Seasonal Augmentation of Myoglobin in the Snowshoe Hare.

Its prominence has long made myoglobin a substance of general interest. Distributional surveys have provided useful and suggestive correlations with repetitive types of muscle, notably cardiac, but a direct demonstration of the function of myoglobin has not been made except in the special case of oxygen storage in diving mammals. The concentration of myoglobin in muscle has been reported as increased by hypoxia in white rats and guinea pigs (1, 2), and it is also higher in native rodents from high altitude as compared with sea level subspecies (3). Exercise can increase its concentration (4), but it has also been recently cited as “a stable substance, not subject to wide variations even in conditions of anemia and deficient diet-in contrast with the labile character of circulating hemoglobin” (5). Modification of the concentration of myoglobin in a natural population has apparently never been described. Seasonal acclimatization may induce physiological differentiation between summer and winter animals (6, 7), and our object in this study was to measure levels of myoglobin in a species subjected to extreme ambient seasonal changes. The snowshoe hare, Lupus americanus, is a common mammal in central Alaska. It does not dig burrows underground which could reduce the effect of ambient factors such as light or temperature, being, in fact, the smallest species not relying on microclimatic evasion to avoid the full impact of the Alaskan winter (8).