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Friday, November 30, 2012

Selfish huddling for the common good

Emperor penguins (Aptenodytes forsteri) have perfected the art of the huddle. It’s this tight bunching of up to ten of these large birds per square meter that lets them survive the harsh Antarctic winter storms. It’s not hard to see how the birds in the center benefit. Not only is the temperature at least twenty degrees warmer in the center, but even more importantly, birds located in that region are not exposed to the grueling winds. But what about the unfortunate birds on the periphery? Do they get a turn on the inside?

You’ll be happy to hear that penguins are surprisingly egalitarian. All members get an equal chance to enjoy the shelter of the huddle’s center. What may surprise you is how the penguins achieve this fairness. According to Aaron Water, Fran├žois Blanchette and Arnold Kim from the University of California, Merced, the flow of penguins within a huddle is entirely self-centered and can be predicted mathematically.

To be clear, the authors did not observe actual penguin huddles. Their data is based on computer simulations. You can see an example below:


Temperature distribution around a huddle of 100 penguins. Here red and blue correspond to warmer and cooler temperatures, respectively. Individual penguins are shown in black, as is the boundary of the huddle, while the polygonal interior of the huddle is shown in white.

The scientists began with the assumption that each penguin’s movements are based solely on whether such movement would benefit it personally. They then threw different wind and temperature conditions at their huddle model. The ‘bird’ with the greatest virtual heat loss (which would correspond to the bird in the most exposed position of the huddle) is relocated to a new position along the edge of the huddle. This is then repeated with the next most vulnerable penguin, which eventually causes the whole group to shift. Birds on the windward side end up on the leeward side, and birds that had been in the center are gradually edged out to the windward side.

You can see a real Emperor penguin huddle in the timelapse video below.

Aaron Waters, Fran├žois Blanchette, & Arnold D. Kim (2012). Modeling Huddling Penguins PloS ONE : doi:10.1371/journal.pone.0050277