Abstract
The wild rat poses a universal problem in urban a well as rural areas. It is estimated that as much as 10% of this country's entire grain crop is consumed or contaminated by rats and mice (U.S. Food and Drug Administration, 1957) and that 50% of our grain shipped to India is destroyed by these pests before it can reach the people of that country. Efforts to control these pests with poisons have been inadequate. The more potent rodenticides have a repugnant taste co the rat, and consequently the animals fail to take a lethal dose. The development of 'warfarin1, a tasteless anticoagulant rodenticide, has been followed by the development of genetically transmitted 'warfarin' resistance in the rat (Pool et al., 1958) . It has been suggested that chemosterility may be an effective method of regulating the sizes of these animal populations. Although chemosterilization would not eliminate these rodents, it is probable that their numbers could be significantly reduced to non-problematical levels by natural population mortality factors coupled with reduced recruitment from reproduction. This concept is based upon the premise that the sterile individuals exert a more potent controlling influence upon a population than if the same number of fertile individuals were removed, since the sterile individuals presumably continue to compete for territory, food, social order and sexual partners (Howard, 1967). If sterilized individuals possess unimpaired libido, they will mate within the social framework and will exert a stabilizing influence on the local population. Stabilization and subsequent reduction of the rodent population from the carrying capacity to a lower level results from a decrease in the size of the female breeding pool. The decrease in numbers of breeding females is based on an increase in pseudopregnancies induced from coitus with sterile males. The promiscuous behavior of the female wild rat, (Calhoun, 1962), however, requires sterilization of approximately 90-95 percent of the males in the population to insure a high percentage of sterile-male matings and effective reduction in the size of the female breeding pool (Johnson, personal communication).