Why do animals in the alpine generally have higher levels of hemoglobin?

High-altitude environments present a number of physiological challenges for endothermic animals, as they are characterized by a lower partial pressure of oxygen () and lower ambient temperatures compared to low-altitude environments at similar latitudes. The reduced  at high altitude results in reduced oxygen loading in the lungs such that the blood may not carry a sufficient supply of oxygen to the cells of respiring tissues (Bencowitz et al. 1982; Bouverot 1985; Turek et al. 1973). This reduced level of tissue oxygenation can impose severe constraints on aerobic metabolism and may therefore influence an animal's food requirements, water requirements, the capacity for sustained locomotor activity, and the capacity for internal heat production.

Very hard to understand