Managing Rat Populations With Birth Control
Rats pose a problem to human societies by destroying and contaminating food and spreading disease. For these reasons, they are typically considered “pests.” One common method of rat population control is the use of poisons known as “rodenticides.” These are often anticoagulants, which work by preventing blood clotting. Over the span of 5-7 days, this can cause death by organ failure or blood loss due to internal bleeding. Other non-anticoagulant rodenticides can cause brain or organ damage.
Ingestion of rodenticides by rats causes a slow and painful death. These chemicals can also have indirect effects, as predators who prey on inflicted rats can themselves be poisoned. Furthermore, these chemicals can be accidentally ingested by humans or companion animals in the home. There is therefore good reason to find safer and more humane alternatives to rodenticides. In this paper, the author considers the potential use of birth control as a humane method of decreasing rat populations.
When talking about rodent birth control, we normally mean chemicals that can reduce fertility. Like poisons, birth control methods could be relatively convenient to use. Both can be placed in tactical locations where rats are likely to find and consume them. In theory, birth control could decrease the number of rats in an environment without causing suffering or death. The first commercially available rodent contraceptive, ContraPest, appeared in the U.S. in 2018.
Chemical birth control methods involve little contact with rodents, and they might help protect the welfare of wild animals. This is true both for rats and for their predators. Beyond this, reducing fertility would probably cause less disruption to rats’ social lives and territories, compared to killing a colony and allowing another to move in. If birth control methods can be shown to be effective, this might help in banning more cruel methods of population control.
However, there are some downsides to this approach. While rodenticides can work over the span of days, birth control will take months to reduce rat populations. This also needs to be a long-term and dedicated strategy, which might become expensive over time. All of this might be unappealing to consumers. Like poisons, these chemicals may be indirectly ingested by predators, leading to reduced fertility. New birth control products also need to go through an extensive and time-consuming approval process.
There are other methods besides ContraPest that might be effective in humanely reducing rat populations. “Resource reduction” involves making it harder for rats to access garbage and food sources that are used to sustain colonies. This basically means removing opportunities to reproduce, without actually killing rats. While possibly humane, this method only works with a lot of ongoing coordination. “Botanic sterilants” are plant extracts that reduce rat fertility. These are probably more humane than poisons, but we don’t know how safe they are (e.g., for dogs). Approval of these products could also be quite slow and expensive.
Many animal advocates hope for a single-dose “sterilant” that could make rats permanently infertile. This could potentially be a quick and humane way of reducing populations. At the moment, there is no single-dose solution available. There may be some benefits here, but a multi-dose approach may be favored in situations where people want to manage rat populations without reducing them too much (for example, where local predators rely on eating rats).
Rodent birth control could lead to large-scale reductions in suffering. The author predicts that if New York City were to switch from rodenticides to entirely ContraPest, around 1.5 million rats (3/4 of the population) could be spared from poisoning each year. However, SenesTech, the company behind ContraPest, recommends first using rodenticides followed by ContraPest for the best results. The author argues that there are a number of other, potentially less cruel methods that could be used in place of rodenticides if a lethal method is needed. These include carbon monoxide, bolt traps, snap traps, electronic traps, dry ice, and drowning. Unfortunately, many of these methods still cause suffering, and some may have unwanted effects on predators or humans. However, they are still likely preferable to rodenticides.
In the city of Newton, Massachusetts, a surge in rat populations during the COVID-19 pandemic has been combated using a combination of lethal carbon monoxide, rat-proofing new constructions, and contraception. The author acknowledges that animal advocates may not feel comfortable supporting any scheme that uses lethal methods. However, in terms of reducing rat suffering as much as possible, methods like this are arguably much better than just using rodenticides. Many cities will likely use lethal methods anyway, so supporting the “least bad” option here might be the most practical way to reduce suffering.
Overall, advocating for the wide-scale use of rodent birth control methods could be an effective way to campaign for animal welfare in rat management. This could pair well with advocating for the ban of inhumane methods of pest control, like rodenticides.