Beaver Colony Response to Fertility Control: Evaluating a Concept

Although breeding activity in a colony of beaver (Castor canadensis) usually is confined to an adult pair, sexually mature male and female progeny are present and may reproduce if 1 or both adults are removed. To assess behavior and reproduction in such progeny, 1 adult in each of 18 colonies was either surgically sterilized or sham-operated in summer 1975. Sterility was induced by either tubal ligation (56, 59 in 10 colonies) or castration (26, 29 in 4 colonies); sham operations were performed in 4 colonies (2d, 29). Behavioral and reproductive responses were normal in sham-operated controls, but 11 of 13 sterilized colonies had no reproduction by beaver. The 14th colony dispersed before parturition could have occurred. Reproduction occurred in 1 of 10 colonies with ligated adults, 1 of 4 with castrated adults, and 4 of 4 with sham-operated adults. Sterilization of either adult in a colony significantly reduces fecundity, yet permits maintenance of an established colony with its associated benefits. Breeding inhibition is discussed with respect to population density, dispersal of 2-year-olds, and beaver management. J. WILDL. MANAGE. 44(3):568-575 The reestablishment of beaver over much of their ancestral range has precipitated an array of man-beaver conflicts that will intensify as beaver increase their range and numbers. Fecundity control through induced sterility has been advocated for a variety of vertebrate pest species (Davis 1961, Knipling and McGuire 1972), including beaver (Arner 1964). Although there have been several attempts to develop this technique for beaver control (Blanchard 1964, Harper 1968, Nevers 1968, Gordon and Arner 1976), results were inconclusive and feasibility was untested. Theoretically, beaver should respond well to reproductive control because they are long-lived (Larson 1967), are believed to be monogamous (Novakowski 1965, Boyce 1974), and form discrete family units consisting of an adult pair and progeny of 2 successive years (Novak 1977). Yearlings and old r progeny, though sexually mature, usually do not breed while in the parent colony unless 1 or both parents are removed (Payne 1975). Our study (Brooks 1977, Fleming 1977) was designed to evaluate colony response when 1 member of the breeding pair is sterile. Technical assistance of C. E. Boardman and W. E. Dodge; and cooperation of the Metropolitan District Commission, Boston, administrators of the study area, are appreciated.