How Do Animals Cope with City Life?

Finding Food

Many animals behavior differently in cities than in natural areas. One major challenge for urban animals is finding food: many natural food sources are scarce, and a variety of new food resources (think trash and pet food!) are available (1). Urban animals may have altered diets, which include human trash, bird seed, and/or pet food, and they may be more willing to explore new foods. Some animals, such as racoons and coyotes, may have smaller home ranges because they can find abundant food in cities without traveling far (1).

Altered food resources and temperatures in urban environments may lead to changes in the timing and success of animal breeding (2). Some birds start breeding earlier and breed for a longer time period in urban areas. Many species suffer reduced success during breeding, perhaps due in part to scarcity of special food sources needed to feed offspring, such as insect prey for bird nestlings (3).

Building nests

Urban animals may also take advantage of other novel resources in cities, such as using artificial structures as nesting sites (2). The Black Phoebe is a clear example–these birds were once restricted to building their mud nests on sheltered banks and rocks near water, but they have now expanded into many urban areas, where they nest on human-made buildings and bridges. Similarly, Barn Swallows and Cliff Swallows often build mud nests on human-made structures.

Avoiding Risk

Urban environments present very different risks from natural ones. Many large predators, like mountain lions, are much rarer in cities. This means that prey animals like deer may spend less time watching for and escaping from potential predators (1). Other animals are exposed to higher densities of predators: small animals like songbirds and lizards are killed in staggering numbers by domestic cats (4). Exposure to these predators may cause a variety of behavioral changes, such as visiting their nest or young less frequently (5). Urban animals are also frequently near humans, and they may become habituated to human presence (1). However, many animals alter the timing of their activities to avoid people. For instance, many mammal species are more active at night in human-disturbed areas, which may allow them to avoid encounters with people (6).

Coping with pollution

Cities are polluted with light, noise, and chemicals, which may disrupt communication among animals. Many animals make sounds to attract mates, recognize each other, and alert others to danger. To avoid city noise drowning out these signals, animals may sing or call more loudly, for longer durations, at different times, or even at different frequency ranges (1). For example, many birds sing at higher frequencies in urban areas, which may make their songs more audible over low frequency traffic and industrial noise (7). Fascinatingly, one study reported that when streets where quiet during the COVID-19 pandemic, White-crowned Sparrows returned to singing at lower frequencies (8). Light pollution may also impact animal behavior. Sea turtle hatchlings rely on light from the open horizon to find the ocean after they hatch, but in cities they may instead migrate inward toward lighted structures (9).

Individual differences

Individual animals show consistent differences in behavioral traits (sometimes called animal personality), such as boldness, aggression, exploration, and sociality (10). Some personalities may be better suited to urban life than others (2). For instance, urban brown anoles are bolder, less aggressive, and spend more time exploring new habitats (11).

Stress Hormones

Hormones play a critical role in regulation of physiological functions–how bodily systems function. A stressful situation, such as escaping from a hungry predator, triggers a cascade of “stress hormones” in humans and animals alike. This hormonal response evolved to help us fight off or run from danger.

Urban environments present many challenges for wildlife, leading scientists to predict that city-living leads to increased baseline stress hormone levels (12-13). Some studies have found that urban birds have higher baseline stress hormone levels than those in non-urban areas, but others have found no difference at all or even the opposite pattern (12-13).

City-living might also lead to reduced responsiveness to stressful events, but scientists haven’t found any consistent patterns in how urban and non-urban animals respond to acutely stressful events (12-13). This is perhaps surprising–if urban environments are challenging for organisms, why don’t we see consistent differences in stress hormone levels? We don’t really know yet! But scientists are hard at work to understand how responses to urbanization vary among different cities, species, and individual animals.

Reproductive Hormones

Reproductive hormones are responsible for regulating sexual development and reproduction. You have likely heard of testosterone and estrogen–these hormones are also present in other mammals, and most animals have something similar!

Scientists have observed that urban animals often breed earlier and for longer, perhaps due to warmer temperatures and availability of food (e.g., trash, breed seed). These reproductive differences are likely driven by changes in reproductive physiology. Studies of the reproductive hormones of urban animals are still few and far between, but those that exist haven’t found any consistent differences between urban and non-urban animals (12-13). For instance, sometimes urban European Blackbirds have lower testosterone levels during breeding, but the testosterone levels of urban Dark-eyed Juncos are elevated over a long period of time than those of their non-urban counterparts (12). More research in this area is urgently needed!

Thermal Physiology

Urban areas are hot! The downtown of a city is often several degrees Celsius warmer than nearby natural areas, which is called the urban heat island effect. Buildings, asphalt, and other dark-colored urban surfaces absorb a lot of heat, and it gets trapped bouncing around among tall buildings.

Thermal physiology refers to the physiological strategies that organisms use to respond to changes in temperature. The thermal -physiology of urban animals may change to allow them to tolerate hot urban conditions. For example, crested anoles in cities can function with higher temperatures than those from rural areas (14). This may be due to changes in their genes that help prevent their proteins from becoming damaged when exposed to high temperatures (14).

Urban birds may also be better able to tolerate hot conditions: a study found that Great Tits (a European songbird) in forests suffer from reduced growth during heat waves, but the growth of city birds is not affected by extreme heat (15). We don’t yet understand why these urban nestlings were less impacted by heat, but it may have to do with changes in thermal physiology!

Oxidative Balance and Inflammation

You’ve likely heard that antioxidants are good for you. This is because molecules called reactive oxygen or nitrogen species (RS), which are natural by-products of our bodily functions, can cause damage to our cells if they aren’t neutralized. These RS contribute to the progression of many diseases in humans and wildlife alike, including cancer. Antioxidants neutralize these reactive molecules and protect our body from damage. When we don’t have enough antioxidants to neutralize all of these RS, we are said to experience oxidative stress and our cells can be damaged or even killed.

Urban animals may experience greater oxidative stress than those in rural or natural areas. Some urban chemical pollutants, such as nitrogen oxides, may react to form RS in the body, leading to inflammation and oxidative stress (16). More generally, chemical, noise, and light pollution may disrupt the body’s ability to combat RS, leading to oxidative stress (16). In combination with greater exposure to RS, urban animals may often have a poor quality, junk food diet that is lower in antioxidants (16). However, more research is needed in this area.

Body size

Body size is one of an animal’s most fundamental traits, affecting temperature regulation, mobility, the food an animal can eat, and much more. Animals may be able to cope with urbanization and changing climates by shifting their body sizes (17). Animals living in cities are often larger than those in rural or natural areas. These may be because they have to travel large distances between suitable patches of habitat in cities, making it advantageous to have large bodies and long legs (17). Animals that eat human trash or food might also be able to grow larger (17). However, this isn’t always the case. Poor quality urban food might lead to nutritional stress, reducing body size of some animals, like songbirds (17). Being small may actually be advantagageous in sweltering hot urban centers because small animals can dissipate heat more quickly (17).

Arms, shoulders, knees, and toes

A variety of other morphological traits may differ between city living animals and their non-urban counterparts. For instance, many species of lizards have longer legs and toes in cities than nearby rural and natural areas, which may improve their ability to grip smooth surfaces like concrete (18). House Finches, a common urban dwelling songbird, have longer and narrower bills in cities than in nearby deserts. These changes in bill shape may be adaptations to eating larger, harder-to-crack seeds at backyard birdfeeders (18). On the other hand, urban-living foxes have shorter, wider muzzles than those in rural areas, which may be the result of developmental changes linked to “friendly” or “tame” behavioral traits (20).

Feathers and hair

Feathers and hair serve a variety of functions, including insulating an animal from extreme temperatures. Cities are often hotter than nearby rural and natural areas, and changes in feathers and hair may allow urban animals to cope with this “urban heat island” effect. Along these lines, urban songbird chicks have fewer feathers than their rural counterparts, leaving larger patches of bare skin that may help them lose body heat faster (21). Feathers and hair come in a range of colors, which may help animals stay hidden from predators or communicate with others. Cities are often predominantly gray, and city-living animals seem to follow suit: animals in urban areas are darker in color, they have duller colors, and are often gray (22-23). These color changes may help birds hide from predators, or they may be driven by limited access to high quality food in urban areas (22-23).