Condensed penguins

[Img: Antarctic Connection]

During my time in Canada, the coldest temperature I recall reading off the digital display on the way back home was -28°C. I couldn't help asking myself why did humans ever settle in such hostile environment (and wtf was I doing there). But if you think Canadians are though (and Germans wimps), hear the story of the Emperor Penguin (Aptenodytes forsteri), that lives in Antarctica.

The Emperor Penguin's adaption to the cold, which can drop down to -50°C during the Antarctic winter, is plainly amazing. Feathers, fat, and their ability to increase the metabolism rate at low temperatures allow the penguins to survive. Equally amazing, but also bizarre, is the Emperor Penguin's breeding behavior.

Penguin colonies up to some thousands have nesting areas inland that are, depending on the annual ice thickness, 50-120 km away from the edge of the pack ice. At the beginning of the Antarctic Winter, some time in March or April, the penguins get out of the water and travel to their nesting areas, mostly walking or sliding on the ice. After mating, the female lays a single egg in late May or early June and passes it on to the male for incubation while she walks back to the shore.

In an environment of ice, snow, and the occasional rock the penguins can't build nests, so they balance the egg on their feet in their brood pouch. And, since there isn't much fish to find on the pack ice, they don't eat. Yes, you read that correctly: They walk a hundred kilometers, the female lays an egg and walks back a hundred kilometers, while the male sits on the egg for another 2 months, during the Antarctic Winter, in the dark, without the female, and all that without eating a thing. By the time the egg hatches, the male has fasted for almost 4 months, lost half of his body weight, and hopes for the female to return because he has nothing to feed the chick. And then he still has to walk back to the shore so he doesn't starve. But hey, my husband assembled the baby cribs!

There's a great documentary, "The March of the Penguins," telling this story:



But the penguins know some physics too!

While a single penguin is able to maintain its core body temperature in the freezing cold, this costs a lot of energy which he can't afford during his Winter fast. So what the Emperor penguins do is they form huddles. The density of the huddles increases with falling air temperature. If the huddle gets very dense, the penguins are in a nearly hexagonal arrangement. In a study from about a decade ago, researchers glued measuring devices to some penguin's lower back. They found that the temperature inside huddles can reach as much as 37.5° (Gilbert et al, arXiv:q-bio/0701051v1 [q-bio.PE]).

The penguins in these huddles do not stand still, but they move in occasional small steps which have recently been subject of another study (Zitterbart et al, PLoS ONE 6(6): e20260). The researchers shot movies of the penguin huddles and tracked the position of the birds. As you will easily notice if you read the paper, David Zitterbart is a condensed matter physicist who compares the huddling penguins to particles with an attractive interaction and the tight huddling to a jamming transition in granular materials. Just that the penguins manage to prevent jamming by coordinated movements. The little steps of the penguins propagate through the huddle like density waves. They measured densities up to 21 birds per square meter.

According to their paper, the penguins' little steps have a three-fold benefit. One is that they help the packing to get denser, much the same way like tapping a bag with ground coffee. The second is that they move the whole huddle, allowing huddles to merge and adjust position and direction. The third one is a turnover of penguins in the huddle, moving those from the outside towards the warmer inside. Though one might argue that what actually is responsible for the turnover is not the little forward steps, but the penguins on the front leaving the huddle and joining it (or another huddle) on the back. But without the forward steps, the turnover would make the huddle move backward.

The below movie from Zitterbart et al's paper (Yes, we live in the age of Harry Potter, where paper has moving pictures!) shows a time lapse of the penguin huddling (actual time about 1h):



I was very confused about the penguin turnover because in all the Emperor Penguin huddles in "The March of the Penguin" and photos I had seen only penguin backs and could not for the hell of it figure out where the penguins are supposed to go if they are all facing towards each other. So I wrote an email to David Zitterbart who kindly explained that there's two different kind of huddles that have been observed. Those that they've described in their paper, which have a forward direction, and circular ones that I had seen images of. He writes that no one really knows how the circular ones work, but they hope to find out with the next experiment.