We tend to think of Darwinian survival as applying exclusively to wild animals in wild settings, such as jungles. But why can’t it just as easily apply to animals living in major cities?

Spend enough time in a major city, and you’ll notice animals stop acting with their natural animalistic instincts and instead start acting like opportunists, feeding on scraps they find lying around. Nearly every animal living in the city is being turned into its own version of Pizza Rat, the once virally famous rat spotted dragging a gigantic slice of pizza along a New York City sidewalk.

And not just those opportunistic rats. It’s birds, it’s monkeys, it’s squirrels.

According to an essay by researchers Daniel T. Blumstein, Peter Mikula, and Piotr Tryjanowski, it’s part of a broader shift called “behavioral homogenization.” All over the planet, urbanized animals were starting to act the same, even if they’re completely different species living on totally different continents, because cities, no matter where you go and no matter how different their cultures make them, are all basically the same.

Animals that call them home are often adaptable and willing to stick much closer to humans as a means of survival, feeding on our scraps.

Animals in Human Cities Are Starting to Act Very Strange. Here’s Why.

These animals tend to be bolder and riskier; they learned behaviors and survival tactics that their wild brethren would never. A wild animal will never learn to open a trash can lid or memorize human routines so they can be in a specific spot at a specific time, because they know there will be food there. Cities were creating a whole new, specific type of less fearful, more opportunistic, and increasingly human-dependent animal.

The three researchers argue that we are even seeing how city life is affecting the way these animals communicate with one another, with urban birds starting to sound more alike because city noise forces them to sing louder to be heard over the din of human hustle and bustle, and to sing earlier and at higher frequencies. It’s evolution happening in real time, and just around the block from our apartments.

It’s a fascinating development, but one that is reducing diversity both between species and within them, the trio argues. All this sameness is leading to less genetic flexibility, making animals more vulnerable to future environmental changes. It’s also altering learned behaviors that have been ingrained in these animals over thousands of years, such as migratory routes and feeding strategies that don’t seem particularly useful in a major city with relative abundance.

It all looks like a lot of animals cleverly adapting to their human-dominated surroundings, but it’s really whittling away at diversity, simplifying animal populations to the point where it may one day become a hindrance to their own survival.