Humans have done a lot to help rats become one of the most prolific mammals on the planet. We leave out food waste and trash. Our buildings and sewers provide cozy homes. We inadvertently help them cross oceans on ships. Now, burning fossil fuels and heating up the climate can be added to that list.
Rising temperatures seem to be helping rat populations grow faster, according to a new study in the journal Science Advances. Researchers found cities that have seen the most warming had more sightings of rats, an indication that the population has grown.
"Rats are fascinating creatures, but they come with very big issues," says Jonathan Richardson, professor at the University of Richmond and an author on the study. "One of the key takeaways I hope cities glean is that it's a problem that's going to require more resources."
Why rats are increasing
The main reason is one that many humans can relate to: when it's colder, rats are likely to spend less time outdoors searching for food.
"When the weather gets cold, that's a bigger challenge for these guys maintaining their own stable body temperature," Richardson says.
That means in the winter, rats are limited in how much time they spend above ground searching for food. Richardson and his colleagues looked at 16 cities to see what factors, including warming winters, affected rat population growth.
They found that denser urban environments and higher human populations helped boost the number of rats. Warmer temperatures seemed to be the biggest influence. Cities with the biggest growth rates were Washington D.C., San Francisco, Toronto, New York City and Amsterdam.
Richardson says it's likely because rats have more of a chance to reproduce if winters are getting shorter.
"If the weather stays warmer a little bit later into the fall or into the winter, or if the spring starts a little bit earlier, just by a week or two, that should allow the rats to be above ground, forage for a little bit longer, secure more food resource and presumably turn that into baby rats," he says.
Even a week or two of abnormally warm temperatures during the winter could give rats a chance to replenish their food stores and search for that extra slice of pizza, helping them be more successful.
Richardson says climate change could pose an additional challenge for cities trying to manage rats, many of which have programs that are already under-resourced and understaffed. Rats are known to spread pathogens, as well as damage infrastructure. They can also cause a mental health burden for people living with rat infestations.
Garbage in = rats out
"I think this study is a great first look," says Kaylee Byers, assistant professor of health sciences at Simon Fraser University in Canada, who was not involved in the research. "I definitely think we need to be thinking about how our changing cities are going to impact rat populations and climate change is one part of that."
Byers says the challenge of pinpointing what's going on with rats is that no one knows exactly how many there are. This study used the number of complaints filed by the public as a gauge for how much rat populations were increasing, but it's an imprecise proxy.
"Not everyone is going to call in a rat complaint to the city," Byers says. "If you live in an area where you see rats all the time, you're probably not calling everyday. Whereas if you're in an area where there aren't a lot of rats and you see one, maybe that's noteworthy enough for you to call."
Tracking and understanding rat populations is key for managing them, Byers says. Rats can be spread unevenly throughout cities and much like people, are very loyal to their neighborhoods, some never traveling more than 100 feet away from home. And today's management strategies are falling short in many places.
"We're often focused on this 'see a rat, kill a rat' mentality and — spoiler — that's not working," she says.
The key for controlling rats is controlling ourselves, Byers says, by better managing food waste and restricting access to trash ("garbage in = rats out"). New York City, long-known for its piles of plastic trash bags and chronic rat problem, has instituted new rules about minimizing the amount of time trash is sitting on the curb and is piloting new trash containers.
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