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City dwelling mammals are getting bigger

A new study published in Communications Biology shows that urbanization plays a key role in determining mammalian body size. Scientists at the Florida Museum of Natural History have found that city life causes many mammals to grow larger. This contradicts the findings of previous research which claimed that city dwelling mammals get smaller over time. 

Until recently, experts believed that a classic biological principle called Bergmann’s Rule could be applied to urban animals. This is the idea that animals living in warmer climates are smaller than members of the same species from colder environments. This would certainly make sense, since cities have higher temperatures than green landscapes (a phenomenon called the heat island effect).

“In theory, animals in cities should be getting smaller because of these heat island effects, but we didn’t find evidence for this happening in mammals,” said Maggie Hantak, lead study author and a postdoctoral fellow at the Florida Museum. “This paper is a good argument for why we can’t assume Bergmann’s Rule or climate alone is important in determining the size of animals.”

The analysis of nearly 140,500 measurements of body mass and length from over 100 North American mammal species conducted by Dr. Hantak and her colleagues leaves no space for doubt: city dwelling mammals clearly grow larger than their non-urban counterparts, regardless of climatic factors.

“This wasn’t what we expected to find,” said Robert Guralnick, curator of biodiversity informatics at the Florida Museum. “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.”

While the impact of climate change on mammal body size has been acknowledged since a decade ago, the effects of urbanization have been ignored. 

“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,” said Guralnick. “What if that isn’t the biggest effect? What if it’s that urbanization is going to lead to fatter animals?”

The abundance of food, water, and shelter, together with a relative lack of predators, can certainly offer more dining opportunities for many city dwelling mammals.

Although bigger is generally better from a biological and evolutionary standpoint, more research is needed to assess the long-term effects on urban mammals of eating a rich diet of human food waste. 

By Andrei Ionescu, Staff Writer

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