Photo: iStock
Photo: iStock

Heavy lifting in the ant world

Researchers in Israel found that ants can collectively carry a large load with a high degree of coordination and focus

All three of you long-time readers of this column will have noted that I have a liking for ants. Or at any rate, for how scientists have used ants to further the boundaries of human knowledge. Thus my earlier columns about chopping off ant legs, about left-leaning ants, about what ants do with pheromones.

I like to think I’m not the only one who finds these little creatures fascinating, especially when they tell us things about the ways of the world. Though there is something brutal about chopping off their legs, I’ll admit. Still, today I want to tell you about what researchers in Israel found when they induced some ants to carry home, among other things, a large, heavy, doughnut-shaped object.

Well, “large" and “heavy" for the ants. This was a single Cheerio, taken from a box of the famous cereal brand. Might seem trivial to you, but the scientists estimated that the little ring is about 350 times heavier than an individual ant. Here’s something to put that number in perspective. The typical car on the road weighs about 1,500kg. That’s heavy, but only about 20 times as heavy as the average human is. So if you want to know about ants carrying home a single Cheerio, think about carrying home, on your back, 17 cars.

And yet I’m sure you have seen ants lift and move a piece of food. How do they do it? Even you and a couple of dozen friends would find it impossible to lift 17 cars and transport them long distances (how long? I’ll come to that). How do ants do it?

Ah, but that’s what this Israeli team set out to answer. Their paper (Ant Groups Optimally Amplify The Effect Of Transiently Informed Individuals by Aviram Gelblum and colleagues, Nature Communications, 28 July 2015) is a tour-de-force of close observation and analysis, but is also filled with insightful, often charming, nuggets of information. In what they teach us, ants can be like that.

The paper starts with this remark: “Collectively carrying a large load requires a high degree of coordination and is rare outside humans and ants." Certainly you can imagine that it would be hard to lift one car, let alone 17, if you and your fellow-lifters don’t work together. But ants seem to instinctively know how to cooperate. They surround the Cheerio and within seconds, it will be moving steadily to where they want to take it (the paper in Nature links to two videos showing this).

Gelblum’s team worked with an ant species called Paratrechina longicornis. In their experiments, they placed a Cheerio up to 7.5m from the ants’ nest—about 2,500 times the length of a P. longicornis ant. Put that in perspective again: 2,500 times the length of a tall human is 5km. Try to think of you and a couple of dozen (tall) pals carrying 17 cars from Churchgate to Bombay Central.

Once they come upon the Cheerio, up to 15 ants attach themselves all around it. The more the ants, of course, the faster they can move the food. But how the ants are distributed around it has little bearing on their speed. Several ants, as you can imagine, are at the “front" of the morsel, meaning they must walk backwards towards the nest. The scientists realized that ant anatomy allows only for lifting or pulling, but not for pushing. The analysis showed that it’s the front runners who actually pull the load along, even as they move backward. The ones at the “back" help largely by lifting.

All fine, but you have got to move the load to where you want it—in this case, in the direction of your nest. If you have ever watched ants transport food like this, you will have also noticed other ants who are constantly on the move around their hard-working mates, sometimes even joining the load-bearing detail. Over time, the Israeli scientists noticed a method to the apparent random motion of these roaming ants. The ones carrying the Cheerio essentially don’t know where they are going—because their antennae are blocked by the load itself. So they need other ants to inform them. That’s the role the roamers play, by monitoring the surroundings and constantly bringing new information about it to the load-bearers. And they do that not by somehow whispering to every ant holding the Cheerio: “Hey, gotta bear left now!" Instead, they join in the task and lead from the front.

So when one of these roamers attaches herself to the Cheerio, she “tends to change the load’s velocity towards her own pulling direction". It’s as if she has noticed that the load is veering off course, and joins in specifically to make the required correction. But after about 20 seconds, her influence on the direction of motion wanes—after all, the load blocks her antennae too. So, in fact, she then “ends up at the trailing edge of the load from where she cannot guide the motion". The Cheerio moves on, waiting for another ant to direct it.

In other words, the paper says, “all useful steering is provided by newly attached ants". Seems like a possibly useful prescription for life itself.

There’s plenty more to chew on in the Gelblum paper, including plenty of detailed mathematics. But importantly, the researchers also found that the ants had no interest in the Cheerio unless it was “previously incubated with cat food". So the Cheerios were stored overnight in a bag of the stuff. The scientists tell us helpfully that they used “either Royal Canin or Happy Cat brands".

Furthering the boundaries of knowledge, always.

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

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