Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)


Wildlife and Fisheries Science


One hundred sixty two beaver (Castor canadensis) harvested by trappers from the free-running Missouri River in southeastern South Dakota were examined from fall 1974 to spring 1976. Ninety two were males and 70 were females (131:100 sex ratio). Age specific sex ratios were not significantly different (P >0.05) from a 1:1 ratio for all age classes. The age structure of the sample was 69:26:45:15:4:3 for age classes zero through five, respectively. Minimum breeding age for female beaver was 2.5 years. Seventy-eight percent of all females (>2.5 years old) collected after the breeding season contained corpora lutea and viable fetuses. The mean number of corpora lutea was 4. 9 1 per breeding female; the mean number of viable fetuses was 4. 62 per breeding female. No statistical difference (P > 0.05) in corpora ]utea or viable fetuses existed between the age classes. The observed overall resorption rate was 5.8 percent. Survival estimates were calculated using time specific life table analysis treating the age structure of the harvested sample as representative of ages at death in the beaver population. Female survival was 60 percent in age class 0, 69 percent in age class 1, and 34 percent in age classes 2-5. Male survival was 55 percent in age class 0, 74 percent in age class 1, and 31 percent in age classes 2-5. Three estimates of the annual rate of population change were made: +17 percent (using a life equation), -39 percent (using a population model) and -15. 3 percent (using a colony count index). Two estimates of colony size were applied to the mean number of colonies in the study area to estimate population density at 185-272 beaver in the study area. Mean colony density was significantly higher (P < 0. 01) in the unstabilized subsection. Comparison of physical features between the subsections of the river showed a significantly higher (P < 0. 01) current speed 2 m from the shoreline in the stabilized subsection and no significant difference (P > 0. 05) in river depth at 2 m from the shoreline. Distance from beaver dens to standing willows (Salix spp.) and cottonwoods (Populus deltoides) was not significantly different (P > 0.05) between the stabilized and unstabilized subsections. Beaver were selective for shoreline areas with slow current speed and deep water when choosing a den site, and beaver preferred low-tapered banks when compared to a high-cut, eroding banks or a granite-lined, stabilized banks.

Library of Congress Subject Headings

American beaver -- South Dakota
Missouri River Watershed



Number of Pages



South Dakota State University