Mammals are some of the most easily recognizable and endearing critters to share our cities with us. Who doesn’t love a cute bunny or squirrel in a city park? Or a fox or raccoon slinking through the streets at night. Even city rats can be endearing. Despite the commonality of these species, urban areas are inhospitable for many other species, leading to local extirpation and loss of biodiversity in metropolitan regions.
So why are these the species that persist in our cities and what makes an urban mammal able to adapt to city life? In urban birds, several recent studies have found that traits such as behavioral flexibility, dispersal ability, and generalist ecological requirements predispose birds to occupy cities. Maybe similar factors are at play in determining urban tolerance and adaptability in mammals? This is exactly what Luca Santini et al. set out to answer in their recent paper: One strategy does not fit all: determinants of urban adaptation in mammals.
190 species of urban mammals…
Santini and colleagues analyzed 190 mammal species commonly found in urban environments worldwide. Their dataset included an impressive array of mammals from all different lifestyles: carnivores, bats, rodents, primates, rabbits, shrews, and more. This compiled list alone makes this work a wonderful contribution to urban evolutionary research (check out the full list of species included). So many more species of mammals are found in cities than we see on a daily basis (which means so many species to study!). Using phylogenetic path analysis, they then examined which behavioral, ecological, and life history traits were correlated with urban tolerance across species. Specifically, they considered traits related to dispersal ability, behavioral and cognitive plasticity, diet, and reproduction and asked to what extent these traits increased the probability that species are occasionally found in urban areas (“visitors”) versus living in and using the urban habitat (“dwellers”).
So what does it take to thrive in the city?
Perhaps unsurprising, the most successful mammals in cities are the ones that produce the largest litters. This effect was consistently associated with urban presence across all orders of mammals analyzed. Other traits they examined had important effects for some but not all species, suggesting different factors influence urban visitation and dwelling in different groups of urban mammals. These included:
- Larger brain size for carnivores, bats, primates, and rodents
- Larger body size for carnivores, ungulates, primates, and rodents
- Greater diet diversity for carnivores, primates, and rodents
- Later weaning age in carnivores and rodents, but earlier for ungulates, primates, and insectivores
- High dispersal ability for bats
Is this adaptation?
Santini and coauthors rightfully point out that their results likely identify the traits that predispose a species to occupy the urban environment rather than indicate the presence of evolutionary adaptation. Nevertheless, it is easy to see how many of these traits may also be the target of natural selection in urban environments. For example, the production of a large number of offspring may enable urban colonizers to overcome the high mortality rates in cities while selection may act to further increase reproductive output. Similarly, larger brains enable urban species to exhibit more behavioral flexibility, which in turn allows them deal with unpredictable conditions in urban environments. This too could be the target of natural selection to further increase brain size if the ability to tolerate unpredictable conditions also results in higher fitness. And maybe the most persuasive argument for urban adaptation presented: in bats, species with long narrow wings tend to be the ones in cities, which happens to be the same pattern observed in birds and suggests a convergent selection pressure for flight in the open urban environment.
In general, we know so little know about why different species tolerate or thrive in urban environments while others are excluded entirely. This study is a great start at understanding the factors that predispose mammals to occupy urban environments and highlights several intriguing areas for future phenotypic analyses of urban adaptation.
Want to read more? Check the full manuscript:
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