Wildlife is increasingly having to deal with our expanding urbanization and the warmer climate we’ve caused. Will wild animals be able to come up with enough coping strategies? ©Brad Josephs

Today, more than half of the global population—55 percent, or 4.2 billion inhabitants—live in cities. By 2050, two-thirds of the world’s people are expected to reside in urban areas. And when we talk about our increasingly metropolitan lifestyles, we usually speak in terms of how they’re changing our culture and our health. In the same way, when we bring up the topic of climate change, we tend to first discuss our coastal cities and the human refugees a warming climate will create.

But growing urbanization spurs a unique set of issues for nonhuman animals, as well. They also need to adjust to the changes we instigate. The problem is that if they are to survive, their adaptation must happen over a far shorter timescale than would have occurred through most of evolutionary time. For example, the rapid climate change that we have caused is heaping a whole lot of pressure on them. And it’s generally agreed that some species will be able to acclimate, but others will not.

So, in a sense, we are causing the world’s wildlife to shape-shift.

How? Animals are getting bigger.


To better regulate their body temperatures as the planet warms, some animals are developing larger ears and legs. Wood mice are growing longer tails.

Our warming climate creates bigger beaks

In a review published in November 2021 in the journal Trends in Ecology and Evolution, bird researcher Sara Ryding of Deakin University in Geelong, Victoria, Australia, reported that some warm-blooded animals are getting larger beaks, ears and legs to better regulate their body temperatures as the planet gets hotter. Other researchers have reported tail length increases in wood mice, and tail and leg size increases in masked shrews.

And while the increases in appendage size seen so far are somewhat small—less than 10 percent—prominent appendages, such as ears, are predicted to have even larger increases in the not-so-distant future. That means, say the researchers, that we might soon have real-life Dumbos in our midst.

The strongest shape-shifting, however, has been documented in birds. Several species of Australian parrots have shown, on average, a 4 percent to 10 percent increase in bill size since 1871, and this is positively correlated with the summer temperatures each year. In North America, dark-eyed juncos—a type of small songbird—show a link between increased bill size and short-term temperature extremes in cold environments.


Since the late 1800s, bill sizes in some Australian parrots have increased 4 to 10 percent, which positively correlates with annual summer temperatures.

Researcher Ryding is careful to note that climate change is a complex and multifaceted phenomenon that’s been occurring progressively, so it’s difficult to pinpoint just one cause of the shape-shifting. But, she adds, these changes have been occurring across wide geographical regions and among a diverse array of species, so there is little in common apart from climate change.

In the future, she hopes to investigate shape-shifting in Australian birds by making 3D scans of museum specimens from the past 100 years. It will give her team a better understanding of which birds are changing the size of their appendages due to climate change and why.

Our cities cause longer lengths

Concurrent with the Australian bird review is another study that seems to bear out the fact that wild animals are getting larger due to climate change.

North American dark-eyed juncos are evolving at a heightened rate. Their bills are increasing in size, as well. ©Joseph Gage, flickr

In general, animals in warmer climates tend to be smaller than the same species in colder environments, a biological principle called Bergmann’s Rule. Because of that, scientists have long speculated that spreading urban areas 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.”

But a study published in the science journal Communications Biology in August 2021 tipped that hypothesis on its head. It showed that urbanization is, in truth, causing many mammal species to grow bigger.

After analyzing nearly 140,500 measurements of body length and mass from more than 100 North American mammal species collected over 80 years, Florida Museum of Natural History researchers discovered an unexpected pattern: city-dwelling mammals are both heftier and longer than their rural counterparts.


Bison in Yellowstone National Park have learned to grapple with our roads. But our negotiating more tolerance for them outside Yellowstone is going to take a long time.

The scientists then created a model that examined how climate and the density of people living in an area—a proxy for urbanization—influence the size of mammals. As temperatures dropped, both body length and mass increased in most of the mammal species studied, evidence of Bergmann’s Rule at work. However, the trend was stronger in areas with more people.

Surprisingly, mammals in cities generally grew larger regardless of the temperature, suggesting that urbanization rivals climate in driving mammal body size. The scientists theorize that one possible reason is that food is readily available in places packed with people.

Our history can catalyze change

Wildlife deals with a lot in this world: finding food and shelter; keeping safe from predators; and navigating diseases and broken bones and wings. Wild animals also need to negotiate our expanding developments and traffic, breathe the air we contaminate, drink the water we pollute and try to escape the laws we enact that harm them. Now, it seems, we can add the skyrocketing temperatures that we cause to that list.

Will we be able to make room on our planet and in our lives for the growing sizes of wildlife? Or do we see largeness—in any form—a threat? ©Colin McNulty

We don’t have a good history regarding our dealings with large animals. Bears, bison and wolves: we’ve tried our best—and almost succeeded—in extirpating them all.

But now that wild animals have turned to this creative strategy for coping with the rising temperatures and increasing urbanization that we’ve brought, perhaps, this time, we can make room—in our hearts and in our home landscapes—for their growing size.

Here’s to finding your true places and natural habitats,