A gentoo penguin (Pygoscelis papua) with a double layer of feathers (left) and a second penguin that has just begun to lose its old plumage (right). ©Agnès Lewden.
Many creatures moult, changing their pelt or feathers completely when the old coverings are worn and tatty, depriving them of insulation and leaving them vulnerable to the cold. But gentoo penguins (Pygoscelis papua) moult in style. They don't slough off the old before growing the new. Instead, gentoos keep hold of their old feathers, becoming super-fluffy as the new ones emerge from beneath, doubling their insulation. But the process is not cost free. The birds lose up to 45% of their body mass as they burn more energy to produce their new plumage when stranded on land or sea ice and unable to feed. But does doubling their feather insulation while turning up their heat place gentoos at risk of overheating during their feather change? Intrigued by the possibility, Agnès Lewden from the University of Brest, France, headed over to the Océanopolis aquarium in Brest to find out how the captive birds there cope during their ‘catastrophic moult’.
Over a period of 8 weeks in late summer and early autumn of 2022, Lewden painstakingly photographed every penguin each day as they moulted, using a thermal camera, making sure that she captured images of each bird's left and right, to ensure that she had a clear view of the heat being lost from the beak, eye, flipper and other body parts on the two sides. ‘The penguins were definitely more fearful during the moult, probably because they are more vulnerable as they can't go into the water’, she says, adding that thermally imaging the birds reduced how much they were disturbed. Teaming up with Tristan Halna du Fretay (University of Brest) and Antoine Stier (University of Strasbourg, France), Lewden then compiled the images of each bird and calculated the temperatures of the feet, beak, eyes and flippers, as well as the average temperature over the trunk of the bird's body, to find out how they changed during the moult.
Comparing the temperatures of the birds' different body regions as they moulted, the team could see that the penguins radiated less heat from the trunk of the body – with their surface temperature falling from ∼16 to ∼15°C – when their new feathers came through adding to their insulation as the birds puffed up before the old feathers fell out. However, when they checked the temperature of the eyes, which should be a reasonable reflection of the bird's internal body temperature, it increased from ∼30 to ∼32°C. The penguins were definitely warming significantly as their feathery insulation doubled. The team could also see the feet, beak and flippers glowing white in the thermal images, showing how the penguins radiated heat from these body sections to keep their temperatures down as much as possible, with the beak temperature increasing from ∼20 to ∼22.5°C.
‘Early moult stages may therefore make it difficult for penguins to dissipate extra heat’, says Lewden. And, as the old feathers began to come away, the temperature of the birds’ eyes began to return to normal as they were able to lose more heat through their skin, having shed the additional insulation from last year's plumage.
So, gentoo penguins are paradoxically at risk of overheating during their moult, when other birds that lose their feathers might struggle to remain warm. And Lewden warns that penguins in the wild could suffer overheating even more in direct sunlight in the southern hemisphere, especially if global temperatures continue to rise.
