Mammals acclimatize to reduced oxygen at high altitudes through physiological and genetic adaptations, including increased red blood cell production, enhanced lung capacity, and changes in haemoglobin structure and function, all aimed at improving oxygen delivery to tissues. 

In order to compensate for lower oxygen levels, the body increases the production of red blood cells (erythrocytes), which carry oxygen in the blood. This allows for a greater capacity to transport oxygen to the tissues.