Essay by Eric Worrall

My question – if warmer temperatures cause more rats, why aren’t the tropics permanently overrun with vermin?

Rats worldwide are enjoying the perks of climate change

A new study has linked increasing temperatures to growing rat populations.
By Dr. Christopher Wachuku
February 1, 2025, 7:23 AM

Climate change is contributing to a global rise in urban rat infestations, according to a new Science Advances study.

As temperatures increase, rats are better able to thrive — even in inclement weather that would typically deter the population’s growth, the article explained.

…

“So we imagine it probably increases their survival over the winter. And we’re pretty sure that that increased food intake also will lead to more reproductive bouts for these rats, which can accelerate population growth,” Jonathan Richmond, the study’s lead author and professor of biology at University of Richmond, told ABC news.

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In Washington, D.C., for example, the rat population is growing 1.5 times faster than it is in New York City, because the more northerly city has taken notable steps to keep rodents in check. In New Orleans and Tokyo – two cities with robust rodent response teams and good citizen reporting systems – the rat populations appear to be shrinking.

Richardson said that cities experiencing declines might be outliers, rather than part of a broader trend, though he did note that in Tokyo, residents seem willing to post rat sightings on social media in a “name and shame” approach to getting businesses to clean up their act.

…

Read more: https://abcnews.go.com/Health/rats-worldwide-enjoying-perks-climate-change/story?id=118284253

The abstract of the study;

Increasing rat numbers in cities are linked to climate warming, urbanization, and human population

JONATHAN L. RICHARDSON, ELIZABETH P. MCCOY, NICHOLAS PARLAVECCHIO, RYAN SZYKOWNY, ELI BEECH-BROWN, JAN A. BUIJS, JACQUELINE BUCKLEY, ROBERT M. CORRIGAN, FEDERICO COSTA, RAY DELANEY, RACHEL DENNY, LEAH HELMS, WADE LEE , MAUREEN H. MURRAY, CLAUDIA RIEGEL, FABIO N. SOUZA, JOHN ULRICH, ADENA WHY, AND YASUSHI KIYOKAWA

Urban rats are commensal pests that thrive in cities by exploiting the resources accompanying large human populations. Identifying long-term trends in rat numbers and how they are shaped by environmental changes is critical for understanding their ecology, and projecting future vulnerabilities and mitigation needs. Here, we use public complaint and inspection data from 16 cities around the world to estimate trends in rat populations. Eleven of 16 cities (69%) had significant increasing trends in rat numbers, including Washington D.C., New York, and Amsterdam. Just three cities experienced declines. Cities experiencing greater temperature increases over time saw larger increases in rats. Cities with more dense human populations and more urbanization also saw larger increases in rats. Warming temperatures and more people living in cities may be expanding the seasonal activity periods and food availability for urban rats. Cities will have to integrate the biological impacts of these variables into future management strategies.

Read more: https://www.science.org/doi/10.1126/sciadv.ads6782

Reading the study, the only subtropical location studied, New Orleans, is an exception to the rule that warmer temperatures have caused a rise in rat numbers. The study doesn’t attempt to explain why New Orleans is flowing counter to the claimed trend, other than mentioning New Orleans’ proactive rat control measures, and stating “… Insights from nontemperate cities nearer to the equator will be important to fully understand the latitudinal climate links to rat population dynamics. …”

If the study authors had bothered to include a few more warm climate cities, they might have discovered how with minimal effort rat populations can easily be managed.

In my subtropical home town, rats are taken seriously, but they are not a significant nuisance, except on rare occasions when a wet year in the desert followed by a drought drives a temporary surge in numbers.

So why isn’t my subtropical home town full of rats?

Probably for the same reason as New Orleans. The same conditions which provide better survival conditions for rats also provide better survival conditions for creatures which hunt rats.

Trash in my hometown is stored in sturdy plastic bins. Lots of householders in my area own dogs and cats, the sky is full of crows and eagles, and at night normally timid snakes and large lizards gorge themselves on anything small and unwary enough to venture within striking distance.

An even more interesting example of rat control is practiced in the Turkish city of Istanbul.

Residents of Istanbul practice an ancient rat management tradition, they encourage feral cats. The cats don’t live in homes, they live outside, they take care of their own living arrangements, though locals provide boxes and other spaces which cats can use as dens. Locals leave food for cats, and even sometimes take them to the vet, but they aren’t pets. Cats live alongside humans, but are not part of the family.

The cats are feral, they can’t be domesticated. They accept friendly contact from humans, but only on their own terms. My friends in Istanbul once tried to domesticate one of the friendlier cats, but it didn’t want to stay. The cat wasn’t happy until they let it return to the streets.

It took me a while to figure out a possible explanation. Centuries ago, Istanbul, or Constantinople as it was once known, was hit harder than most Eurasian cities by the Plague. Istanbul suffered repeated devastations, a repeating cycle of pain which started many centuries before the Plague reached Western Europe.

Although airborne transmission is possible, the Plague is mostly transmitted by contact with rat fleas. Cats are susceptible to the Plague, and can transmit the illness to humans, so close contact with cats was dangerous. But you need cats to control rats. Not only do cats hunt rats, the very presence of a cat can deter rats from settling in the area.

So out of this period of horror and death, the survivors of Istanbul’s repeated devastations developed a tradition of keeping cats close, but not too close – a tradition which has survived into modern times.

The west also has a tradition which involves cats, but sadly our ancestors may have been a little slower on the uptake than the hard pressed residents of plague stricken Istanbul / Constantinople. “Witches” who spread plague in Western tradition were frequently associated with cats. It is easy to imagine that elderly cat lady who stayed healthy while fit young people were dropping in the streets, to the ignorant such unnatural good health could obviously only be explained by magic. And of course, if one of the old lady’s cats got sick, that just proved the old witch was using her devil familiar cats to spread disease.

Cats eventually became widely accepted in Western households, which may have contributed to the eventual decline in US, British and West European plague outbreaks.

Note there is significant historical disagreement about the evolution of attitudes towards cats, in both the West and the East, and the evidence in some cases is sparse, so it is possible my analysis about the history of cats is completely wrong. For example, there is a persistent rumour that Pope Gregory IX ordered the death of cats in medieval Western Europe in the 13th century, but there appears to be no solid historical evidence he ever did such a thing, which leads to the intriguing possibility that medieval people killed cats because they falsely believed that Pope Gregory had instructed that cats were the agents of the devil. Or perhaps stories of medieval killing of cats for religious reasons are themselves revisionist falsehoods.

As the great Terry Pratchett once said, In the bathtub of history the truth is harder to hold than the soap and much more difficult to find. That applies with a vengeance to figuring out how our ancestors treated cats, and the relationship between possible cat purges and plague outbreaks.

One thing we know for sure, places where cats are common don’t have a rat problem. If you don’t want to be overrun by rats and rat borne diseases, forget about trying to control the weather, the right solution is to provide space in your community for animals which hunt rats. Allow cats to do some rat control, either by owning a cat, or by embracing the Istanbul solution – treating feral cats as welcome co-habitants of our urban spaces.