Ants That Count!
Can ants count? Not out loud they can't. Not the way you and I count. But an ingenious experiment conducted in the Sahara suggests maybe ants do count.
Harald Wolf of the University of Ulm and his assistant Matthias Whittlinger proposed that ants have "pedometer-like" cells in their brains that count the steps they take.
How Do Ants Get Home?
Most ants get around by leaving smell trails on the forest floor that show other ants how to get home or to food. They squeeze the glands that cover their bodies; those glands release a scent, and the scents in combination create trails the other ants can follow.
That works in the forest, but it doesn't work in a desert. Deserts are sandy and when the wind blows, smells scatter.
So how do desert ants find their way home?
It's already known that ants use celestial clues to establish the general direction home, but how do they know exactly the number of steps to take that will lead them right to the entrance of their nest?
Wolf and Whittlinger trained a bunch of ants to walk across a patch of desert to some food. When the ants began eating, the scientists trapped them and divided them into three groups. They left the first group alone. With the second group, they used superglue to attach pre-cut pig bristles to each of their six legs, essentially putting them on stilts.
The third group had their legs cut off just below the "knees," making each of their six legs shorter.
After the meal and the makeover, the ants were released and all of them headed home to the nest while the scientists watched to see what would happen.
The "Pedometer Effect"
The regular ants walked right to the nest and went inside.
The ants on stilts walked right past the nest, stopped and looked around for their home.
The ants on stumps fell short of the nest, stopped and seemed to be searching for their home.
It turns out that all the ants had walked the same number of steps, but because their gaits had been changed (the stilty ants, like Monty Python creatures, walked with giant steps; the stumpy ants walked in baby steps) they went exactly the distances you'd predict if their brains counted the number of steps out to the food and then reversed direction and counted the same number of steps back. In other words, all the ants counted the same number of steps back!
Does that mean ants have something like pedometers that do something like counting?
Says professor James Gould of Princeton, commenting on the experiment: "These animals are fooled exactly the way you'd expect if they were counting steps."
Gould says it's pretty clear ants don't have maps in their heads and don't recognize markers along the route. This experiment strongly suggests that ants do have internal pedometers that allow them to "count" their way home.
Special thanks to OddTodd, our animator, and to comedian Jessi Klein, who provided ant voices in our video.
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STEVE INSKEEP, host:
It's MORNING EDITION from NPR News. I'm Steve Inskeep.
Okay, our next report is going to begin in ten, nine, eight - just a few more seconds. And before we start, our science correspondent Robert Krulwich wants to remind you that the only creatures that we know of who can count down -seven, six, five - are us; people - or so we thought.
ROBERT KRULWICH: Suppose you're an ant and you want to show another ant how to get to some food or how to get home. What you do is you squeeze some glands on your body. And those glands produce a chemical odor and the other ants sniff and know that if they follow the trial of that smell, it'll led to food or it will led home. But the thing is there are places on earth where this system simply doesn't work.
(Soundbite of wind blowing)
In the Sahara Desert, for example, if you're an ant, the sandy surface is constantly being blown around by the wind. So if you put down a chemical scent�
Professor JIM GOULD (Biology, Princeton University): It would not work in the desert.
KRULWICH: Says Princeton biologist Jim Gould. Smells just blow away. And yet even though an ant in the Sahara can't leave a smell trail, none.
Prof. GOULD: It gets back home just fine.
KRULWICH: But how? Well, scientists know that ants use celestial clues - the position of the sun, for example - to find the direction home. But how do they know the exact distance - the exact distance back to the nest? Well, Harold Wolf, at the University of Ulm in Germany, proposed, well, maybe ants count. Not out loud, of course.
Professor HAROLD WOLF (University of Ulm): They certainly don't do that.
KRULWICH: No. But maybe they have little pedometers in their brains. So, when they march across the desert to get food certain cells in their brains record every step they take - every step until they reach their food. And then when they're done eating and it's time to turn around and go home, the pedometer in their brain simply reverses and counts them down all the way until they get�
Unidentified Group: Home. Yay.
KRULWICH: �home. They call this the pedometer theory. Now, to find out if there are actually pedometers inside ants' brains, scientists came up with an ingenious experiment that should let us see - and I mean literally see - those pedometers at work if they exist. So they led a bunch of ants across a bit of Sahara desert to some food. And while the ants were eating, the scientists trapped them into three separate groups. Now, for the first group, they took out an adhesive.
Prof. GOULD: They're using superglue.
KRULWICH: Yeah, he said superglue. Here are the instructions.
Prof. GOULD: Glue a little pig bristle on to make them twice as tall so their steps take them twice as far.
KRULWICH: Meaning they attach stiff pig hairs, or pig bristles, to each and every leg of each ant, turning this first group of ants into kind of Monty Python sort of critters, with ridiculously large steps. By the way, what lucky person gets to put six pig bristles on six legs?
Prof. GOULD: Some Swiss graduate student.
KRULWICH: Ok. So that's group number one. For the second group, what do you do?
Prof. GOULD: You trim their legs.
KRULWICH: You what?
Prof. GOULD: Just cut them off at the knees. It's very quick. And you'd be surprised how amenable insects are to having things like this done to them.
KRULWICH: Oh, come on. Like you want to be snipped. The point is, with their legs suddenly smaller every step the ants take becomes shorter.
Prof. GOULD: They're taking little baby steps, maybe only half as far.
KRULWICH: And the third group you just kind of leave alone?
Prof. GOULD: You leave them alone.
KRULWICH: Ok. Now, comes the fun part. When everybody was ready and it was time for the ants to go home they were released and, says Professor Gould�
Prof. GOULD: These animals are ruled(ph) exactly the way you would expect it. They were counting steps.
KRULWICH: The stilty ants, they took big steps home and they stopped well past the nest. The stumpy ants took little itty-bitty steps home and they ended up shy of the nest. And the regular ants took regular steps home and they ended up right at the nest. But they all took the same number of steps.
So can we say that all these ants, they all counted?
Prof. GOULD: Yes.
KRULWICH: Wait a second. Before you say yes, have we eliminated all the other possibilities?
Prof. GOULD: As far as I'm concerned all the other possibilities are off the table.
KRULWICH: Professor Gould says it's pretty clear ants don't have maps in their heads, don't memorize markers along the route. In fact, the best evidence that ants do have pedometers is, after their walk, the scientists took all three groups of ants and pushed them into the nest where they were going to spend the night. And the next morning, hungry again, all three sets of ants with their new legs marched off to have a meal.
Now, remember the pedometer formula. The steps you take out should equal the number of steps you take back. So the ants ate, and this time when they turned around to go home�
Prof. GOULD: It's a matter of counting steps. And if you are taking giant steps, well, that's fine. It's now ten giant steps to the food source and ten back. And if you're taking baby steps, maybe it's 40 to the food source and 40 back. But the point is, because you're counting the steps, you know exactly how far to come back.
KRULWICH: So all three sets of ants got home perfectly. And the only explanation, really, is that their pedometers adjusted to their new leg sizes. Or in a pedometer-like way, ants do count.
Dr. GOULD: That's right.
INSKEEP: Another report from the mind and the explorations of NPR's Robert Krulwich. And Robert, I understand we can see this experiment close up and full-color.
KRULWICH: Yep, we have recreated it, the whole thing, in cartoon form, but we should warn you, for the fun of it, we've made it a very scary cartoon.
Unidentified Man: Impossible? We thought so.
KRULWICH: A very scary cartoon.
Unidentified Man: Unimaginable? We thought that too.
KRULWICH: We call it...
Unidentified Group: Pedometers in the brain.
KRULWICH: And you can see it all, the marching ants, the stilts, everything, just go to npr.org. Transcript provided by NPR, Copyright National Public Radio.
