The beaver (Castor canadensis Kuhl) is subjected to a widely fluctuating energy regime in the northern portion of its distribution. During the summer the animal has free access to an abundant food supply in the form of growing plant material, while during the winter the food supply is limited to a store of cached saplings. The working hypothesis of this study was that seasonal shifts occur in energy expenditure such that it is highest during the summer when an abundant food supply is readily

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... ilable. In the Mackenzie Delta, Northwest Territories, growth was found to be rapid in the summer and absent in the winter. A winter weight loss characterized immature animals. Fat was deposited in the autumn, maintained during the winter and mobilized in the spring. Animals were lean during the summer. Thyroid gland weights were high in the summer and low in the winter. It was concluded from these data that metabolic energy expenditure is high during the summer and low during the winter. A consideration of possible extrinsic causes of this annual pattern and the finding that the beaver ceases to grow during the winter when on a constant ration made available ad libitum led to the conclusion that the pattern is an inherent property of the beaver at northern latitudes. The thyroid gland was hypothesized as the major effector of the annual pattern within the organism. Light intensity was hypothesized as the environmental factor that times the level of energy expenditure to environmental conditions. No major seasonal changes in thyroid activity, food intake or growth were observed in California beavers maintained under Vancouver climatic conditions and a constant ration made available ad libitum, but Arctic beavers maintained under the same conditions showed a growth cessation, a 40% reduction in food intake and a depression in thyroid activity during the winter. This is consistent with the conclusion that the annual metabolic pattern observed in northern beavers in the field is an inherent attribute. Manipulation of lig [...]