What If? Lab
What if ants left no trails?
Have you seen ants walking in a line? They follow a smelly path to food! What if ants left no trails? Let's find out with Olly.
Many ants mark the way to food with a scent trail, and their nestmates follow it. Because old scent fades, the colony often ends up on the shortest path. Computer scientists even copy the trick. Imagine ants that left no trails and follow what might change.
Trail pheromones let ant colonies find short routes without any ant seeing the whole map: positive feedback on busy paths and evaporation on slow ones. This thought experiment removes the trails and explores scent paths, collective problem solving and the computer algorithms inspired by ants.
The big question
Ants often walk in a long line. Each ant follows the one in front.
They are following a smell you cannot notice. What if there was no smelly path?
Drop a crumb outside and, before long, a line of ants may be marching toward it. Each ant seems to know exactly where to go.
The secret is a path of scent. So what if ants left no trails? This lab is a thought experiment. We can't switch the scent off, but we can follow the clues, including some clever experiments with mazes.
Britannica notes that some ants lay scent pheromones along a trail to a food source so that other members of the colony can find it. Pheromones are chemicals released in tiny amounts that trigger a response in other members of the same species.
This thought experiment asks what would happen to a colony if those trails vanished, and uses the question to explore how simple chemical signals let thousands of small-brained insects solve route-finding problems that also challenge computers.
Explore the idea
Make a prediction
How do ants show other ants the way to food?
How do many ants show their nestmates the way to food?
What mechanism lets many ant species recruit nestmates to food?
- They leave a smelly trail
- They draw arrows in the dirt
- They make loud calls
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Established fact
Some ants leave a smelly path to food.
Some ants lay scent trails that their nestmates follow and strengthen.
Trail pheromones recruit nestmates and are amplified by followers (Britannica; Scientific American).
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Established fact
Fading smell helps ants pick the short way.
Because scent fades, short trails stay strongest, so colonies often end up on the shortest route.
Evaporation leaves longer, thinly used trails weaker, so shorter routes win (Scientific American).
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An idea scientists discuss
Ants with no trails are pretend.
What ants would do without trails is an idea, built from what trails do today and from ants that already navigate alone.
Consequences of removing trail pheromones are inferred, not observed.
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Established fact
Some ants leave a smelly path to food.
Some ants lay scent along a trail to food so that other ants from their colony can find it.
Some ants lay scent pheromones along a trail to a food source so that colony members can find the food (Britannica).
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An idea scientists discuss
Without the path, ants might have to search alone.
Without trails, each ant might have to find food on its own.
Without trail pheromones, foraging would rely on individual search; how much that would cost a colony is speculation.
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Established fact
Each ant adds more smell to the path.
Nestmates follow a scout's trail and lay their own scent, making the trail stronger.
Nestmates follow the trail and lay their own pheromones, amplifying the markings along the path (Scientific American).
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Established fact
Ants have smells for danger too.
Ants and termites use several different scents to send different messages, including warnings of danger.
Social insects such as ants use several pheromones to coordinate colony activities; pheromones also signal danger (Britannica).
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An idea scientists discuss
A big pile of food might be missed.
Without trails, a big find might be collected slowly, because the scout couldn't lead a crowd to it.
Mass recruitment to large resources would be lost; the colony's harvest of big finds might fall. This is reasoned, not measured.
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Established fact
Old smell fades, so the short path stays strongest.
Scent slowly fades, so long trails where ants are spread out thinly smell weaker than short ones, and ants choose the stronger trail.
Because pheromones evaporate, longer trails carry lower concentrations and are abandoned in favor of shorter ones (Scientific American).
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Established fact
Ants solved a giant maze!
In a maze with 32,768 possible paths, about 83 percent of 12 Argentine ant colonies found one of the two shortest within an hour.
About 83 percent of colonies used at least one shortest path within an hour; after barriers, almost 86 percent established trails on the new shortest route (Scientific American).
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An idea scientists discuss
Without smell, ants might take the long way.
Without trails, a colony might keep using long, slow routes because nothing would make the short ones stand out.
Without reinforcement and decay, there would be no mechanism for route selection at the colony level; an inference from how trails work.
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Established fact
Computers use pretend ants to find short paths.
Some systems that route phone calls through busy networks have sent out virtual ants that leave virtual scent on the routes they travel.
Systems that route telephone calls while minimizing connection times were deploying virtual ants that deposit virtual pheromones by 2010 (Scientific American); ACO began with Dorigo's 1992 Ant System (ACO metaheuristic site).
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Established fact
Desert ants count their steps to get home.
Desert ants given tiny stilts took longer steps and walked past their goal, which showed they measure distance by counting steps.
Desert ants (Cataglyphis fortis) fitted with stilts took the right number of steps but overshot their goal, showing that stride counting acts as a pedometer (Live Science, on a 2006 study in Science).
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An idea scientists discuss
Maybe trail-less ants would count steps too.
If ants left no trails, more of them might have to find their way like desert ants do, using the sky and counting steps.
A trail-free world might favor individual navigation of the kind desert ants use, with celestial cues and step counting; whether other species could switch is speculation.
How a scent trail works
A scout ant finds food. On the way home she leaves a smelly path.
Other ants follow the smell to the food. They add more smell as they go.
When a scout ant finds food, she lays a trail of a special scent on her way back to the nest. These scents are called pheromones. Her nestmates follow the trail and add their own scent as they walk, so a busy trail grows stronger.
Ants use different pheromones for different messages. Some lead to food, and others warn of danger. The scent doesn't last forever, though: it slowly fades away.
Among social insects such as ants and termites, several different pheromones transmit the messages needed to coordinate a colony, including trail and alarm signals. Scouts deposit trail pheromone between food and nest; nestmates follow and lay their own, amplifying the marking.
This is positive feedback: the more ants use a path, the stronger and more attractive it becomes. Because pheromones gradually evaporate, a trail that stops being reinforced fades. Reinforcement and decay together let a colony's route change as conditions change.
Wipe the trails away
Now pretend there is no smelly path. A scout finds food, but no one can follow her.
Every ant might have to search alone. Lots of food might be missed.
Without trails, a scout who found a pile of crumbs would have no way to lead others there. Each ant might have to wander and search by itself, and a big find might be collected slowly or not at all.
The colony might lose its knack for choosing short paths, too. These are ideas, worked out from what trails do today. Some ants, though, already manage without trails, which gives us a real clue.
Remove trail pheromones and recruitment to a find would collapse; foraging would rely on individual search. The colony would also lose its distributed memory of good routes and its ability to converge on short paths through reinforcement and evaporation.
Those consequences are reasoned, not observed. Some ants navigate largely as individuals: a 2006 study in Science found that desert ants, which use cues in the sky to head home across barren land, also count their steps to judge distance. Trail-free life is possible, but it is a different strategy.
Why the short path wins
Smell fades away over time. On a long path, the ants are spread out.
So the long path smells weaker. More ants pick the short path!
Here's the clever part. Imagine two trails to the same food, one short and one long. On the long trail, the ants are spread out more thinly. Because scent fades, the long trail ends up smelling weaker than the short one.
So the short trail smells stronger, more ants choose it, and it gets stronger still. No single ant compares the routes. The whole colony finds the shortcut together.
Because pheromones gradually evaporate, longer trails, on which ants are spread more thinly, carry lower concentrations than short ones. Ants follow stronger pheromone, so short routes are reinforced and long ones abandoned.
In a test reported by Scientific American in 2010, Chris Reid, then at the University of Sydney, and colleagues turned the Towers of Hanoi puzzle into a maze with 32,768 possible paths and two shortest ones. Within an hour, about 83 percent of 12 Argentine ant colonies used at least one shortest path; when barriers blocked them, almost 86 percent established trails on the new shortest route.
Ants that inspire computers
Computers can copy ants! Pretend ants inside a computer leave pretend smell.
They help find short paths for phone calls.
Computer scientists borrowed the ants' trick. In a method called ant colony optimization, "virtual ants" explore possible routes inside a computer and leave virtual scent on each route they try. The best routes build up the strongest scent. Marco Dorigo described the first such system in 1992.
By 2010, some systems that route telephone calls through busy networks were already using virtual ants. Chris Reid, who studied how Argentine ants get around obstacles, said he hoped to give virtual ants something like a compass and a step counter, so they cope better when a route is blocked.
Ant Colony Optimization (ACO) uses artificial systems inspired by real colonies to solve discrete optimization problems. The first, Ant System, came from Marco Dorigo's 1992 PhD thesis and was applied to the traveling salesman problem; the ACO metaheuristic was defined in 1999 by Dorigo, Di Caro and Gambardella.
Scientific American reported in 2010 that virtual ants were already routing telephone calls through busy networks. Reid found Argentine ants blocked by a barrier search locally instead of retreating, and said he intended to give virtual ants "compasses and odometers" so they work around blockages in networks better. Ant research keeps feeding back into computing.
Amazing facts
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Ants solved a puzzle maze!
Argentine ants found the shortest routes through a maze built from a famous puzzle, the Towers of Hanoi.
The maze turned the possibilities and dead ends of the Towers of Hanoi puzzle into 32,768 possible paths, two of them shorter than all others (Scientific American).
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Scientists gave ants tiny stilts!
To test whether desert ants count steps, scientists glued tiny stilts to their legs. The ants took the right number of steps but marched past their goal.
The idea of an ant pedometer was first proposed in 1904 but stayed untested until the 2006 stilts study in Science (Live Science).
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Blocked ants look around nearby.
When Argentine ants hit a barrier, they don't just turn back. They search the area close by.
Blocked Argentine ants began a localized search rather than retracing their steps, which Reid attributes to internal "compasses and odometers" (Scientific American).
Words to know
- pheromone
- a chemical an animal gives off in tiny amounts to send a message to others of its kind
- scout
- an ant that goes out first to search for food
- colony
- a group of ants that live and work together
- evaporate
- to slowly turn from a liquid into a gas and drift away
- algorithm
- a set of step-by-step rules a computer follows to solve a problem
- shortest path
- the quickest route between two places
Discovery challenge
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What do some ants leave on the way to food?
What do some ants leave behind to show the way to food?
What do some ants deposit to recruit nestmates to food?
- A scent trail
- Footprints in mud
- Bits of leaf
Show the answer
A scent trail
A smelly trail.
A trail of scent called a pheromone.
Trail pheromone, reinforced by each follower.
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Why do ants pick the short path?
Why do colonies often end up on the shortest trail?
Why are shorter pheromone trails selected?
- Its scent stays strongest
- It is always downhill
- The queen chooses it
Show the answer
Its scent stays strongest
Its smell stays strongest.
Scent on long, thinly used trails fades, so short trails smell strongest.
Evaporation leaves thinly used long trails weaker, so reinforcement favors short ones.
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How do desert ants know how far they walked?
How do desert ants measure how far they have walked?
What did the stilts experiment suggest desert ants use to gauge distance?
- They count their steps
- They look at clouds
- They smell the sand
Show the answer
They count their steps
They count their steps.
They count steps: ants on stilts took longer steps and walked too far.
A step count: lengthened strides made ants overshoot their goal.
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What do pretend computer ants help with?
What do virtual ants help computers do?
What kind of problem does Ant Colony Optimization solve?
- Find short routes
- Draw pictures
- Predict the weather
Show the answer
Find short routes
Finding short paths.
Finding short routes, for example for phone calls in busy networks.
Discrete optimization problems such as routing and the traveling salesman problem.
Experiments to try
- Watch What If? Lab What if a forest grew back after a fire? In 1988, fires passed through about a third of Yellowstone National Park. Today, the young trees that sprouted afterward have grown into new forests. Follow the forest's comeback: the first plants, the pine cones that open with heat, and the animals that return.
- Watch What If? Lab What if honeybees could not dance? When a honeybee finds great flowers, she flies home and dances on the honeycomb. The dance tells other bees which way to fly and how far. Scientists have even tested hives where the dance can't point the way. Imagine bees that could not dance and follow what might change.
Explore next
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Watch
Animal
Leafcutter ant
Leafcutter ants are farmers. Workers cut pieces of leaves, carry them home along busy trails and use them to grow a fungus garden underground, and the fungus is their food. This page is about one well-known kind, Atta cephalotes, which lives in Central and South America in giant nests of hundreds of thousands to millions of ants.
- Watch What If? Lab What if a forest grew back after a fire? In 1988, fires passed through about a third of Yellowstone National Park. Today, the young trees that sprouted afterward have grown into new forests. Follow the forest's comeback: the first plants, the pine cones that open with heat, and the animals that return.
- Watch What If? Lab What if honeybees could not dance? When a honeybee finds great flowers, she flies home and dances on the honeycomb. The dance tells other bees which way to fly and how far. Scientists have even tested hives where the dance can't point the way. Imagine bees that could not dance and follow what might change.
- Watch Try it What If? Lab What if elephants talked with very low sounds? Elephants really do talk with very low rumbles, some too low for people to hear. Turn the pitch down in our sound model and see what changes about the wave, then find out how far real elephant calls can carry.
- Watch Try it What If? Lab What if fish had no swim bladder? Most bony fish carry a gas-filled pouch called a swim bladder. It makes the fish almost exactly as heavy for its size as the water around it, so it can stay at one depth without sinking or floating up. Use a floating-and-sinking model to see what happens when the gas is taken away.
- Watch What If? Lab What if there were no decomposers? Bacteria, fungi, earthworms and millipedes break down dead leaves, wood and dead plants and animals, and give their nutrients back to the soil. National Geographic calls them Earth's cleanup crew. Imagine a world without them, and follow what might pile up, what might run short, and who does the work.
- Watch What If? Lab What if we could understand animal sounds? Animals pass messages with sounds, smells, movements and touch. Scientists have already worked out what a honey bee's dance means, and they are listening hard to whales. Imagine understanding three animals, a bee, a whale and a dog, and find out how much we really know.
- Watch What If? Lab What if there were no fungi? Fungi are a whole kingdom of living things, with about 144,000 known species, from tiny yeasts to big mushrooms. They puff up our bread, ripen some cheeses, rot dead wood and team up with plant roots. Imagine a world with no fungi at all and follow what might change.
For parents & teachers
What children will learn
- Some ants lay scent trails called pheromones that nestmates follow and strengthen.
- Because scent fades, short trails stay strongest, so colonies often find the shortest route.
- Some desert ants find their way without trails, using the sky for direction and counting steps for distance.
- Computer scientists copy ant trails to solve routing problems.
Recommended ages: 5 to 17 · About 15 minutes
Talk about it
- How is an ant trail like a path worn across a grassy field?
- Why might it help that the scent fades away?
- What could people design by copying how ants work together?
Try at home or in class
- With a grown-up, put a crumb of bread on the ground outdoors near some ants. Check every few minutes from a respectful distance, without touching them. Does a line form? Does it take a straight or a winding route?
- Play the fading trail game: two routes of paper dots lead to a 'food' box, one short and one long. Walkers add a dot each trip, and every minute remove one dot from each route. Which route ends up with more dots?
Key vocabulary: pheromone, scout, colony, evaporate, algorithm, shortest path
Where these facts come from
- Pheromone , Encyclopaedia Britannica
- Ants Lead Way to Speedier Computer Networks , Scientific American
- About Ant Colony Optimization , Ant Colony Optimization (aco-metaheuristic.org)
- When Ants Go Marching, They Count Their Steps , Live Science
Checked, not yet human-reviewed. Last updated October 27, 2026. Spotted a mistake? Tell us.