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Doing the Penguin Huddle

January 20, 2014

Penguins have always been popular cartoon characters. In my generation, it was Chilly Willy, who joined Woody Woodpecker in the Walter Lantz anthropomorphic cartoon character repertoire. Chilly lived in Alaska, as it was a common misconception that penguins lived at the nearby North Pole.

Today's children are more sophisticated, and Chilly Willy would play well only with the pre-kindergarten crowd. That's why cartoon penguins have morphed into things like The Penguins of Madagascar. I rarely use the word, "sophisticated," since it has traditionally not meant what people think. The Greek sophists were philosophers who won their arguments using logical ploys, so "sophisticated" had a derogatory sense through the later 19th century.

When I did a post-doc in Pittsburgh in the mid-1970s, their hockey team, the Pittsburgh Penguins, was going through some rough times. Recently, they won the 2009 Stanley Cup against the Detroit Red Wings after losing to them the year before. Now, my favorite penguin is Tux, the Linux mascot (see figure).

Tux, the penguin mascot for Linux.

The idea of a penguin mascot for Linux came from its originator, Linus Torvalds. Its first rendition was by Larry Ewing, who created Tux with the popular (and free) image editing program, GIMP.

Tux supposedly has the satisfied look of a well-fed penguin.

(Image by Larry Ewing, Simon Budig and Anja Gerwinski, via Wikimedia Commons.)

It's amazing that penguins can survive in the extreme cold of Antarctica. A scientific tabulation of the low temperatures of that continent have recently been released by scientists at NASA and the U.S. Geological Survey.[1] Scientists have used Earth-monitoring satellites, including Landsat 8, to learn how cold Antarctica really is. In measurements between 2003-2013, they found that one region along a remote ice plateau had a record low temperature of about -93°C.[1]

The recorded low temperature of -93.2°C occurred on August 10, 2010. This was just slightly lower than the previously recorded low, at Vostok Station in Princess Elizabeth Land, Antarctica, of −89.2°C on July 21, 1983.[1] A world away, in Greenland, it's only gotten as cold as -75°C. Alaska, the home of Chilly Willy, has only gotten as cold as -62°C.[1] New Jersey, where I'm writing this, has had a temperature as low as -37°C, but that record was set more than a hundred years ago.

Four Emperor Penguin chicks (Aptenodytes forsteri), part of a National Science Foundation study, in San Diego, 1988. These penguins were native to McMurdo Station, Ross Island, Antarctica.

(Via Wikimedia Commons.)

The idea of huddling to keep warm has been a plot point of many television sitcoms. It makes sense from some "spherical cow" physics and the fundamental geometrical principle that an agglomerated mass will present a smaller area to the outside world than a set of its component masses if separate. Heat is lost at the interface between a body and its environment, and the smaller that area can be made to be, the less heat that will be lost.

Although penguins are not physicists, they've grasped that principle, and they huddle in extreme weather to keep warm. The huddle pattern is a triangular lattice. Instead of standing in place, individual penguins will move every 30-60 seconds, and their movement causes adjacent penguins to move, also. Physicists, who are accustomed to analyze the motions of interacting particles, have decided to use this approach to analyze the motion of penguins in a huddle.[2-3]

Scientists from the University of Erlangen-Nuremberg (Germany), the University of Greifswald (Greifswald, Germany), the Université de Strasbourg (Strasbourg, France), CNRS (Strasbourg, France), and the Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung (Bremerhaven, Germany) have published their findings in an open access paper in the New Journal of Physics.[2]

They were able to create a mathematical model of penguin motion in a huddle. The model revealed that a motion as small as two centimeters by a penguin will cause its neighbors to move also to maintain close contact. The motion is much like the stop-go movement of cars in a traffic jam, and the mathematical model had been used to study traffic jams.[3] Such motion propagates like a wave throughout the huddle to keep it as dense as possible. This wave motion also aids in merging smaller huddles into larger ones, and it serves to remove lattice defects.[2-3]

Doing the penguin shuffle.

A kymograph showing the merger of two penguin huddles.

The merger proceeds from top to bottom as the gap is closed, and the wave movement of the leftmost huddle then propagates to the rightmost huddle.

An animation of this is available at the online paper.[2]

(Fig. 5 of Ref. 2, licensed under the Creative Commons Attribution 3.0 license)

The two centimeter threshold distance is just twice the compression thickness of a penguin's feather layer, so they penguins pack closely without touching. This is important, since compressing the feather layer lowers its thermal insulation.[3] The wave motions in a huddle can originate at any penguin, and it can propagate in any direction, depending on where the gap arises. Says study coauthor, Daniel Zitterbart of the Alfred Wegener Institute,

"We were really surprised that a traveling wave can be triggered by any penguin in a huddle, rather than penguins on the outside trying to push in... We also found it amazing how two waves, if triggered shortly after each other, merged instead of passing one another, making sure the huddle remains compact."[3]

The emperor penguin is the only vertebrate species that breeds in the -50°C temperatures and 200 km/h winds of the Antarctic winter at their particular nesting areas.[2-3]

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