Final answer:
The improved fitness of non-native visitors to high altitudes is due to physiological adaptations like increased red blood cells and hemoglobin concentration, which help compensate for the low oxygen levels in such environments.
Step-by-step explanation:
The secondary responses of increased fitness observed among non-native visitors to high altitudes are often attributed to physiological adaptations to the low-oxygen environment. These adaptations can include an increase in the number of red blood cells, hemoglobin concentration, or breathing rate to compensate for the reduced oxygen availability.
Different populations worldwide have developed unique adaptations. For example, Tibetans may increase their breathing rate while Andean highlanders have higher hemoglobin levels. These responses aid in maintaining oxygen delivery to tissues and can contribute to the improved fitness of individuals who spend extended periods at high altitudes.
However, it is essential to note these adaptations may not be as robust in non-native visitors compared to local indigenous populations who have had generations to genetically adapt to these conditions.