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Rats thrive around humans, and for good reason: They feed on crops and garbage and adapt easily to many environments, from farms to the world’s largest cities. To keep them under control, humans often resort to poisons. But chemicals that kill rats can harm other animals too.

The most commonly used poisons are called anticoagulant rodenticides. They work by interfering with blood clotting in animals that eat them. These tempting-tasting bait blocks are placed outside buildings in small black boxes that only rats and mice can enter. However, the poison remains in the rodents’ bodies and threatens larger animals that prey on them.

My colleagues and I recently reviewed studies from around the world that sought to document exposure of wild carnivores to anticoagulant rodenticides. Many of the animals tested in these studies were already dead; others were still alive and participating in other studies.

The researchers found rodenticides in about a third of the animals they examined, including bobcats, foxes and weasels. In a third of the animals, they directly linked the poisons to death – usually by finding the chemicals in the animals’ liver tissue.

Most of the poisons these studies discovered were so-called second-generation anticoagulant rodenticides, developed since 1970. These products are used exclusively in residential and urban areas and can kill a rat or mouse after just one night. First-generation rodenticides, typically used only on farms, require multiple doses to kill.

These poisons are widespread and their use is largely unregulated in most countries. Rodenticide use is expected to increase and could contribute to the decline of many carnivore species around the world.

Climbing the food chain

When wild animals ingest rat poison – typically by eating a poisoned rat – the consequences can include internal bleeding and injury, lethargy, and a reduced immune response, which can make them more susceptible to other diseases. In many cases, the animal dies. Sometimes these deaths are so large that they reduce the local predator population.

We began our investigation by compiling a list of 34 animal species known to be exposed to rat poison. These included members of the weasel and canid families, such as stoats, polecats and red foxes, as well as wild cats and other carnivores.

Some of these predators, such as mountain lions and gray wolves, do not normally hunt rodents. Rodenticides have even been detected in semi-aquatic predators such as river otters, which normally eat crustaceans and fish.

It is likely that large carnivores such as wolves ingest rat poison by eating other poisoned carnivores such as raccoons and bobcats.

This movement of poisons up the food chain is called bioaccumulation. The most well-known example is bald eagles and other birds of prey that were exposed to the pesticide DDT in fish they consumed before the United States banned DDT in 1972. Many affected species, including bald eagles, ospreys, and peregrine falcons, suffered drastic declines in their populations for years due to the effects of DDT on their populations.

Carnivores in danger

We found dozens of previous studies that attempted to quantify exposure risk, typically by examining the animals’ habitats. Some studies found an increased risk of consuming rat poison in urban and agricultural areas, but many also found a high correlation with natural spaces.

For example, a 2012 study found rodenticides in martens and rodent martens living near illegal cannabis cultivation areas in Humboldt County, California, because the growers protected their fields with rat poison.

Other possible factors for infection were the sex and age of the animal. All in all, further studies are needed to find out which animals are at risk.

Most research on this topic is conducted in North America and Europe. Only a handful of studies have focused on South Africa, New Zealand or Australia, even though over half of the world’s threatened carnivore species are found in Asia, Africa or South America.

In Africa, for example, anticoagulant rat poisons could threaten species such as the endangered black-footed cat. These poisons are also widely used in Asia, particularly on palm oil plantations. These forested agricultural areas are home to many wild species, including carnivores that hunt rodents, such as the palm civet and Bengal cat.

Our study found that 19% of predator species on the International Union for Conservation of Nature’s Red List of Threatened Species have their ranges partially or completely overlapping with countries where rat poison has been detected in wildlife. However, rodenticides are listed as a recognized threat in only 2% of Red List species, and none of these are among the 19% that our research suggests may be threatened by rodenticide exposure. This suggests that wildlife researchers and conservationists are not fully aware of the reach of these poisons.

Kiawah Bobcats

I am conducting my dissertation research on Kiawah Island in South Carolina, where biologists have detected rodenticides with anticoagulants in bobcats. The island’s bobcats have been GPS-collared and monitored since the early 2000s as part of one of the longest multigenerational studies of a carnivore in the world.

In late 2019 and early 2020, three bobcats were found dead from rodenticide poisoning, including two females who died in childbirth. The bobcat population plummeted from an estimated 30 to just 10 individuals. These deaths attracted media attention, sparked efforts to restrict poison use on the island, and spurred research to understand how rat poison affects bobcats.

Kiawah is a popular vacation destination, but these bobcats have persisted despite decades of development. Part of my work is trying to figure out how rodenticides and urbanization affect the cats.

In 2020, Kiawah residents volunteered to stop using rodenticides on the island, and the city government conducted educational campaigns to inform the public about the threat to wildlife. Today, there are about 20 bobcats on the island, and work to end rodenticide use continues.

These poisons have contributed to the deaths of other charismatic animals, including urban pumas in Southern California and Flaco, an eagle owl that escaped from New York’s Central Park Zoo and lived in the park for months. In Europe, rodenticides have been found in the carcasses of Italian wolves.

Rats damage property, contaminate food and spread disease, making their control a human health concern. However, my research provides further evidence that better control methods are needed to reduce the need for anticoagulant rodenticides.

Community-based efforts like those on Kiawah Island can help. So can urban trash cleanup. But many places around the world may need better regulation and oversight of rat poison use.