There’s physics to having your ducklings in a row. By paddling in an orderly line behind their mother, baby ducks can take a ride on the waves in her wake. That saves the ducklings’ energy, researchers report in the December 10th issue of the Journal of Fluid Mechanics.

Earlier measurements of duckling metabolism showed that the youngsters saved energy when swimming behind a leader, but the physics behind that savings wasn’t known. Using computer simulations of waterfowl waves, Zhiming Yuan, an expert of the University of Strathclyde in Glasgow, Scotland, and his colleagues found that a duckling cruising in just the right spot behind its mother gets an assist.

When a duckling swims on its own, it kicks up waves in its wake, using up some energy that would otherwise send it moving ahead. That wave drag resists the duckling’s move. But ducklings in the sweet spot experience 158 percent less wave drag than when swimming alone, the researchers calculated, meaning the duckling gets a push instead.

Like good partners, the ducklings share with one another. Each duckling in the line passes along waves to those behind, so all the baby ducks get a free ride.

But to get the benefit, the youngsters need to keep up with their mom. If they fall out of position, swimming gets harder. That’s fair punishment for ducklings that dawdle.

Anyone who commutes（通勤）by car knows that traffic jams are an unavoidable part of life. But humans are not alone in facing potential traffic jam. Ants also commute一between their nest and sources of food. The survival of their colonies（群体） depends on doing this efficiently.

"The more they are, the more food they're going to bring back. But at the same time, they might end up with traffic jam because there are too many of them.” Sebastien Motsch, a mathematician in Arizona State University said. When humans commute, there's a point at which cars become dense enough to slow down the flow of traffic, causing congestion. Motsch and his colleagues wanted to know if ants on the move could also get blocked. So they control traffic density by constructing bridges of various widths between a colony of Argentine ants and a source of food. Then they waited and watched.

The flow of ants did increase initially as ants started to fill the bridge and then stabilize at high densities. But it never slowed down, even when the bridge was nearly filled with ants. The researchers then took a closer look at how the behavior of individual ants impacted traffic as a whole. That meant cautiously tracking thousands of separate ants as they made their way across the bridge.

Motsch and his team found that when ants sense overcrowding, they adjust their speeds and avoid entering high-density areas, which prevents congestion. These behaviors may be made by pheromones （外激素），chemicals that tell other ants where a trail is. The ants also manage to avoid colliding with each other at high densities, which could really slow them down.

Can ants help us solve our own traffic problems? Not likely, says Motsch. That's because when it comes to getting from point A to point B as fast as possible, human drivers put their own goals first. Individual ants have to be more cooperative in order to feed the colony. But the research could be useful in bettering traffic flow for self-driving cars, which can be designed to be less like selfish humans一and more like ants.

Monkeys seem to have a way with numbers

A team of researchers trained three Rhesus monkeys to associate 26 clearly different symbols consisting of numbers and selective letters with 0-25 drops of water or juice as a reward. The researchers then tested how the monkeys combined or added-the symbols to get the reward.

Here’s how Harvard Medical School scientist Margaret Livingstone, who led the team, described the experiment: In their cages the monkeys were provided with touch screens. On one part of the screen, a symbol would appear, and on the other side two symbols inside a circle were shown. For example, the number 7 would flash on one side of the screen and the other end would have 9 and 8. If the monkeys touched the left side of the screen, they would be rewarded with seven drops of water or juice; if they went for the circle, they would be rewarded with the sum of the numbers—17 in this example.

After running hundreds of tests, the researchers noted that the monkeys would go for the higher values more than half the time, indicating that they were performing a calculation, not just memorizing the value of each combination.

When the team examined the results of the experiment more closely, they noticed that the monkeys tended to underestimate a sum compared with a single symbol when the two were close in value—sometimes choosing, for example, a 13 over the sum of 8 and 6. The underestimation was systematic: When adding two numbers, the monkeys always paid attention to the larger of the two, and then added only a fraction of the smaller number to it.

“This indicates that there is a certain way quantity which is represented in their brains,” Dr. Livingstone says. “But in this experiment what they’re doing is paying more attention to the big number than the little one.”