The demographic consequences of fertility reduction in rats and voles

Rodent population control is a global problem, complicated by evolved non-responsiveness to rodenticide treatment. Contraceptives could help mitigate this challenge, but questions remain about their efficacy, especially for rodenticide-resistant populations. We used an age-dependent demographic model to generate two hypotheses: Fertility reduction applied early in female lifetimes (1) is more effective in controlling rodent populations than when applied later in female lifetimes, and (2) is effective in controlling rodent populations that are expanding. Compared to controls, fertility reduction applied early, in mid-life, and late in female lifetimes, decreased, matched, and accelerated, respectively, the rates of population growth. Fertility reduction was effective in reducing population size only when sustained over multiple generations and was ineffective when application was episodic. Substituting classic Rattus norvegicus and Microtus agrestis life history data into our simulation framework confirmed that early fertility reduction was effective in controlling population growth, including expanding populations in both species. These simulations generated two additional hypotheses for field applications of fertility control: Over treatment durations, (3) the fraction of the population consisting of juveniles, and (4) the overall population size, will both decrease. We tested these predictions using a 12-month contraceptive bait application on rats in two urban US locations (Washington, DC) where rodenticides were already deployed. Consistent with our predictions, these populations showed marked decreases in the proportion of juvenile to adult rats, and in the total number of rats observed in camera traps over the study period. Our results support fertility control as an effective method for managing rodent populations.