NY Times, November 13, 2007
From Ants to People, an Instinct to Swarm
By CARL ZIMMER
If you have ever observed ants marching in and out of a nest, you
might have been reminded of a highway buzzing with traffic. To Iain D. Couzin,
such a comparison is a cruel insult — to the ants.
Americans spend a 3.7 billion hours a year in congested traffic.
But you will never see ants stuck in gridlock.
Army ants, which Dr. Couzin has spent much time observing in
Panama, are particularly good at moving in swarms. If they have to travel over
a depression in the ground, they erect bridges so that they can proceed as
quickly as possible.
ÒThey build the bridges with their living bodies,Ó said Dr.
Couzin, a mathematical biologist at Princeton University and the University of
Oxford. ÒThey build them up if theyÕre required, and they dissolve if theyÕre
not being used.Ó
The reason may be that the ants have had a lot more time to adapt
to living in big groups. ÒWe havenÕt evolved in the societies we currently live
in,Ó Dr. Couzin said.
By studying army ants — as well as birds, fish, locusts and
other swarming animals — Dr. Couzin and his colleagues are starting to
discover simple rules that allow swarms to work so well. Those rules allow
thousands of relatively simple animals to form a collective brain able to make
decisions and move like a single organism.
Deciphering those rules is a big challenge, however, because the
behavior of swarms emerges unpredictably from the actions of thousands or
millions of individuals.
ÒNo matter how much you look at an individual army ant,Ó Dr.
Couzin said, Òyou will never get a sense that when you put 1.5 million of them
together, they form these bridges and columns. You just cannot know that.Ó
To get a sense of swarms, Dr. Couzin builds computer models of
virtual swarms. Each model contains thousands of individual agents, which he
can program to follow a few simple rules. To decide what those rules ought to
be, he and his colleagues head out to jungles, deserts or oceans to observe animals
in action.
Daniel Grunbaum, a mathematical biologist at the University of
Washington, said his field was suddenly making leaps forward, as math and
observation of nature were joined in the work of Dr. Couzin and others. ÒIn the
next 10 years thereÕs going to be a lot of progress.Ó
He said Dr. Couzin has been important in fusing the different
kinds of science required to understand animal group behavior. ÒHeÕs been a
real leader in bringing a lot of ideas together,Ó Dr. Grunbaum said. ÒHe has a
larger vision. If it works, thatÕll be a big advance.Ó
In the case of army ants, Dr. Couzin was intrigued by their
highways. Army ants returning to their nest with food travel in a dense column.
This incoming lane is flanked by two lanes of outgoing traffic. A three-lane
highway of army ants can stretch for as far as 150 yards from the ant nest,
comprising hundreds of thousands of insects.
What Dr. Couzin wanted to know was why army ants do not move to
and from their colony in a mad, disorganized scramble. To find out, he built a
computer model based on some basic ant biology. Each simulated ant laid down a
chemical marker that attracted other ants while the marker was still fresh.
Each ant could also sweep the air with its antennas; if it made contact with another
ant, it turned away and slowed down to avoid a collision.
Dr. Couzin analyzed how the ants behaved when he tweaked their
behavior. If the ants turned away too quickly from oncoming insects, they lost
the scent of their trail. If they did not turn fast enough, they ground to a
halt and forced ants behind them to slow down. Dr. Couzin found that a narrow
range of behavior allowed ants to move as a group as quickly as possible.
It turned out that these optimal ants also spontaneously formed
highways. If the ants going in one direction happened to become dense, their
chemical trails attracted more ants headed the same way. This feedback caused
the ants to form a single packed column. The ants going the other direction
turned away from the oncoming traffic and formed flanking lanes.
To test this model, Dr. Couzin and Nigel Franks, an ant expert at
the University of Bristol in England, filmed a trail of army ants in Panama.
Back in England, they went through the film frame by frame, analyzing the
movements of 226 ants. ÒEverything in the ant world is happening at such a high
tempo it was very difficult to see,Ó Dr. Couzin said.
Eventually they found that the real ants were moving in the way
that Dr. Couzin had predicted would allow the entire swarm to go as fast as
possible. They also found that the ants behaved differently if they were
leaving the nest or heading back. When two ants encountered each other, the
outgoing ant turned away further than the incoming one. As a result, the ants
headed to the nest end up clustered in a central lane, while the outgoing ants
form two outer lanes. Dr. Couzin has been extending his model for ants to other
animals that move in giant crowds, like fish and birds. And instead of tracking
individual animals himself, he has developed programs to let computers do the
work.
The more Dr. Couzin studies swarm behavior, the more patterns he
finds common to many different species. He is reminded of the laws of physics
that govern liquids. ÒYou look at liquid metal and at water, and you can see
theyÕre both liquids,Ó he said. ÒThey have fundamental characteristics in
common. ThatÕs what I was finding with the animal groups — there were
fundamental states they could exist in.Ó
Just as liquid water can suddenly begin to boil, animal swarms can
also change abruptly thanks to some simple rules.
Dr. Couzin has discovered some of those rules in the ways that
locusts begin to form their devastating swarms. The insects typically crawl
around on their own, but sometimes young locusts come together in huge bands
that march across the land, devouring everything in their path. After
developing wings, they rise into the air as giant clouds made of millions of
insects.
ÒLocusts are known to be around all the time,Ó Dr. Couzin said.
ÒWhy does the situation suddenly get out of control, and these locusts swarm
together and devastate crops?Ó
Dr. Couzin traveled to remote areas of Mauritania in Africa to
study the behavior of locust swarms. Back at Oxford, he and his colleagues
built a circular track on which locusts could walk. ÒWe could track the motion
of all these individuals five times a second for eight hours a day,Ó he said.
The scientists found that when the density of locusts rose beyond
a threshold, the insects suddenly began to move together. Each locust always
tried to align its own movements with any neighbor. When the locusts were
widely spaced, however, this rule did not have much effect on them. Only when
they had enough neighbors did they spontaneously form huge bands.
ÒWe showed that you donÕt need to know lots of information about
individuals to predict how the group will behave,Ó Dr. Couzin said of the
locust findings, which were published June 2006 in Science.
Understanding how animals swarm and why they do are two separate
questions, however.
In some species, animals may swarm so that the entire group enjoys
an evolutionary benefit. All the army ants in a colony, for example, belong to
the same family. So if individuals cooperate, their shared genes associated
with swarming will become more common.
But in the deserts of Utah, Dr. Couzin and his colleagues
discovered that giant swarms may actually be made up of a lot of selfish
individuals.
Mormon crickets will sometimes gather by the millions and crawl in
bands stretching more than five miles long. Dr. Couzin and his colleagues ran
experiments to find out what caused them to form bands. They found that the
forces behind cricket swarms are very different from the ones that bring
locusts together. When Mormon crickets cannot find enough salt and protein,
they become cannibals.
ÒEach cricket itself is a perfectly balanced source of nutrition,Ó
Dr. Couzin said. ÒSo the crickets, every 17 seconds or so, try to attack other
individuals. If you donÕt move, youÕre likely to be eaten.Ó
This collective movement causes the crickets to form vast swarms.
ÒAll these crickets are on a forced march,Ó Dr. Couzin said. ÒTheyÕre trying to
attack the crickets who are ahead, and theyÕre trying to avoid being eaten from
behind.Ó
Swarms, regardless of the forces that bring them together, have a
remarkable ability to act like a collective mind. A swarm navigates as a unit,
making decisions about where to go and how to escape predators together.
ÒThereÕs a swarm intelligence,Ó Dr. Couzin said. ÒYou can see how
people thought there was some sort of telekinesis involved.Ó
What makes this collective decision-making all the more puzzling
is that each individual can behave only based on its own experience. If a shark
lunges into a school of fish, only some of them will see it coming. If a flock
of birds is migrating, only a few experienced individuals may know the route.
Dr. Couzin and his colleagues have built a model of the flow of
information through swarms. Each individual has to balance two instincts: to
stay with the group and to move in a desired direction. The scientists found
that just a few leaders can guide a swarm effectively. They do not even need to
send any special signals to the animals around them. They create a bias in the
swarmÕs movement that steers it in a particular direction.
ÒIt doesnÕt necessarily mean you have the right information,
though,Ó Dr. Couzin pointed out.
Two leaders may try to pull a swarm in opposite directions, and
yet the swarm holds together. In Dr. CouzinÕs model, the swarm was able to
decide which leaders to follow.
ÒAs we increased the difference of opinion between the informed
individuals, the group would spontaneously come to a consensus and move in the
direction chosen by the majority,Ó Dr. Couzin said. ÒThey can make these
decisions without mathematics, without even recognizing each other or knowing
that a decision has been made.Ó
Dr. Couzin and his colleagues have been finding support for this
model in real groups of animals. They have even found support in studies on
mediocre swarmers — humans.
To study humans, Dr. Couzin teamed up with researchers at the
University of Leeds. They recruited eight people at a time to play a game.
Players stood in the middle of a circle, and along the edge of the circle were
16 cards, each labeled with a number. The scientists handed each person a slip
of paper and instructed the players to follow the instructions printed on it
while not saying anything to the others. Those rules correspond to the ones in
Dr. CouzinÕs models. And just as in his models, each person had no idea what
the others had been instructed to do.
In one version of the experiment, each person was instructed
simply to stay with the group. As Dr. CouzinÕs model predicted, they tended to
circle around in a doughnut-shaped flock. In another version, one person was
instructed to head for a particular card at the edge of the circle without
leaving the group. The players quickly formed little swarms with their leader
at the head, moving together to the target.
The scientists then sowed discord by telling two or more people to
move to opposite sides of the circle. The other people had to try to stay with
the group even as leaders tried to pull it apart.
As Dr. CouzinÕs model predicted, the human swarm made a quick,
unconscious decision about which way to go. People tended to follow the largest
group of leaders, even if it contained only one additional person.
Dr. Couzin and his colleagues describe the results of these
experiments in a paper to be published in the journal Animal Behavior.
Dr. Couzin is carrying the lessons he has learned from animals to
other kinds of swarms. He is helping Dr. Naomi Leonard, a Princeton engineer,
to program swarming into robots.
ÒThese things are beginning to move around and interact in ways we
see in nature,Ó he said. Ultimately, flocks of robots might do a better job of
collecting information in dangerous places. ÒIf you knock out some individual,
the algorithm still works. The group still moves normally.Ó The rules of the
swarm may also apply to the cells inside our bodies. Dr. Couzin is working with
cancer biologists to discover the rules by which cancer cells work together to
build tumors or migrate through tissues. Even brain cells may follow the same
rules for collective behavior seen in locusts or fish.
ÒOne of the really fun things that weÕre doing now is
understanding how the type of feedbacks in these groups is like the ones in the
brain that allows humans to make decisions,Ó Dr. Couzin said. Those decisions
are not just about what to order for lunch, but about basic perception —
making sense, for example, of the flood of signals coming from the eyes. ÒHow
does your brain take this information and come to a collective decision about
what youÕre seeing?Ó Dr. Couzin said. The answer, he suspects, may lie in our
inner swarm.