Humans have become one of the planet’s most powerful evolutionary forces. From urban foxes and birds to lizards that can climb glass, animals are changing their behaviour and, in some cases, their biology to survive in an increasingly human-dominated world.

In Puerto Rico, anole lizards living in cities look noticeably different from their forest-dwelling counterparts. Researchers have found that urban lizards have evolved longer limbs and more toe lamellae — the tiny scales on the underside of their feet — which help them cling to smooth surfaces such as glass and metal.

What is particularly striking is how quickly some of these changes have emerged.

Cities have effectively become a new ecosystem, forcing wildlife to cope with unfamiliar conditions, including concrete surfaces, traffic, artificial light, noise, pollution and reduced access to natural habitats.

In some cases, animals can respond within just a few generations. Such rapid changes can be driven by climate change or direct human activity, including antibiotic use, hunting and the spread of invasive species. Scientists refer to this phenomenon as “contemporary evolution”.

“Some animals in cities are responding well,” says Andrew Sih, who studies the evolution of animal behaviour at the University of California, Davis.

Their responses can include recognising new predators, finding alternative sources of food and shelter, adapting to noise and artificial light, and developing greater tolerance to toxins and pathogens.

City living

For some animals, urban life can come with unexpected advantages. Studies have found that species capable of adapting to cities can benefit from greater food availability and fewer predators, potentially allowing them to produce more offspring.

British red foxes are a well-documented example. Urban populations can have higher reproductive rates than their rural counterparts, partly because of the abundance of food in cities.

In the US, urban raccoons are also showing signs of physical and behavioural change. Research suggests they may be displaying early signs of domestication, including shorter snouts and reduced aggression towards humans.

In Denmark, urban hares have also begun tolerating human disturbance more readily. This could result from changes in gene expression or natural selection favouring individuals that are less sensitive to people.

Research published in 2025 found that animals living in urban areas, where they face higher temperatures and greater pollution, showed significantly greater resilience to stressful conditions than animals from protected habitats.

So why do some animals manage to adapt while others struggle?

Sih’s research suggests that an animal’s ability to change its behaviour in response to a changing environment can be crucial. Behavioural flexibility can sometimes provide enough time for genetic changes to emerge.

“Step one is that they need to respond well behaviourally,” Sih says. “If they do that well enough, they survive well enough to have a chance to later evolve genetically to adjust even better.”

In Illinois, for example, armadillos have increasingly been spotted near major roads. Researchers believe the animals may be using roads to regulate their body temperature because asphalt absorbs and retains heat.

During winter, when temperatures in the region can fall to -34C, the warmth from roads could provide a useful advantage.

Such behavioural changes may give animals time to adapt to environments increasingly shaped by humans. But not every behavioural adjustment leads to evolutionary change.

Urban birdlife

City birds have also been forced to adjust.

If you hear birds singing above the sound of traffic, you may be hearing a song that has been reshaped by the urban environment. Studies have shown that many birds alter the frequency, volume and timing of their songs to make themselves heard over city noise.

The dark-eyed junco offers another example of rapid adaptation.

In the early 1980s, a group of juncos settled year-round on the University of California, San Diego campus, giving researchers an opportunity to observe how the birds responded to urban life.

The birds became less aggressive and developed fewer white feathers on their tails, a feature associated with aggressive displays. They also began breeding earlier, raised more young and changed their migration patterns.

Another urban population at the University of California, Los Angeles, began nesting in trees and on buildings instead of on the ground.

The birds’ physical characteristics changed too. Urban juncos developed smaller wings, which researchers believe may be an adaptation to navigating densely built-up environments rather than flying long distances over open land.

North America has lost an estimated 168 million birds over the past 50 years. For species such as the junco, the ability to adapt quickly could prove critical to their survival.

Beyond city limits

Rapid evolution is not confined to urban environments.

In some parts of Africa, a growing proportion of elephants are being born without tusks. Poachers tend to target elephants with tusks, meaning tuskless animals are more likely to survive and reproduce. Over time, tusklessness has therefore become an evolutionary advantage in areas where poaching is widespread.

Human activity is also changing fish populations.

Some fish species are becoming smaller because fishing tends to remove the largest individuals. Smaller fish that escape capture are then more likely to survive and reproduce, gradually favouring smaller body sizes.

Other animals are evolving ways to cope directly with pollution. Atlantic killifish, for example, have rapidly developed resistance to toxic industrial chemicals. Genetic changes in the biochemical pathways involved in responding to these chemicals have made some populations less sensitive to them.

Across the animal kingdom, species are being pushed to change as the planet around them changes.

But evolution cannot always keep pace.

“Other animals — often in the same species — simply ‘don’t get it’, and do not respond well behaviourally to these new challenges,” Sih says. “If anything, I note that for many animals, they don’t really have time to evolve, as things have changed too much too quickly.”

Even animals that successfully adapt to life alongside humans, buildings, pollution and a changing climate may face further challenges.

For those species, the next few decades could provide another test — and another opportunity to evolve.