Humans are dramatically changing the environment of the Earth in many ways. Only about 23% of the land surface (excluding Antarctica) is considered to be “wilderness”, and this is rapidly decreasing. What wilderness is left is also mostly managed conservation areas. Meanwhile, about 3% of the surface is considered urban. I could not find a statistic for suburban, and there is currently no specific definition of what qualifies as suburban.

This, of course, is having an impact on animal life. One interesting question is how animals are adapting to these changes, and specifically are there species of animals that are adapting to urban life. Some species clearly do better than others in cities. Rats and pigeons are doing well. Peregrine falcons have also adapted well – the tall buildings give them excellent perching locations, and the pigeons provide an ample food source. Bats do well in Australian cities. In Africa some hyena populations are adapting to human food sources, and are believed to be in the process of self-domesticating.

Coyotes have also set up populations in many cities, feeding on the small animals that also live in cities. A significant part of the diet of urban coyotes is small rodents, including rats. Coyotes are well established in NYC, probably partly due to the abundant rodent population.

Racoons are another mammal that has adapted well to urban life. They tend to follow humans as a convenient food source. This has created a bit of an arms race between humans trying to keep racoons out of their garbage bins and racoons trying to find a way in. Racoons are smart critters to begin with. They have a relatively high neuronal density and good problem-solving skills. In one study city racoons displayed greater problem solving than their rural cousins.

But let me dive a bit deeper into the neuronal density question. This refers to the number of neurons per unit volume. Larger mammals have larger brains, but they also have larger neurons, and therefore they do not necessarily have more neurons. Brain size alone is therefore not a good measure of intelligence or processing capacity. In the study I linked about they compared many carnivores, and found they had a similar brain size to body size ratio as other mammals. Primates, of course, have the highest. But when they looked at neuronal density they found some interesting outliers. The single highest neuronal density was found in a golden retriever. Also peaking about the average were racoons and lions. There obviously can be lots of reasons for a species to evolve greater intelligence and problem-solving, such as cooperative hunting.

What is the effect of domestication? This is complicated, but generally domesticated animals have smaller brains than their wild cousins, although about the same neuronal density. This is attributed to the easy life of domesticated animals and pets, and so the trade-off with a metabolically demanding brain is not there. But animals that are not domesticated but live in human settings, like racoons, may have the strongest selective pressure for intelligence. It may also be that already smart animals adapt the best to urban niches.

This is an area that requires a lot more study, but we are seeing that not only are some animals, by luck, already well adapted to cities, others are adapting or evolving to be more competitive in a city environment. One recent study is interesting in how it approached this question. The Chicago museum has over 100 years of animal specimens in its drawers, including many local species. Researchers took advantage of this trove by doing a morphological comparison of specimens over this time period. They compared the skulls of chipmunks and voles and found intriguing differences.

The skulls of the chipmunks became larger while their teeth became smaller, for example. The researchers hypothesize that this is because they are relying on more human food sources, which are softer than the nuts they generally eat, but also provide more calories. So they can grow larger, but don’t need as large teeth. Of course, this can also be mostly genetic drift. It is difficult to prove a specific selective pressure is at work.

The voles has progressively smaller inner ear bones. Again, there is no way to know if this is just genetic drift or a response to a specific selective pressure. But the researchers hypothesize that this may be an adaptation to noisy urban environments.

It is interesting to think about how urban environments are affecting animal evolution. Cities can provide not only extreme selective pressures, but ones that are rapidly changing. It is a good reminder that evolution is not just the slow steady process that Darwin envisioned. Rapid evolutionary change is possible, and may even be the norm, as populations move into new or changing environments. It is also another reminder of how much we are impacting the environment of the Earth. Human activity in now the dominant selective pressure on the planet.