A new study finds many mammal species living in cities are growing bigger. Scientists suspect the mammals - like mice and rats - are bulking up due to the overabundance of food in tightly packed urban settings.
It was once thought city animals would get smaller over time due to a phenomenon called the 'urban heat island effect.'
Buildings, roads, and human activity emit a lot of heat, making cities warmer than surrounding areas. Animals in warmer climates tend to be smaller than animals of the same species in colder locales - except in cities, apparently.
Upon analysis of nearly 140,500 measurements taken from more than 100 North American species collected over 80 years, researchers at the Florida Museum of Natural History found urban mammals are longer and heavier than their rural counterparts.
City-dwelling mammals tended to grow larger regardless of temperature, suggesting urbanization may be a larger determinant of size than climate.
“That wasn’t what we expected to find at all,” Robert Guralnick, Florida Museum curator of biodiversity informatics, said in a statement.
“But urbanization represents this new disturbance of the natural landscape that didn’t exist thousands of years ago. It’s important to recognize that it’s having a huge impact.”
Not all mammals reacted to the heat the same way - and it's not clear what the long-term effects of animals eating human trash will be. Still, researchers say the findings could help city planners devise ways to manage urban landscapes in the decades to come.
“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 wildlands in 100 years.”
The findings have been published in the journal Communications Biology