Cats are no strangers to killing rats. Their work as vermin exterminators helped them to sail the seven seas, eliminating rats on ships as far back as 9,000 years ago.

Centuries of domestication have not dulled cats' desire to kill rats, and as rat populations swell in some cities, those cities are turning to feral cats to keep the rats in check.

Last November, one Chicago alderman, for instance, suggested that the city's Department of Streets and Sanitation bring on feral cats to deal with the city's rat population.

And the Windy City's rat population is a legitimate issue. Not only has the city been named the U.S.'s rattiest city by the Orkin pest control company for three years running, but Mayor Rahm Emanuel announced his 2018 budget would dedicate $1 million and five crews dedicated to exterminating rats. But would those proposed feral cats be worthwhile? Could managed bands of cats help a city recover the same way one or two cats might keep a ship vermin-free?

Give feral cats jobs

A cat carries a rat in its mouth Feral cats can be helpful in controlling rat populations, according to some. (Photo: Romuald Cisakowski/Shutterstock)

Plenty of individuals seem to think so, and a few organizations agree and are keen on the idea as a "green" way to exterminate rats while also keeping shelter cats alive.

The Tree House Animal Society in Chicago and the Humane Rescue Alliance in Washington, D.C., both offer feral cats backyard rat killers, with programs called Tree House Cats at Work and Blue Collar Cats.

Both programs supply businesses or homes with a small colony of two to three feral cats that have been vaccinated and either spayed or neutered. The "employers" then help the cats acclimate to their new surroundings by housing them in long dog crates, feeding them tasty food and providing them with comfy shelters. This is done to ensure the cats, once they're roaming around, will feel incentivized to return to a home base instead of simply running off or joining another colony.

After about a month, the cats can be released from the crates to begin their "work." In addition to killing the rats, the very presence of the cat can deter rats from infesting an area. The cats' urine and as well as leaving their scent on things by rubbing against them marks a territory, and these smells cause rats to think twice about even stepping near such an area.

Once the cats are out and about, it falls to the "employers" to keep the cats happy and healthy, routinely feeding, housing and supplying a safe place to use a bathroom, in addition to covering any vet visits they may need.

One Chicagoan, Victoria Thomas, told CNN in 2016 that once she hired Tree House Cats at Work, she "instantly saw the rat holes just, they were vanishing." Prior to housing the colony, Thomas spent close to $4,000 on various efforts to stop rats from overtaking her yard to no avail, including wire fences and poisoned cinnamon rolls.

Advocates see these programs as a boon to the environment in a number of different ways. It keeps feral cats out of shelters and doing what they do best; it controls the rat population and it does so without relying on things that might hurt the environment, like rat poisons, which can infiltrate the food chain and cause harm to other animals like owls and hawks.

"As far as rodent control goes, it's nearly 100 percent effective," Liz Houtz, the community manager for Tree House Cats at Work, said to CNN. "It's the only long-term, permanent solution there is."

Too much of a good thing?

A cat waits atop a bird house for some prey Cats can be opportunistic hunters. And, apparently, clever ones, too. (Photo: HildeAnna/Shutterstock)

Not everyone is convinced that letting feral cats have the run of a neighborhood is for the greater good, however.

When the alderman inquired about feral cats working for Chicago, the city's Streets and Sanitation Commissioner, Charles Williams, had this say:

"What you're talking with feral cats, you're putting an animal into an environment to get rid of an animal. It may get rid of that other animal, but then you're stuck with that animal," he said. "Plus, the feral cats can be somewhat aggressive. I wouldn't want the city to be associated with putting an aggressive animal on the street that could end up hurting someone's child."

And those who engage the services of Cats at Work or Blue Collar Cats are "stuck" with them. One of the conditions of taking on the colony is that you maintain the cats for the rest of their lives.

As for Houtz's claim that feral cats are effective, the science is iffy on that point. A 2009 study published in PLOS One found that Baltimore's feral cats had a minimal impact on the city's rat population and would scavenge from the same resources as the rats they should have been hunting. One of the reasons? The rats may simply be too big for the cats to hunt. But it's worth noting that the study focused on house cats in addition to feral cats, potentially skewing the results.

Still, the study was used by Grant Sizemore, the director invasive species programs at the American Bird Conservancy in 2017. Writing in the Washington Post, Sizemore contends that since cats are "opportunistic predators," they may hunt and exterminate more than just rats; they're also more than happy to hunt birds that may require conservation protections. Sizemore also argues that cats pose public health risks, including rabies and infection from the parasite Toxoplasma gondii.

Like Houtz, who runs a feral-cat-for-hire program and wants it to work, Sizemore has a vested interest in protecting birds from an admittedly difficult to regulate and control feral cat population. Regardless of perspectives, the debate surrounding feral cats as backyard and business guardians highlights how complicated this particular issue can be. A city isn't a ship, after all. A ship is a small, controlled environment with limited resources and limited impacts. A city is a complex ecosystem with a number of different interests and links in it, all of which can effect one each other.

It's important to weigh all those considerations before embarking on a course of action that significantly alters that ecosystem.