Every winter (spring in the Northern Hemisphere), after having produced a chick, female Emperor penguins (Aptenodytes forsteri) head off to sea for two months to fatten up, while the males stay behind, foodless for over 100 days, to tend the chicks. (The males get their turn to eat later, but often walk about 100 km to get to the water.) With air temperatures as low as -40° C (same in Fahrenheit), and the winds blowing as hard as 140 kph (90 mph), it gets deadly cold. And that’s when the penguins huddle together for warmth.
Here’s a short PBS video of penguin huddles. Note the constant shifting of the birds.
And the huddle really keeps them warm. A 2012 paper in PLOS ONE (screenshot below, pdf here, reference at bottom) reported that the temperature inside the huddle can reach 20°C-37.5°C (68°F-100°F). Individuals outside the huddle, exposed to the air and wind, however, don’t get as warm. As the video shows above, the penguins move constantly, with exterior penguins going into the middle and then being expelled to the margins again. These huddles appear to last only a few hours—during bad storms.
If you click on the screenshot below, you’ll go to a paper in which the three authors, using several simple assumptions, try to predict what shape the huddle will assume, and how it will change over time. There’s also a shorter but easier-to-understand summary that’s just been published in Quanta Magazine.
The authors’ assumptions are these:
- The penguins form a huddle, and they’re packed in the most efficient way: hexagonally. (This corresponds to how they appear to be packed in nature.)
- The huddle is assumed to be on a plane, and the penguins have a uniform size and shape. They encounter wind.
- The penguins have an assumed rate of heat loss, with the ones on the outside losing more heat.
Then the authors then do their simulation, which involves one chilly penguin moving at a time to get warmer. Here are the steps they simulate:
- Generate a huddle and determine the huddle boundary.
- Compute the wind flow around the huddle.
- Compute the temperature profile around the huddle.
- Compute the local rate of heat loss for each penguin.
- Add random variations to the rate of heat loss (optional).
- Identify the penguin with the highest rate of heat loss (the “mover”) and move it to a location on the boundary where heat loss is minimal.
- Determine the new huddle boundary.
- Repeat over the desired number of iterations by going back to Step 2.
Quanta shows a diagram of how the simulation works:
You’ll notice immediately that some of these assumptions are oversimplified—especially that the coldest penguin is the one that moves, and he (it’s a male) moves to the warmest spot on the periphery, not to just a warmer spot. The video of the huddle above doesn’t seem to show only one penguin moving at a time, nor does it seem realistic that the mover can find the warmest possible spot. Note, though, that as the coldest penguin (on the windward side, of course) moves, an interior penguin becomes an exterior penguin, and that generates a new mover, and so on and so on. Eventually, the shape of the huddle changes.
As the diagram from the paper shows below, the model shows the huddle changing from irregular to roughly rectangular, with one small side of the rectangle facing the wind. (The short faces of the rectangle are actually rounded, not straight.) After about 50 iterations of the model, the steady-state shape begins to emerge. Here’s a diagram from the paper showing the shape change over time (iterations) with the wind coming from the left.
One interesting outcome of the model is that, in the end, all penguins have experienced about the same loss of heat and have had roughly equal access to the warmth inside the huddle. This is an example of a selfish behavior acting to produce heat equity for all. The authors also note that the model does not produce the least heat loss for the colony as a whole, which I think would come from a sphere. (I’m just guessing here.)
Now the value of this simulation is only as good as its predictive power. Do penguin huddles really assume these shapes over time? The answer appears to be, well, not really: they are not elongated rectangles but are more irregular in shape. (One prediction that was confirmed previously, though, is that in the huddle the penguins are packed hexagonally.)
The authors tweaked the model by allowing random variations in the heat loss of individual penguins, which generate more irregular shapes that, say the authors, produce a shape of the huddle “qualitatively similar to that of real huddles.” They point to a figure that supposedly shows the qualitative similarity (Fig. 5a), but it doesn’t show that: it shows that huddles become more irregular in shape as the degree of random perturbation increases. It would have been better had they shown some real shapes of huddles evolving over time.
The Quanta article links to studies of heat loss and penguin movement in progress that may eventually yield this data, but it doesn’t exist now. But the data do show that, as predicted, individuals tend to move from the windward to the leeward side of the huddle, and that this movement is more pronounced when wind speed is stronger.
A lot remains to be done, including observations of colonies to see how individuals move. But that’s very hard given the terrible weather conditions and the problems of putting researchers on the site near these fragile colonies (One of the cited papers describes a remote-controlled observatory.) And of course, if the assumptions of the model prove to be wrong, as a couple seem to be, then the model needs to be severely refined. But at least the authors have singled out a cool problem that may eventually have a fairly simple solution. But I think that solution will have to involve more than one penguin moving at a time!
Article: A. Waters, F. Blanchette F, and A. D. Kim. (2012) Modeling huddling penguins. PLOS ONE 7(11): e50277. https://doi.org/10.1371/journal.pone.0050277