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Business News/ Opinion / The ants go marching
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The ants go marching

As far back as 1904, biologists had suggested that ants have some kind of internal pedometer that allows them to gauge distance

Photo: ThinkStock Premium
Photo: ThinkStock

There’s a certain cruelty in some experimental findings I first heard about some years ago. Be warned: this column will mention it. To prepare you, let me remind you of an experiment I wrote about in my column Dead Ant! Dead Ant!

That experiment was the idea of the great biologist E.O. Wilson. He suspected that when an ant dies, its body takes a day or two to release a particular chemical, called a pheromone. Only when that happens will the other ants pick up the body—even though it’s been lying there motionless for two days—and fling it out of their colony.

To confirm this, Wilson isolated this pheromone and smeared a smidgeon of it on a living ant in the colony. Promptly, the ant’s colleagues picked the little fellow up—no matter that he struggled and kicked—and flung him out. Probably befuddled by this unkind treatment, he came back in, only to be thrown out again. It may not have occurred to him that he had just proved Wilson’s theory: the pheromone, and not his protestations, drove his friends’ actions. Because pheromones control certain ant behaviours.

But ants behave in so many intriguing ways that there are plenty of opportunities to investigate theories about them. For instance, they seem to have a pretty good idea of distances. They’ll go out foraging for food, wandering here and there. When they find something tasty, they’ll pick it up and march straight back—no wandering—to their colonies. How?

There are two aspects of that homeward march that ants must work out: the direction, and the distance. How they determine direction is by now pretty well-understood: we believe they are able, like bees, to use the sun’s position in the sky to orient themselves. But the distance? One idea was that they estimated it by how much energy they used up in their outward journey. Use that same amount in the reverse direction and you’re home. Except that on the return journey, the ant might be ferrying a large piece of food, thus expending more energy on his travels—meaning you’d expect him to stop some distance short of the colony in the belief he had reached it. But no: ants actually make it back unerringly. Really, how?

As far back as 1904, biologists had suggested that ants have some kind of internal pedometer that allows them to gauge distance. Imagine that you know your stride is 1m long, and it takes you 150 strides to reach the closest place to stock up on vodka. So if you crave vodka on some rainy night, you know you have to go about 150m. Thus, better take an umbrella.

But how can we verify this with ants?

Three German biologists, Matthias Wittlinger, Rüdiger Wehner and Harald Wolf, set out to do just this in 2004. They worked with ants that live in the Tunisian desert, a species called Cataglyphis fortis, who are pretty good at finding their way home, even in darkness. The biologists let the ants run about 10m from their colonies to a source of food (their paper actually calls this “booty", which I thought was a nice touch). Then, to understand if and how these ants relied on their legs to make distance calculations, they manipulated them in two ways before sending them homewards.

With some ants, they pasted bristles from a pig to their legs, thus increasing their length. How I’d have liked to watch as they actually attached these makeshift stilts to each of six legs of a struggling ant! But never mind that. The ants-on-stilts still moved their legs just fine, but because their legs were suddenly longer, they were taking giant strides instead of normal scurrying steps.

With some other ants, the scientists shortened their legs. The cruel part, remember. Using a pair of scissors, they snipped off the lower sections of the ants’ legs, so that they now had to wander about on stumps. These poor fellows also had no problems moving their legs as they headed home. But they were now reduced to mincing along, besides perhaps being in some residual pain too.

In both cases, the ants travelled a particular number of steps—what they remembered from the outbound journey—and began “pacing back and forth around the assumed position of the nest entrance". Unfortunately for them, this assumed position was wrong: about 5m beyond home for the stilted ants, about 4m short of home for the stunted ones.

This evident confusion was proof of the pedometer hypothesis. And it was further confirmed the next day, when these ants did both their food and home runs on their modified legs. No confusion this time, because the ants were, in effect, “recalibrated" to their new legs.

Thus they were able to calculate distance and make their way home just as accurately as any unmutilated brethren would.

Ingenious experiments, both. Yet I wonder if the furthering of scientific knowledge is consolation enough for an ant suddenly lamed.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers will explore the joy of mathematics, with occasional forays into other sciences.

Comments are welcome at dilip@livemint.com. To read Dilip D’Souza’s previous columns, go to www.livemint.com/dilipdsouza

Follow Mint Opinion on Twitter at https://twitter.com/Mint_Opinion

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Published: 31 Jul 2014, 07:24 PM IST
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