Penguins are far better swimmers than they are walkers, but sometimes they have to trek vast distances on land by waddling on their short legs. Now, researchers studying king penguins walking on treadmills reveal that heftier males (fresh from their feeding sojourn at sea) are less steady on their feet. The findings are published in PLOS ONE this week. 

King penguins (Aptenodytes patagonicus) spend a long time chasing prey in the sea, and after they return ashore, they often have to journey across large swathes of land to reach their breeding colony – all while carrying a heavy accumulation of fat as well as food in their stomach to provision their chicks. Once they reach their destination, they typically have to fast for up to a month, relying only on their energy stores, and they end up losing about a quarter of their body mass.

To see if natural variations in body mass impact the penguin’s walking gait, a team led by Astrid Willener from the University of Roehampton captured 10 male king penguins during the austral summer from a colony at Baie du Marin on Possession Island in the Crozet Archipelago. They then trained the penguins to walk on a treadmill with an acceleration data logger attached to the feathers on their back. The penguins walked for 10-minute intervals at a speed of 1.4 kilometers (0.9 miles) an hour. The researchers collected data for each penguin at two body masses: On the first day, the penguins weighed an average of 13.2 kilograms (29 pounds), and on day 14 of their fast, they weighed 11 kilograms (24 pounds) on average. 

King penguins, the team found, don’t exhibit major differences in their walking biomechanics whether heavy or light – not with stride frequency nor body acceleration. There were, however, differences between skinny and fat penguins when it comes to how much the body leans: When the penguins are slimmer, there’s less variability in the leaning angle and amplitude. Skinnier birds also showed a decrease in waddling amplitude. When they gain weight, it becomes more difficult to control the larger mass and the momentum associated with it.

When the birds are heavier, their frontal fat accumulation seems to shift their center of mass forward (see image to the right). And this results in a less stable posture when they’re upright, which may cause them to fall forward during each step. 

^{Check out a video of one of the penguins walking on a treadmill above. Science Magazine}

^{Image in the text: The theoretical change in the location of the center of mass between a lighter (left) and heavier (right) king penguins in a standing posture. A.S.T. Willener et al., 2016 PLOS ONE}