Monday, June 13, 2011

The Penguin Wave

If you like the 2005 movie March of the Penguins then you will like today's post. The movie, and today's post, is about Emperor penguins.

Emperor penguins (Aptenodytes forsteri) are the only vertebrates that breed during the austral winter, in Antarctica. At the approach of winter these penguins journey in large numbers, single file, from the sea inland to their breeding grounds. There they find mates through courtship song and dance. Once they have paired off into monogamous couples they mate and the females produce a single egg. The females transfer the egg to the tops of the males feet where it can be kept warm and off of the ice. Exhausted and having gone weeks without nourishment the females return to the sea leaving the males to guard and hatch the egg. The males must survive the brutal Antarctic winter by huddling in large groups to keep warm and protect the eggs.

To say the males survive some pretty harsh conditions would be an understatement. We are talking temperatures of −45°C (−49°F) and winds of 50 meters per second (111 mph)! All during this time (110-120 days) the males are fasting, waiting for the females to return, and trying to stay warm. This is when they huddle. The penguins aggregate in huddles where ambient temperatures are above 0°C (32°F) and can reach as high as 37°C (98.6°F). But huddling poses a couple of problems. The penguins on the outside are colder than the penguins on the inside. Some some kind of rearrangement is necessary. But, if the huddle density is too high an internal rearrangement becomes impossible, preventing the penguins on the outside from getting to the warmth on the inside. If the huddle density is too low the penguins won't stay warm enough.

A new paper in the journal PLoS ONE looks at the coordinated movements of an Emperor penguin huddle. To study this they first picked a medium sized penguin colony consisting of approximately 2000 animals. Then, from an elevated, high, distant position they set up a camera that took high resolution time lapse images every 1.3 seconds for 4 hours. During that time they recorded environmental conditions such as air temperature and wind speed. Once they had the images they analyzed them in the lab, detecting and tracking penguin positions.

They found that Emperor penguins have a surprising strategy to prevent jamming while still remaining in a densely packed (and so warm) configuration. As the sun sets and the temperatures drop from −33 to −43°C (-27 to -45°F) the penguins aggregate into multiple huddles. These multiple huddles are tightly packed and remain relatively motionless, with the penguins within a single huddle all facing the same direction. The jammed state of the huddle is interrupted every 30 to 60 seconds by small 5-10 cm coordinated steps of the penguins. Sort of like doing the wave at a sports event if each time you sat down you sat in the seat next to you. It slowly moves the crowd in a single fluid direction and rotates every bird through the warmest parts of the huddle.

The authors propose that these small, regular steps serve a three-fold purpose. First, they help the penguins pack together in the highest density. Second, the small steps lead to a forward motion of the entire huddle. This allows smaller huddles to merge into larger huddles. Third, the steps lead over time to a slow large scale huddle reorganization. The huddle is reorganized but individual penguins do not change positions relative to their neighbors and do not force their way out of a huddle. It is unclear whether the the penguin wave is started by a single individual or a few leading penguins and follows a well-defined hierarchy among group members. But similar wave patterns have been seen in other grouping/flocking animals such as pigeons, fish, and locusts.

Check out a video of the time lapsed penguin huddle. And because the article is published in PLoS ONE (linked at the bottom of the post) it is free to everyone, and you can access additional movies at the bottom of the article in the Supporting Information section.

Daniel P. Zitterbart, Barbara Wienecke, James P. Butler, Ben Fabry. (2011) Coordinated Movements Prevent Jamming in an Emperor Penguin Huddle. PLoS ONE: 6 (6): e20260 (DOI: 10.1371/journal.pone.0020260)

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