterministic way
than smaller colonies
well the hypothesis that gordon and her
colleagues put forth is that ants
in larger colonies basically can get
better statistics
on interaction rates because they have
more
samples each ant has on average more
samples
of interactions with other ants and so
they get better statistics
so next time you see ants in your
kitchen or 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 if we could
somehow take advantage of their
statistical abilities