The Population Dynamics and Behavior of Beavers in the Sierra Nevada

DOCTORATE DISSERTATION: The population dynamics and behavior of a populationof beavers, Castor canadensis, at Sagehen Creek, Nevada County, CA, were investigated from June 1977 through October 1979. A total of 29 beavers from four colonies were live-trapped, assigned to age classes, sexed, marked with color coded ear tags for individual identification, fitted with radio transmitters if older than kits, and released. The primary objective of the research was to examine the similarities and differences in behavior among members of the various age-sex classes. Resting associations and locations for all radio-tagged animals were also ascertained. The population size increased by 6 individuals, and increases in population density, average length of stream occupied per colony, total length of stream occupied by all colonies, and intercolonial movement occurred during the research. The sex ratio increased from 1 male:1 female in 1977 to 1.5 males: 1 female in 1979, which resulted from the presence of four new young adult males living at the borders of the existing colonies during the summer of 1979. The age class composition also changed, with more adults and two-year olds and fewer kits present in 1979 than in previous years. Three of the four colonies were typical family groups, consisting of the mated adult pair, several yearlings and the young of the year. Reproduction occurred all three years, but not in every colony, and the average litter size was 2.33 kits. Only one known mortality occurred and this was a male kit, although several subadults of both sexes disappeared from the population. Dispersal of the subadults is thought to regulate population size, although four two-year old individuals remained with their parent colonies at the termination of the research. The observed population changes suggest that the carrying capacity of the habitat for beavers has been reached and that the growth of the population should slow down or cease during the next few years. Seventeen types of behavior, which were grouped into three categories based on biological function, were recorded. Younger animals had a higher rate of occurrence of personal maintenance and social behaviors, while older animals had a higher rate for colony maintenance behaviors. Adult males had the highest rate of colony maintenance during both the summer and fall, and males of all age classes (excluding kits) had higher rates of colony maintenance in the fall than females. Adult males are considered to invest in the family group primarily through the performance of colony maintenance behaviors. Kits had the highest rate of social behaviors and initiated the most encounters with other beavers. The adult females were involved in the most interactions and had the lowest frequency of submissive interactions. The adult males had the highest frequency of dominant interactions. An age class dominance hierarchy was present within the families, with older age classes always dominant to younger age classes. No evidence for a sexual hierarchy was found, and the adults within a colony were considered to be codominant. Beavers in all colonies used a number of rest sites, both lodges and bank burrows. Most colonies had one preferred rest site that was used by all members, while some colonies had two preferred rest sites. Use of rst sites and resting associations were affected by season of the year, number of young in the family, and specific age of the adults in the mated pair.