Holding Your BreathHemoglobin and myoglobin are widely responsible for oxygen transport and storage (see the Perspective byRezende). The ability of diving mammals to obtain enough oxygen to support extended dives and foraging is largely dependent on muscle myoglobin (Mb) content.Mircetaet al.(p.1234192) found that in mammalian lineages with an aquatic or semiaquatic lifestyle, Mb net charge increases, which may represent an adaptation to inhibit self-association of Mb at high intracellular concentrations. Epistasis results from nonadditive genetic interactions and can affect phenotypic evolution.Natarajanet al.(p.1324) found that epistatic interactions were able to explain the increased hemoglobin oxygen-binding affinity observed in deer mice populations at high altitude. In mammals, the offloading of oxygen from hemoglobin is facilitated by a reduction in the blood's pH, driven by metabolically produced CO₂. However, in fish, a reduction in blood pH reduces oxygen carrying capacity of hemoglobin.Rummeret al.(p.1327) implanted fiber optic oxygen sensors within the muscles of rainbow trout and found that elevated CO₂levels in the water led to acidosis and elevated oxygen tensions.