These costs are of great importance to penguins because some, such as the Emperor, may walk over 100 miles from their rookeries to the open sea after fasting for four months during the harsh Antarctic winter. An inefficient walk requires more energy and more food.
Evidently, Griffin said, penguins have made an evolutionary tradeoff. Their short legs make them more streamlined swimmers and divers, even if their walking is less efficient. Short legs may also help reduce heat loss, especially while incubating the eggs in winter.
The team's findings also have implications for a general theory of walking. The study provides strong evidence that the cost of generating muscular force to support body weight is an important determinant of the metabolic cost of walking. This supports a general theory that relates the mechanics and energetics of locomotion and which, until now, applied only to running gaits.
Kram and his late Harvard colleague C. Richard Taylor proposed this theory in 1990, before Kram moved to UC Berkeley. They suggested that running animals use energy at a rate inversely proportional to the time the foot applies force to the ground during each stride. Short-legged animals are in contact with the ground for a shorter time than long-legged animals running at the same speed, so their muscles must exert their force more quickly. Because faster muscles are less economical, and because the muscles of smaller animals have to exert larger muscle forces relative to body weight, smaller animals are less efficient runners.