Seasonal metabolic organization in the muskrat (Ondatra zibethica). I. Changes in growth, thyroid activity, brown adipose tissue, and organ weights in nature

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Abstract
Ten muskrats (Ondatra zibethica) per month were removed live from the natural environment for the duration of 1 year, in a preliminary study of seasonal metabolic organization. Growth in total body weight and skeletal dimensions occurs during the spring, summer, and autumn, but is absent during the winter. Thyroid activity, as reflected by 24-h I-125 conversion ratio, 24-h thyroidal uptake of injected I-125, and relative thyroid weight, is low during the winter relative to other seasons. The mass of interscapular brown adipose tissue, plus weights of water, lipids, and protein therein, are inversely related to seasonal changes in temperature. Percentage lipids in the tissue increases from August to April, and decreases from April to July; percentage water follows an opposite pattern. Percentage protein increases slightly during the winter. Dynamics of DNA content in the tissue suggest DNA synthesis occurs during the summer, rather than in the autumn when the tissue is enlarging. Adrenal weights increase slightly during the winter, and greatly during the spring and summer when the animals are about 1 year old. The liver and kidneys decrease in weight during the winter, while heart weight, lung weight, hematocrit, and blood hemoglobin increase considerably during that period. Thyroid activity and the mass of interscapular brown adipose tissue are inversely correlated from August to April (5% level).A number of hypotheses regarding seasonal metabolic organization are set forth on the basis of the above data.

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