Calcium-45 Labeling of Mammals for Use in Population Studies

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Abstract
Techniques are described for labeling young thirteen-lined ground squirrels, cottontail rabbits and snowshoe hares with 45Ca. The isotope was implanted subcutaneously into adult females during the breeding season and subsequently passed to their offspring. The labeled young were used to make spring population estimates by means of the “Lincoln Index,” and to determine movements. This technique gave increased numbers of marked animals per marking effort and reduced the trap bias that is common in trap-retrap census methods. Thirty microcuries of 45Ca implanted in a female thirteen-lined ground squirrel labeled her offspring for a year or more after birth. Sixty microcuries of 45Ca labeled all four litters of snowshoe hares and could be detected until the September following birth. Sixty microcuries of 45Ca were in most cases sufficient to label all cottontail litters, but 175 μCi insured labeling fifth and sixth litters and prolonged the time of detection. The petrosal bone of the inner ear had the slowest turnover of 45Ca and was thus the best bone for detecting the presence of radioactivity. By varying strengths of implanted capsules or times of implant in relation to parturition, littermates among thirteen-lined ground squirrels could often be identified by amounts of radioactivity present. Spring population estimates were obtained on study areas for snowshoe hares and ground squirrels by means of 45Ca labeling.

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