Read with me, if you will, the opening paragraphs of a swell new paper in PLoS One (access free; reference at bottom) about how emperor penguins, Aptenodytes forsteri, keep warm in the Antarctic winters. Just by filming the birds for 4 hours on a single day (August 3, 2008), the four researchers found an amazing group behavior. (I see no sense in rewriting the authors’ perfectly clear scientific prose):
Emperor penguins are the only vertebrates that breed during the austral winter where they have to endure temperatures below -45o C and winds of up to 50 m/s while fasting. From their arrival at the colony until the eggs hatch and the return of their mates, the males, who solely incubate the eggs, fast for about 110–120 days. To conserve energy and to maintain their body temperature, the penguins aggregate in huddles where ambient temperatures are above 0o C and can reach up to 37o C.
(37o C is, of course, human body temperature, so it’s nice and warm in the groups.)
Each colony consists of a group of huddles, and in each one the penguins are tightly packed, and all facing in the same direction. The density of penguins can reach—get this—21 animals per square meter! (And these are not small birds: they weigh between 50 and 100 pounds and can be up to 4 feet tall.) The picture below shows several huddles within a larger group:
Several emperor penguin huddles. Photo by Robyn Mundy.
More from the paper:
Huddling poses an interesting physical problem. If the huddle density is too low, the penguins lose too much energy. If the huddle density is too high, internal rearrangement becomes impossible, and peripheral penguins are prevented to reach the warmer huddle center. This problem is reminiscent of colloidal jamming during a fluid-to-solid transition. In this paper we show that Emperor penguins prevent jamming by a recurring short-term coordination of their movements.
The authors filmed the penguins on a single day, using time-lapse photography at a rate of one image every 1.3 seconds. As the figure from the paper (below) shows, individuals were tracked using the characteristic yellow and white face patch of the breed. Note that the males in these huddles are incubating eggs (nearly all of them had one), and when they move they do so by waddling with the egg balanced on their feet.
Here’s the amazing thing the authors found: penguins keep the huddle “fair”, and move from the periphery to the interior (and vice versa), by episodic but coordinated waves of penguin shuffling:
The jammed state of the huddle is interrupted every 30–60 s by small 5–10 cm steps of the penguins (Fig 1C,1E, Movie S2, S3), reminiscent of a temporary fluidization. These steps are also spatially coordinated and travel as a directed wave with a speed of about 12 cm/s through the entire huddle (Fig. 1E). After the wave has reached the end of it, the huddle re-enters the jammed state. Interestingly the propagation speed of the traveling wave is comparable to the speed of the individual penguins during the step. This is analogous to the propagation of sound waves in an elastic entropic medium (gas or fluid) where typical molecular velocities are comparable to the velocity of pressure waves.
You will of course want to see what this looks like. The links below go to the three movies from the paper (each between 1.5 and 4 minutes long), along with the descriptions. DO NOT MISS THESE STUNNING MOVIES.
Movie S1. Huddle formation and occurrence of coordinated traveling waves. Time lapse recordings (full field of view) over 2 h (resolution reduced from 10 MP to 480 p), showing about half of the penguin colony during the aggregation and huddling process. At the beginning of the movie (~12 p.m. with temperatures above −35°C), only few penguins aggregated in smaller huddles. As the temperatures gradually fell, larger and more stable huddles formed until nearly all the penguins aggregated in one large huddle.
To see the Sphenisciform Shuffle in the next two videos, keep your eye on the penguins’ white face patches. You’ll see them advancing in a jerky but coordinated way as the penguins step forward.
Movie S2. Huddle formation and occurrence of coordinated traveling waves (detail). Time-lapse recordings (detail of S2 over 1 h) showing multiple huddles. The penguins in a huddle mostly face in the same direction which defines a rear end and a front end of the huddle. When a penguin joins the huddle, it does so by aligning itself first in the direction in which the other penguins are facing, and then moving closer to the huddle. As a result, penguins tend to join a huddle at its rear (trailing) end and leave it at the front (leading) end. During the periodic traveling wave, the huddles move in the forward direction (in the direction in which the majority of the penguins are facing).
The shuffling is most evident in this video:
Movie S3. Coordinated traveling waves in a densely packed huddle. 21 min sequence from S2 (detail corresponding to Fig. 1B) at reduced speed. The movie shows the travelling wave of small steps every 30–60 sec.
The authors show that this type of movement is not unique to penguins, but has been seen in locusts and fish schools, as well as in tissue-cultured cells. They also show that it resembles “fluid-to-solid gelation of short-ranged attractive colloids.” But returning to the biology, the behavior raises two interesting questions:
- Is this behavior evolved or learned, or a combination of the two? Natural selection could of course favor this “altruistic” behavior since it’s to each penguin’s advantage to participate in a shuffle. The time you lose being cold on the periphery is more than compensated for by the larger amount of time you spend inside the warm huddle. (The behavior is not pure altruism, of course, for individuals gain rather than lose fitness by participating in the shuffle). But what about cheaters, who don’t move along, or didn’t when the behavior evolved? They would benefit by never having to be on the periphery, but they could of course have been punished for such cheating by the other penguins. It would be very hard to test whether this behavior was hard-wired, since it would involve creating large huddles of naive, hand-reared penguins under artificial conditions, and then subjecting them to an artificial winter.
- Mechanically, how does it work? The authors note, “It is also unclear whether the traveling wave in a huddle is triggered by a single or few leading penguins and follows a well-defined hierarchy among group members, similar to the collective behavior in pigeon flocks. Modeling attempts with self-driven agents have explained collective behavior such as temporal and long-range spatial synchronization in bird flocks, fish schools or traffic congestion by evolutionary strategies and a small set of simple interaction rules between neighboring agents. Similar mechanisms may also apply to the collective behavior of penguins in a huddle.
You don’t need fancy machinery or DNA sequencers to discover amazing things about our world. This senational behavior was revealed by four researchers armed only with a question and a video camera.
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Zitterbart D.P., B. Wienecke, J. P. Butler JP, and B. Fabry. 2011. Coordinated movements prevent jamming in an emperor penguin huddle. PLoS ONE 6(6): e20260. doi:10.1371/journal.pone.0020260
Does the behaviour have to be evolved or learnt if it is so widely found in so many different species at different levels of complexity? Could it not be an emergent property of how groups move?
If you are in an approximately circular group that you would get if every creature wants to get keep close, then is your individual path not going to be eventually deflected to the outside as everyone else tries to move inwards?
It brings up a problem for penguin colonies that are devastated by loss of individuals perhaps through human actions (over fishing, oil etc) – there will be a critical mass below which the penguins will suffer far more from the cold, burn more calories & not survive long enough to rear the chicks.
Fascinating stuff!
Agreed- fascinating stuff! “Emergent property”, thank you, just what I was thinking but couldn’t think of a suitable way to phrase it.
Movies were just amazing (took a couple of minutes to download, but worth the wait!).
37c is too warm for me – I would be shuffling to the outside! 😉
Where it’s -50C with 50kph wind? I doubt it.
I’m in the same spot as Dominic – even at minus lots and lots, if I were wearing that much insulation I’d be happy to take my turn at the outside of the huddle. Personally, I like a nice steep temperature gradient from my core to the environment, on the order of 50 or 60 K is about right for me (appropriately dressed, this gives me more options for modulating heat flux).
I’m not a biologist by any means, but I would think that penguins don’t cheat because there is no way for it to go unnoticed. No individual could hide the fact they are cheating and I’m willing to bet that cheaters would be dealt with harshly. If the rest of the flock turned on an individual it could easily be deadly for him and his egg.
What would ‘cheating’ look like? A refusal to shuffle, stubbornly staying put, or pushing salmon-like against the flow?
It may simply be physically impossible amid all that pushing and shoving.
And in a constantly shifting huddle, a stay-put ‘cheater’ is likely to end up on the periphery anyway.
It may be more of a mosh-pit: everybody tries to move to or stay in the center but they are buffeted about by the multitudes trying the same.
It’s also perhaps worth mentioning that occasional muscular activity would help generate more heat for the huddle.
Yes – imagine you were a human in that situation – you will never stay close enough to another individual to see if they were cheating. Think how easy it is to lose a companion if you go into a crowd together.
If it’s just an emergent property of each penguin trying to maximize their own position then all motion should begin at the periphery. The central penguins should be content where they are an therefore attempting to hold their ground. That doesn’t seem to be what’s happening. Firstly, all of the penguins on the edge should be pushing centrally, causing the center to become more densely packed and hurting all of them. This appears to be a more coordinated movement not guided by pushing from the edge and individual utility maximization.
Except that 37 C is really very hot, not only for a naked ape like me, but especially for a cold-adapted, insulated animal. So the center is not the maximal position. I doubt that the central penguins are content, and they are probably trying to move to a more moderate location.
If it’s an emergent property, then the desired condition is emphatically not the 37 C central location, but a more moderate one. It’s not about everyone pushing toward the center, but penguins on the periphery and penguins in the center both pushing toward a more moderate location.
(NB: I’ve never been to the Antarctic, but I imagine that the smell is something else.)
One good way to conceptualize the benefits of huddling: if your body is completely surrounded by objects of the same temperature (e.g. other bodies), then it’s perfect insulation: with no heat gradient, your body heat has nowhere to be lost to.
I too wonder this shuffling-wave is even a ‘behavior’ that needs a nature-and/or-nurture explanation.
I have not read the paper, but it seems to me that we really have no idea whether individual birds in the center are participating ‘intentionally’ in the coordinated shuffling or if, alternatively, it’s more or less forced by birds self-interestedly shoving in constantly from the periphery.
“peripheral penguins” – that must be a rock group
And the penguin shuffle is a new dance craze…
It reminds me of a story of people escaping a working camp in Siberia: they slept like spoons in a drawer; regularly, there was a change and the 2 at the ends came to cuddle up in the middle of the row.
Is that like the Crazy Chicken?
If not, it should be. Don’t we have any choreographers in the audience?
At the beginning of the first video it reminded me of what a large conference or gathering of people looked like. It appears that individuals and small groups are getting together, waiting for friends to arrive, and perhaps waiting for a socially acceptable time to join the throng. Maybe waiting for friends is a good idea, as you don’t want to be stuck with a poor conversationalist penguin for hours on end.
At the beginning of the first video I thought, ‘OMG, they’ve cloned Charlie Chaplin!”
Domestic fowl chicks, which will huddle in corners till the ones on the bottom smother, could learn a lot from penguins.
(Of course, domestic fowl chicks did not evolve to be raised in artificial brooders…)
Loved the vids, esp. the last. The mass of heads-down schleppers, slogging along, and the individualists popping up their heads as if to say ‘Hey, are you sure this is the right way?!’
I haven’t been able to watch the videos, because the download speed was – ahem – glacial.
But I notice that in the paper it is said that wind speeds during the observation were low or moderate. I wonder if this is typical for the Antarctic winter? I recall seeing the penguins ‘huddle’ in one of David Attenborough’s TV documentaries. There was a strong wind blowing, and it was pretty clear (I thought) what was happening: the penguins on the side of the huddle exposed to the wind were shuffling round to avoid it, which exposed a new row of penguins to the wind, who then shuffled round in turn. Although the result was that each penguin ‘took its turn’ on the edge of the huddle (and I think that was what the commentary said, with the implication of altruism), it looked entirely ‘selfish’ to me.
I just found the Attenborough sequence on YouTube:
Amazing vids. Thanks for helping me start the day right!