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N°33 I quarterly I April 2014 Live from the Labs | 13 w Penguins’ Optimal Huddle BY Fui Lee Luk w Drivers’ behavior in a traffic jam is utterly predictable: when one car inches forward, all follow even if traffic is at a standstill. Echoing this stop-and-go motion far from roads, Emperor penguins huddling for warmth in the Antarctic shuffle their positions one by one, in tiny “traveling waves,” every 35 to 55 seconds. Using video analyses and mathematical models, a Franco-German team has unlocked the mechanism of this odd phenomenon.1 Male Emperor penguins incubate eggs perched on their feet when their mates leave for about 75 days to re-feed at sea. To keep their bodies at 37°C in temperatures as low as -30°C, the males form huddles. But how and why do waves of movement ripple through these packs? To find out, biologists filmed 50 tagged males near the French Dumont d’Urville research base during incubation and produced a model for bird motion based on a mathematical model for congested road traffic. Results show that penguins, like drivers, seek to maintain an optimal distance from their neighbors: about 2 cm for the birds, as this is the closest they can crowd without flattening their feathers and reducing their insulation. Unlike cars in a jam however, bird motion is not linear and any individual in a pack can set off a wave with a single step in any direction, thus “moving the huddle forward or sideways,” notes André Ancel of the IPHC.2 Interestingly, huddles adapt to the weather, “thinning or disbanding when conditions improve.” The team suspects that as the cramped birds lack space to turn the eggs with their beaks, the waves aim to rotate the eggs and “keep their temperatures even for sound embryo development.” Further studies scheduled for 2015 will use dummy eggs that can record temperatures and rotation frequency. “Hopefully, these will be adopted by penguins that lose their own eggs,” concludes the researcher. 01. R.C. Gerum et al., “The origin of traveling waves in an emperor penguin huddle,” New Journal of Physics, 2013. 15(125022): doi:10.1088/1367- 2630/15/12/125022. 02. Institut pluridisciplinaire Hubert Curien (CNRS / Université de Strasbourg). Biophysics Contact information: IPHC, Strasbourg. André Ancel > andre.ancel@iphc.cnrs.fr From Soap Bubles to Tropical Cyclones by F ui Lee Luk w What do soap bubbles and tropical cyclones have in common? A lot, according to LOMA 1 physicists who have recently shown2 that the streaky whirls of soap fluid on bubble surfaces are comparable to the spiraling vortices of cyclone winds. To explore vortex properties, the physicists made half-bubbles of soap on brass disks to reproduce the curvature of the atmosphere. By heating them from below, they triggered a vortex-induced thermal convection effect similar to the atmospheric flows observed in cyclone formation. “The soap bubble vortices (2 cm in diameter and 10 μm-thick) can be compared to cyclonic ones (which can stretch over 1000 km and cover a 10-km atmospheric range) in that “both are very flat structures with a life cycle consisting of an intensification and a decline phase,” explains LOMA scientist Hamid Kellay. Backed by digital simulations conducted by the IMB,3 the results reveal “universal behavior” in vortices: a weak rotation rate just after they form, which gradually speeds to a climax before declining. The team has thus defined a model for Strasbourg predicting peak intensity and the time it takes to reach it when a vortex is one quarter into its cycle. Comparison with data supplied by La Réunion University on 150 Atlantic and Pacific cyclones has confirmed the model’s reliability for giant vortices as well. This could help anticipate cyclone behavior—and better prepare populations at risk. 01. L aboratoire ondes et matière d’Aquitaine (CNRS / Université de Bordeaux). 02. T . Meuel et al., “Intensity of vortices: from soap bubbles to hurricanes,” Nature Scientific Reports, 2013. doi: 10.1038/ srep03455. 03. Institut de mathématiques de Bordeaux (CNRS / Université de Bordeaux / Institut Polytechnique de Bordeaux). Contact information: LOMA , Bordeaux. Hamid Kellay > hamid.kellay@u-bordeaux.fr q S oap bubble on a heated rotating brass disk. Temperature variations create movement and swirls on the bubble's surface. q When temperatures reach -30° and during blizzards, Emperor penguins huddle together in “turtle” formation. Bordeaux A photo report and the film Soap-bubble cyclones, can be viewed on the online version of the magazine: > www.cnrs.fr/cnrsmagazine © CNRS/IPHC/DEPE/IPEV


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