Rat infestation seems to be an issue people across the world are tackling, but warming temperatures, urbanisation and other human actions could be why the rodents are thriving in cities. An examination of rat sighting reports in 16 cities across the globe has shown that 11 of these have seen rat complaints increase, according to a study in Friday’s journal Science Advances. Washington leads the rat increases by far, based on individual trends within cities, but San Francisco, Toronto, New York City and Amsterdam have all seen complaints soar.

Only three cities have seen significant decreasing trends including New Orleans, Louisville and Tokyo. The study used between 7 and 17 years of public rat sighting (average of 12.2 years) and inspection data to quantify changes in populations for each city and to evaluate trends. Specifically, it counts complaints made by people. Because of the way the commensal pests live and hide, they are not as easily counted as other critters.

“We’re seeing these increasing trends in rats in cities that are warming the fastest, probably because this is a small mammal that has physiological challenges in the cold weather months,” Mr Richardson said. “If we’re warming the climate and winter starts a week or two later and spring arrives a week or two earlier, that’s one, two, maybe even three or four weeks across the entire year where those rats can be above ground foraging, acquiring more food and maybe squeezing out one or two more reproductive cycles. ”.

Other statistical links observed were less surprising, as they showed that rats liked the built-up environment and accompanying large human populations. This has the ability to create serious issues for people, experts have warned as rats also converge on certain spaces, or cities. “When rodent populations are high, people get sick, motor vehicles become disabled, mental health declines, fires are started and foods fouled,” said rat expert Michael Parsons who is from the US and wasn’t part of the study.

Because the statistics have been taken from figures recorded over year, and only cities that haven’t changed their reporting methods were looked at, Richardson said the trends they spotted have scientific merit. Even though Washington has the highest increasing trend that doesn’t mean it has more rats or even rat sightings, it is just that the numbers within the city are increasing fastest, he said.