Flightless bird that require a significant amount of overall metabolic energy, ratites decrease their basal metabolic rate and conserve energy. A study looking at the basal rates of birds found a significant correlation between low basal rate and pectoral muscle mass in kiwis. On the contrary, flightless penguins exude an intermediate basal rate. This is likely because penguins have well-developed pectoral muscles for hunting and diving in the water. For ground feeding birds, a cursorial lifestyle is more economical and allows for easier access to dietary requirements. Flying birds have different wing and feather structures that make flying easier, while flightless birds'
Ratite as a whole probably had an origin in the northern hemisphere. Early Cenozoic northern hemisphere paleognaths such as "Lithornis", "Pseudocrypturus", "Paracathartes" and "Palaeotis" appear to be the most basal members of the clade. The various ratite lineages were probably descended from flying ancestors that colonised the south in multiple waves, probably initially in South America, and then evolved gigantism. Loss of flight allows birds to eliminate the energetic cost of maintaining flight-enabling pectoral muscle mass. The basal metabolic rate of volant species is much higher than that of flightless terrestrial birds. But energetic efficiency can only help explain the loss