Penguins have evolved a really neat way of staying warm. To conserve energy, they form densely packed huddles with a triangular lattice structure. What's amazing is that these packs — which can include thousands of individuals — actually move. A recent physics study shows how it's done.
Male Emperor penguins have it rough. As keepers of the eggs, they have to endure Antarctic winters featuring temperatures as low as −58 °F (−50 °C) and wind speeds reaching 120 mph (200 km/h). Huddling is obviously a great strategy; when crammed together, their body surface temperature can rise to 98.6 °F (37 °C) in less than two hours. Of course, this strategy can only work if there's a system to keep everything in order. With thousands of individuals packed into tight spaces, there's great potential for chaos to set in.
And indeed, previous studies have shown coordinated movements in regular wave-like patterns within these huddles. Scientists assumed that these waves were triggered by individual penguins that disturb the huddle structure, and that the propagating wave acts as a way to remove defects and to restore order.
But until the recent study, the precise mechanisms that govern these wave transmissions were unknown — or if they were caused by the same individual in the huddle.
But all is now clear after a team of researchers applied a mathematical model frequently used to study road congestion, flocks of birds, and schools of fish (i.e. a coupled system of differential equations that measure the positions and velocities of individuals). It predicted how autonomous individuals can incrementally move step-by-step within a tightly packed huddle — and in a way that's quite reminiscent of how cars inch forward in a traffic jam (characteristic "stop-and-go waves" and intermittent behavior).
Fascinatingly, the waves can emerge from virtually any point in the huddle, and the rest of the huddle will adapt. So it's not a matter of cold penguins at the exterior trying to inch their way in. But this only works so along as their steps don't exceed a two centimeter threshold distance (about the thickness of the penguin feather layer).
The researchers say this allows for perfectly compact huddles that maintain each bird's maximum fluffiness and insulation. "[T]he model confirms that the traveling waves help to achieve an optimally dense huddle structure, and describes how huddles can merge," write the authors in the study.
The physics models were later confirmed by optical flow velocimetry of video recordings taken of the penguins in their natural habitat.
Read the entire study at New Journal of Physics: "The origin of traveling waves in an emperor penguin huddle."