Why rats in big cities are larger than ever before

By William Janes via SWNS

Urbanization is causing city mammals such as rats to grow bigger, according to a new study from the Florida Museum of Natural History. 

The worrying changes could have unpredictable effects on the ecosystem.

The new findings throw into doubt previous research, which suggested that some animals would shrink in size as cities expanded. 

"In theory, animals in cities should be getting smaller because of theheat island effects, but we didn’t find evidence for this happening in mammals,” said Dr. Maggie Hantak, the lead author of the study.

“This paper is a good argument for why we can’t assumeBergmann’s Rule or climate alone is important in determining the size of animals.”

The urban heat island effect is the phenomenon of cities having higher temperatures than their surroundings due to buildings and roads trapping and re-emitting more heat than green landscapes. 

Bergmann’s Rule is a biological principle that states that animals in warmer climates tend to be smaller than the same species in colder environments.

But an 80-year collection of 140,500 body length and mass measurements from over 100 North American species has shown that city-dwelling mammals are both longer and heftier than their rural counterparts.

Over the last decade, some experts have thought that warmer temperatures, caused by climate change, were causing many animal species to grow smaller over time. 

In theory, smaller animals may have smaller or fewer offspring, creating a knock-on effect on the environment.

Dr. Robert Gurlanick, curator of biodiversity informatics atFlorida Museum of Natural History said: “When we think about what’s going to happen to mammalian body size over the next 100 years, a lot of people frame that as global warming causing animals to get smaller."

“What if that isn’t the biggest effect? What if it’s that urbanization is going to lead to fatter mammals?”

The findings also raise questions about what would happen to animals with certain survival tactics.

Researchers found animals that use hibernation or torpor, a temporary way of slowing metabolic rate and dropping body temperature, shrank more dramatically in response to increases in temperature than animals without these traits.

The fate of these creatures in urban environments is still unknown.

 “We thought species that use torpor or hibernation would be able to hide from the effects of unfavorable temperatures, but it seems they’re actually more sensitive,” Hantak said.

As urbanization ramps up, animals could be divided into “winners and losers,” and mammal distributions may shift, the team warns.

The abundance of food, water, and shelter and relative lack of predators in cities may help certain species succeed in comparison with their neighbors in rural areas. 

“Animals that like living in urban environments could have a selective advantage while other species may lose out because of the continued fragmentation of landscapes,” Guralnick said. 

“This is relevant to how we think about managing suburban and urban areas and our wild lands in 100 years.”

While bigger is often better biologically, the long-term consequences to urban mammals of eating a diet of human food waste have yet to be determined.

 “When you change size, it could change your whole lifestyle,” Hantak said.

The findings were published in the journal Communications Biology.