To stay warm in the Antarctic winter, hundreds, or sometimes thousands, of emperor penguins huddle closely together with their flippers held against the body, the head tucked into the shoulders, and the beak hidden between the necks of others. But sometimes it gets a little too hot in there. Just a couple of uncomfortably warm penguins can break up a whole huddle in under two minutes. A study, published in this month’s Animal Behaviour, demonstrates how social thermoregulation is a highly dynamic phenomenon.

The density of emperor penguin (Aptenodytes forsteri) huddles can be as high as 10 birds per square meter (10.7 square feet). While movement is limited, individuals are able to make small steps in a synchronized way. Penguins exposed to the most wind move along the opposite flank of the huddle, for example. Huddling maximizes energy savings by reducing the body surface area that’s exposed: Heat can only dissipate through the inhalation of cold air and through their heads. That means ambient temperatures can reach 37.5°C (99.5°F), which is well above their upper limit of 20°C (70°F). As a result, emperor penguins are faced with the paradoxical situation of having to lose excess heat in a cold environment. It’s unclear how they deal with these contradictory requirements. 

To better understand the dynamics of emperor penguin aggregations, a team led by André Ancel of the University of Strasbourg photographed and videotaped an entire colony at Pointe Geologie Archipelago during 2005, 2006, and 2008, and they examined meterological data obtained from a permanent weather station 500 meters (1,640 feet) away. About 3,000 pairs breed there each year. Pairing takes place in the spring, followed by incubation from mid-May to July and chick rearing through September. A group can contain several huddles as well as loosely aggregated individuals who aren’t part of any; these birds are typically grooming, lifting the abdominal skin fold covering the egg, or performing courtship displays when females were present.

^{On the left, you can see huddles at the center and bottom of the image, with loose aggregations around them. After the breakup (on the right), the birds reach a state of loose aggregation. A. Ancel et al., 2015 Animal Behaviour}

On average, penguins spent between 40 and 60 minutes in huddles, and huddles themselves lasted between 15 minutes and three hours. Wind speed, solar radiation, and air temperature were the main factors driving huddle formation. 

A small number of birds would trigger movement that propagates to the entire huddle, breaking it up in as little as a couple of minutes. The percentage of moving individuals rose from 2.7 percent to 99.9 percent within two minutes, and released birds joined loose aggregations. But while they rapidly left huddles following breakups, they slowly moved from loose aggregations to huddles.

Different parts of the colony appeared to continually exchange individuals in response to environmental conditions: Penguins needing warmth joined huddles, while those hoping to dissipate heat break huddles apart. Sometimes a haze of warm air rises over the colony during a breakup, and afterwards, departing birds have been seen consuming fresh snow. The regular growth and decay of huddles, the team writes, operates as pulses through which birds gain, conserve, or lose heat.

[H/T: Science News]