Emperor penguins are lords of the cold. They thrive in frigid conditions that would make human popsicles out of anyone relying on only the hair nature gave them for protection. Many researchers have bundled up and braved harsh Antarctic winters to study these fascinating birds and their strategies for survival.
In 2011, a team of international scientists reported that tight-knit huddles of Emperor penguins exhibit wave-like motions. Every 30-60 seconds waves will propagate throughout the huddle, which can consist of thousands of penguins at a time, allowing penguins at the huddle’s chilly outskirts to move inward to the warm center and those at the center to relinquish their turn.
The team’s observations generated more questions than answers. Was there a lead penguin that triggers the wave each time? How do the waves compare with the collective behavior of other masses such as bird flocks, fish schools and traffic jams? And why do the waves move throughout the huddle in the direction researchers observe? Another group of researchers, including two of the four scientists who authored the 2011 paper, have answered some of these questions in a recent paper published in IOP’s New Journal of Physics.
By developing a model that replicates observations in nature, the group determined that any individual penguin, regardless of their position in the huddle, could trigger a wave. Moreover, because the penguins are so closely amalgamated, the simple movement of a single step can produce this phenomenon.
In 2005, during the winter months of June and July, the group took time-lapse images at a rate of 25 frames per second of two Emperor penguin colonies located at Pointe Geologie and Atka Bay in Antarctica. While average wind speeds reached 14 meters per second and air temperatures ranged from a bone-chilling -33 to a soul-crushing -43 degrees Celsius, the temperatures toward the middle of the penguin huddles could reach up to 37 degrees Celsius.
So, spending time at or near the center is important for penguins and their young to avoid the icy licks of winter wind, albeit temporarily. And waves act to reorganize the huddle that then allows penguins on the outskirts to move further into the huddle.
By adapting equations of motion that predict traffic jams, the group created a model that can replicate the wave motion throughout a huddle of Emperor penguins. The key difference between a penguin huddle and a traffic jam is the direction that waves travel. In a traffic jam, cars can only move in a forward direction, restricting the wave’s motion to one direction. But in a penguin huddle, penguins can move left, right, forwards, backwards or diagonal, which means the consequential waves can propagate in any direction, too. The video depicts this versatility.
|Group of Emperor penguins. Credit: U.S. Antarctic Program Photo Library|
From their model, the group concluded that any single penguin could trigger a wave. Occasionally, a penguin will perform a step, the authors explained. After a short period of time, neighboring penguins will also move. This time delay that can predict wave velocity, depends on the physical distance between penguins, speed of each penguin’s steps and threshold distance, which is the necessary distance between penguins that is close enough to coax a penguin to move with the others but also large enough to give the penguin room to move.
The group’s model shows a trend between wave velocity and threshold distance: smaller threshold distances should trigger faster waves. The wave speed that the group measured from their time-lapse images was about 12 meters per second. When the group input values they measured from the time-lapse images for distance between penguins, threshold distance and speed of each penguin’s movement into their model, they calculated a wave speed “nearly identical” to what they observed.
Moreover, the threshold distance the group measured for the Emperor penguin huddles they observed was around two centimeters – about twice the thickness of the penguins’ compressive feather layer.
“This suggests that the penguins touch each other only slightly when standing in a huddle, without compressing the feather layer so as to maximize the huddle density without compromising their own insulation,” the authors stated in the paper.
For more videos of the group's time-lapse images and models see the paper's supplementary material.