Flocking together

One of the most mesmerizing sights in nature is the collective motion of large groups of animals.

I remember watching films by Jacques Cousteau as a kid, and being fascinated by his underwater footage of schools of fish swimming along and then turning as one, the light flickering from their silvery sides as if they were each reflective scales on a giant single organism.  Murmurations of starlings barely even look real; the flocks swirl and flow like some kind of weird, airborne fluid.  But the most astonishing example of collective motion I've ever seen was when Carol and I visited Bosque del Apache Wildlife Refuge, in central New Mexico, a few years ago, during the migration of snow geese through the region.

"Get there early," we were told.  "At least a half-hour before sunrise.  You'll be glad you did."

We arrived just as the light was growing in the eastern sky.  The wetland was full of tens of thousands of snow geese, all moving around in a relaxed sort of fashion, calling softly to each other.  The brightness in the sky grew, and then -- without any warning at all...

... BOOM.

They all exploded into the air, seemingly simultaneously.  We have wondered many times since what the signal was; there was nothing we could discern, no handful of birds that launched first, no change in the vocalizations that a human would interpret as, "Now!"  One moment everything was calm; the next, the air was a hurricane of flapping wings.  They whirled around, circling higher and higher, and within ten minutes they were all gone, coursing through the sky toward their next destination.

[Image licensed under the Creative Commons CrunchySkies, Snow goose migration at Squaw Creek National Wildlife Refuge, Feb 2013, 25, CC BY-SA 3.0]

How animals manage such feats, moving as a unit without colliding or leaving members behind -- and seemingly without any central coordination -- has long fascinated zoologists.  Way back in 1987, computer simulation expert Craig Reynolds showed (using software called "Boids") that with only a handful of simple rules -- stay within so many wing-lengths of your nearest neighbors but not close enough to touch, match the speed of your neighbors within ten percent either way, steer toward the average heading of your nearest neighbors, give other members a chance to be in any given position in the group -- he was able to create simulated flocking behavior that looked absolutely convincing.  

Last week, a paper out of the Max Planck Gesellschaft showed there's another factor that's important in modeling collective motion, and this has to do with the fact that flying or swimming animals have a rhythm.  Look, for example, at a single fish swimming in an aquarium; its motion forward isn't like a car moving at a steady speed down a highway, but an oscillating swim-glide-swim-glide, giving it a pattern a little like a Slinky moving down a staircase.

Biologist Guy Amichay, who led the research, found that this gives schools of fish a pulse; he compares it to the way we alternate moving our legs while walking.  "Fish are coordinating the timing of their movements with that of their neighbor, and vice versa," Amichay said.  "This two-way rhythmic coupling is an important, but overlooked, force that binds animals in motion.  There's more rhythm to animal movement than you might expect.  In the real world most fish don't swim at fixed speeds, they oscillate."

The key in simulating this behavior is that unlike the factors that Reynolds identified, getting the oscillating movement right depends on neighboring fish doing the opposite of what their nearest neighbors are doing.  The swim-glide pattern in one fish triggers a glide-swim pattern in its friends; put another way, each swim pulse creates a delay in the swim pulse of the school members around it.  

"It's fascinating to see that reciprocity is driving this turn-taking behavior in swimming fish, because it's not always the case in biological oscillators," said study co-author Máté Nagy.  "Fireflies, for example, will synchronize even in one-way interactions.  But for humans, reciprocity comes into play in almost anything we do in pairs, be it dance, or sport, or conversation,"

"We used to think that in a busy group, a fish could be influenced by any other member that it can see," said co-author Iain Couzin. "Now, we see that the most salient bonds could be between partners that choose to rhythmically synchronize."

So zoologists have taken another step toward comprehending one of the most fascinating phenomena in nature; the ability of animals to move together.  Something to think about next time you see a school of fish or a flock of birds in flight.  Getting it right requires rapid and sophisticated coordination we are only now beginning to understand.

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