As graceful as penguins might be underwater, on land they're not the most efficient walkers, and characteristically waddle around on their super-short feet. But the waddling is actually what keeps them stable - unless they get too chubby.
The sub-Antarctic species of king penguins (Aptenodytes patagonica) sometimes need to trek quite a distance from the shore - where they forage for fish - all the way to their breeding colonies. Any time on land, especially when rearing chicks, is usually spent fasting, so penguins tend to pack on several extra kilograms of fat before proceeding with courtship.
When humans gain weight or become pregnant, our gait changes - but such changes haven't been much studied in animals. Researchers from the University of London wanted to find out whether weight gain in penguins affects their stability, which could impact their agility on land when it comes to predators.
To find this out, the scientists actually built a treadmill - instead of exercise, it was used for measuring the penguins' gait.
The second largest penguin species after Emperor, king penguins average nearly 100 cm in height and tend to weigh around 15 kg.
The scientists, lead by Dr Astrid Willener, captured ten heavy male penguins and monitored their treadmill performance twice - before and after a 14-day fasting period, which caused them to lose nearly a quarter of their body weight.
The treadmill was set at a speed of 1.4 km/h (in the video their movement is greatly sped up), and the researchers used an acceleration data logger to calculate changes in their waddling posture.
The results showed that overall penguins are quite adapted to walking even at a heavier weight, although they did become somewhat less stable on their feet.
“Waddling amplitude, leaning amplitude and leaning angle all remained fairly constant across the two body masses,” the authors wrote in the study recently published in the open-access journal PLOS ONE.
“However, some differences were uncovered; in particular there was good evidence that variability in the leaning angle and leaning amplitude, and some evidence that the waddling amplitude, were lower when the birds were lighter. These results indicate that heavier king penguins have a higher frontal and sagittal instability; they are less stable walkers than when they are lighter.”
“The link between gait and energy expenditure can help to improve penguin protection,” Willener told The Guardian. “The energy expended during their walk, particularly when stressed and responding to predators, may affect their ability to fast and protect their chicks.”