The energy expenditure required for flight increases proportionally with body size, which is often why flightlessness coincides with body mass.[2] By reducing large pectoral muscles that require a significant amount of overall metabolic energy, ratites decrease their basal metabolic rate and conserve energy.[19][20] 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 exhibit an intermediate basal rate.