Cities are making mammals bigger - Florida Museum of Natural History
A new study shows urbanization is causing many mammal species to grow bigger, possibly because of readily available food in places packed with people.
The finding runs counter to many scientists’ hypothesis that cities would trigger mammals to get smaller over time. Buildings and roads trap and re-emit a greater degree of heat than green landscapes, causing cities to have higher temperatures than their surroundings, a phenomenon known as the urban heat island effect. Animals in warmer climates tend to be smaller than the same species in colder environments, a classic biological principle called Bergmann’s Rule.
But Florida Museum of Natural History researchers discovered an unexpected pattern when they analyzed nearly 140,500 measurements of body length and mass from more than 100 North American mammal species collected over 80 years: City-dwelling mammals are both longer and heftier than their rural counterparts.
“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 study lead author Maggie Hantak, a Florida Museum postdoctoral researcher. “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.”
Hantak and her collaborators created a model that examined how climate and the density of people living in a given area – a proxy for urbanization – influence the size of mammals. As temperatures dropped, both body length and mass increased in most mammal species studied, evidence of Bergmann’s Rule at work, but the trend was stronger in areas with more people.
Surprisingly, mammals in cities generally grew larger regardless of temperature, suggesting urbanization rivals or exceeds climate in driving mammal body size, said Robert Guralnick, Florida Museum curator of biodiversity informatics. “That wasn’t what we expected to find at all,” he said. “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 animals respond to human-induced environmental changes in the same way, Hantak added. The researchers also investigated how the effects of climate and urbanization may be tempered or amplified by the behavior and habits of certain species.
They 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 finding could have important implications for conservation efforts, Hantak said. “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,” she said.
The researchers published their findings in Communications Biology.
More on:
Posted On: 17/08/2021