Follow the Leader?
A school of jackfish. Photo © Linda Cline at Dancing-Fish.com
DB: This is Earth and Sky. When visited the Museum of Science in Boston, we got this question.
Dave Goldberg: My name is Dave, and I’m from Boston, Massachusetts and my question is, when birds and fish organize into flocks and schools, do they have a leader to follow or do they act instinctively?
JB: Dave, it appears likely that birds in flocks and fish in schools don’t follow a leader. Instead, they follow their neighbors.
DB: The complex behavior of flocking and schooling seems to emerge when individual birds and fish follow the simple rules of alignment, cohesion, separation, and avoidance. In other words, each creature appears to steer toward the average direction and position of its neighbors.
JB: Flocks and schools help protect their members. There is indeed safety in numbers. The changing shapes and colors in a school of fish confuse predators. Many bird predators have to aim at a specific target, so a hawk is likely to wind up empty-clawed if it swoops into a dense flock. And flocks and schools also have more “eyes” to look out for food and predators. Since flocking and schooling are so important for survival, it makes sense they’d depend on collective instincts – rather than on a single leader who might get eaten or go astray.
DB: Dave, thanks for your question. And special thanks to the Bureau of Land Management and to the National Fish and Wildlife Foundation. We’re Block and Byrd for Earth and Sky.
If you enjoyed this program, you may be interested in the following:
Physics of Flocks (American Institute of Physics)
The Boids (Computer simulation of flocking)
Author’s notes:
Craig Reynolds adds: Worth noting is that herds of land animals (cattle, caribou, gazelles, …) appear to follow the same rules (as birds and fish) except they are restricted to the 2d surface of the earth. Very similar rules apply to people in crowds or cars in traffic.
Dr. John Toner adds: Somewhat amazingly, this purely “local” following can, apparently, lead to an arbitrarily large flock all moving coherently (i.e., in the same direction at the same speed), even if all of the critters are making mistakes (i.e., not following their neighbors perfectly).
This works even in 2 dimensions (e.g., a herd of one million wildebeest moving together across the Serengeti plane), despite a well-known (to physicists) theorem from statistical mechanics called the “Mermin-Wagner theorem (after the guys who proved it), which states that in 2 (or fewer) dimensions, if all the wildebeest stood still and tried to POINT in the same direction as their neighbors (rather than MOVE in the same direction), they could not, if they made errors.
This is empirically NOT true of flocks, and a physicist named Tamas Vicsek showed in computer simulations about 10 years ago that even if the creatures only followed their neighbors, and even if they did move in 2 dimensions, the flock could still fly coherently. Apparently, motion is different from pointing.
Later, a theory was developed by Yu-hai Tu of IBM Research and me which explained this. Basically, we showed using a “hydrodynamic” theory of flocking (e.g., one in which the motion of the flock was treated the way we treat a flowing fluid (though with important differences reflecting the fact that the critters are self-propelled) ) that the Mermin-Wagner theorem did not apply when the critters were moving, rather than pointing. We also found that such a moving flock is infinitely more viscous (i.e., stickier) than any fluid, a result, along with many others, that we confirmed with further computer simulations.
The following people were interviewed for today’s program. Our thanks to:
Dr. Eric Schultz
Department of Ecology and Evolutionary Biology
University of Connecticut
Storrs, CT
Craig Reynolds
Research and Development Group
Sony Computer Entertainment America
Foster City, CA
Dr. David Winkler
Department of Ecology and Evolutionary Biology
Cornell University
Ithaca, NY
Dr. Amy McCune
Department of Ecology and Evolutionary Biology
Cornell University
Ithaca, NY
Dr. Lee Fuiman
Marine Science Institute
University of Texas
Port Aransas, TX
l
Dr. Julia Parrish
School of Marine Affairs
University of Washington
Seattle, WA
Dr. John Toner
Materials Science Institute
University of Oregon
Eugene, OR
Additional Teacher Resources
Craig Reynolds, Boids (Flocks, Herds, and Schools: A Distributed Behavioral Model.): Boids
This site consists of a computer model of coordinated animal motion such as bird flocks and fish schools, as well as a wealth of in-depth information into a variety of facets concerning the social and physical science behind flocks, herds and schools. This site is geared more to older students.
The National Geographic Society, National Geographic News: Unlocking How Flocks Stop, Turn, and Swirl in Unison
It’s one of the wonders of the natural world?to see a flock of starlings pulse, wheel, and ripple as one across an evening sky. Just how do they perform these displays with such precision? This article explores the mystery behind how birds coordinate movement in flocks.
Harvard University, Harvard University Gazette: Scientists Show How Fish Save Energy By Swimming In Schools
This article explains how, using a tank designed to mimic a turbulent waterway, scientists have found that fish employ a unique and clever swimming motion to harness the energy of eddies in flowing water. By essentially hitching a ride and letting these vortices propel them along, the scientists say, fish can swim against a current with considerably less exertion that is required in calmer settings.
Boids (Flocks, Herds, and Schools: Distributed Behavioral Model: Steve’s Aquarium; A Little Boids Applet
This site provides a working model of a school of fish avoding a predator. This site is a great interactive visual aid for students and a great introduction to the science behind the question “why do fish swim in schools?”
