Beaver Productivity in Idaho

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 46.2 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, supporting the belief that beavers leave their home colony when almost 2 years old. This paper summarizes data on beaver productivity collected in Idaho in two separate periods, 1953-56 and 1962-64, and adds comparable data to the beaver literature from many other states. A method is also described for delineating placental scars on uteri that have been hardened and discolored by preservative solutions. Gratitude is expressed to the many beaver trappers who collected uteri and counted fetuses and to conservation officers and other personnel of the Idaho Fish and Game Department who distributed preservatives and solicited trappers' cooperation. Thanks are extended to Dr. Paul D. Dalke for his counsel during the portion of the work done by the senior author and for reviewing this manuscript. Dr. Edson Fichter and Levi L. Mohler also reviewed the manuscript and contributed editorial suggestions. Statistical assistance was provided by Drs. Robert Giles and Dale Everson. MATERIALS AND METHODS Collection of Materials and Data State trappers collected uteri of beavers during the fall trapping seasons of 1953 through 1956. They counted fetuses during the spring trapping season in 1956. The materials and data were collected throughout the state and sent to the junior author for analysis. During the 1962-63 fall through spring trapping season, two beaver trappers in southeastern Idaho, John Kindall and Warren Moore, collected uteri and counted fetuses preliminary to a study of productivity and movements by the senior author (Leege 1964). In the 1963-64 season, additional reproductive tracts, fetus counts, and data on sex and age ratios were gathered by Leege with the assistance of these two trappers. Beavers were livetrapped during the summer months in 1962 and 1963 in portions of Bear Lake and Caribou counties in southeastern Idaho. One hundred ninety1Contribution from the Idaho Cooperative Wildlife Research Unit; The College of Forestry, Wildlife and Range Sciences, University of Idaho; the Idaho Fish and Game Department; the Wildlife Management Institute; and the U. S. Fish and Wildlife Service, cooperating; and Idaho Fish and Game Department P.-R. Project W-108-R. 2 Present address: Idaho Fish and Game De-