The iconic emperor penguin is renowned for braving the Antarctic winter to incubate its young, facing temperatures as low as −40°C. Despite their frosty surroundings, emperor penguins maintain core body temperature near 37°C, in large part due to their impressive, insulating plumage. Intrigued by the complexity of heat transfer dynamics in this warm-blooded creature in such a chilly environment, Dominic McCafferty, from the University of Glasgow, UK, and his colleagues set out to measure the direction and magnitude of heat flux and gain further understanding on the effects of weather and climate on the energetics of this extreme species. In their latest study published in Biology Letters, they decided to measure surface temperature variation of free-ranging emperor penguins.
McCafferty and crew braced for the cold, heading to the Dumont d'Urville emperor penguin colony (Terre Adélie, Antarctica) in the austral winter. The team deployed a thermal imaging camera and a digital camera at the colony to capture infrared and digital images of 40 birds, taking advantage of this non-invasive means of investigating thermoregulation. Using images of birds separated from each other by at least one body length, they used image analysis software to determine mean surface temperature of the front and rear trunk, wings, head and feet. They also logged the surface temperature of the surrounding ice, air temperature, relative humidity, wind speed and cloud cover. The images revealed that nearly the entire penguin exterior was below the temperature of freezing (i.e. below 0°C), with the exception of the eye region. Despite this trend, there were differences in how the temperatures of various body parts compared with that of the surrounding, and well below freezing, air. The head, wings and feet were warmer than the surrounding air temperature, but trunk temperatures were even colder than that of the air. Higher air temperatures meant warmer plumage, and stronger winds made for colder wings and feet.
Next, the team used a heat transfer model to estimate the direction and relative magnitude of heat fluxes. Their model showed that radiative heat loss was greatest from the body trunk, followed by the head, wings and feet. They explain that the cloudless sky can act as a radiative sink, causing the penguin's surface temperatures to drop below that of the surrounding air, as seen in other species under similar environmental conditions. The team predicted that the cool trunk feather surface would then actually gain heat from the surrounding, warmer air via convection. However, because of the low thermal conductivity of feathers, little of this heat will reach the skin. This low heat conductivity of the emperor penguin's tuxedo-like coat works both ways though, and also helps prevent internal heat loss. Only the un-feathered areas (feet, eyes and beak) and sparsely feathered wings lose heat from the body interior.
Other adaptations such as effective heat exchange networks in the blood vessels of these birds and the thick, scaly skin encasing their feet certainly contribute to this bird's ability to retain heat where feathers cannot help. Behaviour is also key to keeping warm; the scientists found that heat loss from the feet was reduced by 15% when the penguins leaned back, lifting their toes off the frozen floor, and during windy, cloudy conditions, the well-known huddle is vital against potential large convective heat losses. Despite its chilly exterior, the penguin's feathered coat remains its most crucial adaptation to the icy Antarctic – as anyone with a feather-filled parka will testify!
