In the heat of a US summer, urban rats’ thoughts focus on one goal: it’s reproduction time, as the media make clear.

They go for the type of headlines which send shivers down the spine. “Rats love climate change”, says one. “Climate change is scary; rat explosion is scarier”, says another.

Reports from several cities in the US indicate that city rat populations are increasing, and many point to climate change as a primary cause of what is a growing rodent problem.

The actual reasons for what some of the more florid commentators refer to as a “ratpocalypse” are difficult to gauge. Little research has been carried out.

Take, for example, New York City. Estimates of the size of the city’s rat population – the human population is 8.6 million – range anywhere between 2 and 32 million.

“The brown rat, or Rattus norvegicus, common on the US east coast, can produce six litters a year with an average of 12 pups per litter”

While the figures might be vague, there’s general agreement that the city’s rat numbers are growing substantially. Calls to vermin controllers have increased. City officials say the incidence of rat sightings has gone up by nearly 40% over the last five years.

Rats can be vectors, or carriers, of disease and can transmit ticks and fleas. They can also do considerable damage to vital infrastructure, gnawing through wires and damaging pavements and drainage systems by their burrowing.

In cold weather rats tend to be inactive and stay in their burrows. With milder winters and rising average temperatures, rats’ breeding season is extended.

Rats reproduce at a prolific rate; the brown rat, or Rattus norvegicus, common on the US east coast, can produce six litters a year with an average of 12 pups per litter.

“Everywhere I go, rat populations are up”, a research scientist told the New York Times newspaper.

More city residents

Other US cities report similar rat infestations; Chicago is said to have a particularly serious rodent problem.

While increased temperatures are believed to be one cause of the growth in rat populations, other factors are involved. In the US and around the world, more and more people are choosing to live in cities.

By 2050, it’s estimated that 70% of the world’s population will be urban-based. More people in tightly-packed areas means more food being accumulated and more rubbish. Inefficient waste collection facilities in many urban areas inevitably lead to a growth in rodent numbers.

Surprisingly, there seems to have been little research into urban rats, their behaviour and the size of populations in various cities.

One problem encountered by researchers is that many urban dwellers, particularly landlords, are reluctant to admit to rat infestation problems – or to highlight the issue by allowing scientists to carry out surveys on their premises.

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