Highland deer mice and their lowland cousins ventured on a simulated seven-week ascent to 6,000 meters. By tracking how the mice responded to cold stress and progressively lower oxygen levels, the study tested whether deer mice have a generalized ability to acclimatize or whether some mice have evolved adaptations to local conditions. The highland mice were better able to regulate their body temperature, an indication that mice living at higher altitudes have evolved distinct ways to acclimatize to low oxygen conditions.