Beaver Productivity in Idaho

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Abstract
Data on the productivity of beavers (Castor canadensis), including sex and age ratios, litter size, and rate of pregnancy, were gathered from livetrapping and fur-trapping operations in 1953-56 and 1962-64. Standard methods were used for determining sex and age of both live beavers and carcasses. Uteri hardened and discolored by preservatives were cut longitudinally to expose breaks on the internal ridge of tissue which pointed out sites of placentation. Counts of placental scars and fetuses indicated average litter size to be 3.1 and 3.4, respectively. The percentage of yearling and older females with placental scars dropped from 45.2 in the mid-1950''s to 32.3 in the 1960''s in southeastern Idaho, possibly owing to increased disturbance of colonies from accelerated fur trapping. Males consistently outnumbered females in kit and yearling age-classes while females were more abundant among the adults. The sex ratio of 352 beavers examined was 113 males per 100 females. Twenty-one percent of these animals were kits, 23 percent yearlings, and 56 percent adults. A disturbed (trapped) beaver population had a lower percentage of kit and yearling beavers than did an undisturbed population (35 percent vs. 49 percent). A highly migratory population consisted of 38 percent late yearlings, supportingthe belief that beavers leave their home colony when almost 2 years old.

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