Adult king penguins sometimes fast for up to a month at a time when protecting their eggs, but they compensate by packing on several pounds of fat in advance. The added weight helps them survive while attending to their parental duties, but a new study suggests that fatter birds may have a tougher time staying upright as they walk to the breeding colony. Researchers captured 10 adult male king penguins (Aptenodytes patagonicus) from a colony in the Crozet Archipelago between Madagascar and Antarctica, intentionally choosing males with high body mass. Then, using an accelerometer that records motion around three axes, they studied the gaits of a group of 10 king penguins as they walked on an enclosed treadmill at 1.4 km/hr (as seen in the video above, which the scientists captured for a different study of penguin locomotion but demonstrates the experimental setup). After the first round, the team placed the plump penguins in a cage to fast for 2 weeks. On average the birds lost a little more than 2 kilograms—about a quarter of their total mass—and then the researchers repeated the treadmill experiment. Previous studies have shown that obese and pregnant humans readily change their walking gaits in response to weight gain, widening their stance and taking shorter steps. But to the scientists’ surprise, the penguins didn’t change their stride frequency or posture after the weight loss. However, they did discover decreases in the penguin’s waddling amplitude, meaning the heavier birds had a more exaggerated waddle. Similarly, scientists saw greater amplitude differences in how far the penguins leaned over relative to the ground. Both findings suggest that the penguins are less stable with all the added weight—teetering more dramatically, like a person carrying a heavy box—and may explain past observations that heavier birds are more likely to take a tumble on their trek to the breeding colony, the team reports today in PLOS ONE.