Our typical urban rats, mainly brown rats (Rattus norvegicus), have undergone adaptations that enable them to thrive in urban environments. These adaptations distinguish urban rats from their wild relatives and highlight the impact of urbanization on species evolution.
Genetic evolution and neighborhood adaptations
In cities like New York City (NYC), with a rat population estimated to be as large as one-third of the human population, urban rats have a different genetic profile compared to their ancestral population in China. Research revealed that there are regions in the urban rat’s genome that are genetically different from their ancestors. These genes are associated with metabolism, diet, the nervous system and locomotory behavior (Harpak et al., 2021).
Besides an overall difference between the ancestral population and adapted urban population, within NYC rats from different neighborhoods display different genetic profiles. Specifically, the genetic profiles between Lower and Upper Manhattan are noticeable within the genome, whereas Midtown Manhattan lies between the two neighborhoods. This neighborhood is less residential, meaning less food, and is a site where intense rat control takes place, functioning as a barrier between the Lower and Upper Manhattan populations (Combs et al., 2018).
So, urban rats have evolved specific genetic changes that are crucial for their survival in cities whereas these adaptations are absent in the ancestral population, suggesting that living in human-dense urban environments has led to rapid evolutionary changes.
Morphological adaptations
Additionally, urban rats have evolved specific morphological changes to survive the urban environment. Rats analyzed in 2010 had a notable longer nose and shorter upper molar tooth row compared to rats in 1890. Longer noses, like in polar bears, can be beneficial for survival in colder environments, very much needed in an urban city like NYC where temperatures during the colder months range from -2 to 4°C with a minimum of -13°C. As for the shorter upper molar tooth row, this is an adaptation for a diet that consists of human leftovers which has a softer quality, making strong and long upper molar tooth row unnecessary (Puckett et al., 2020).
Behavioral adaptations
Another reason why urban rats are so successful is due to the layout of NYC and most of North American cities—the grid layout. Rats that live in a structured grid-like urban layout exhibit more structured and extensive exploration to cover larger areas, whereas irregular streets restrict rat movements. So, they adjust their movements and exploratory strategies depending on the urban layout, a sign of flexible spatial cognition (Yaski et al., 2011).
Dietary and metabolic adaptations
Research conducted in another big city, Toronto, has discovered that rat diets from urban sites were of higher quality and were more homogeneous and stable over time. Urban rats had higher nitrogen isotope values, which is typically a marker for diets richer in animal protein and fat, making the diet more energy-rich or “higher quality”. Since an urban environment offers food that derives from human sources, this suggests that rats in the city were consuming more protein-rich, nutrient-dense human food waste like meat scraps or processed food, rather than scavenging low-nutrient or plant-based material. The range of isotropic values in urban rats was narrower than in wild rats and remained relatively constant across 100 years. This indicates that even as cities grew and changed over time, the type of food waste available to rats remained consistent in composition. This long-term stability implies that human-driven food environments in cities provide a dependable ecological niche for urban rats to thrive (Guiry & Buckley, 2018).
Resistance to rodenticides
As rats are the most widespread urban species worldwide, they are also a big zoonotic source for humans. The extensive use of rodenticides in cities has exerted selective pressure on rat populations, leading to the evolution of resistance. Studies done in French cities, where the rat population has the highest numbers across other European cities, found mutations present in the urban rat’s genome that increase resistance to rodenticides. More than 50% of the rats in France carry the mutation, highlighting the rapid evolutionary response of urban rats to human inventions (Berny et al., 2014; Desvars-Larrive et al., 2017).
In conclusion, the “superpowers” of urban rats are statements of the dynamic nature of evolution in response to urbanization. Through genetic, morphological, and behavioral adaptations, these rodents have become expert urban survivors, also highlighting the influence of human environments on wildlife evolution. These adaptations of urban rats have significant implications for urban ecosystems and public health. Their ability to exploit human environments leads to more interactions with humans, raising concerns about the transmission of zoonotic diseases. However, despite their potential role as zoonotic reservoirs, knowledge of urban rat populations remains scarce. Understanding the evolutionary changes in urban rat populations is essential for developing effective management strategies and detecting potential health risks.
Bibliography
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Combs, M., Puckett, E. E., Richardson, J., Mims, D., & Munshi-South, J. (2018). Spatial population genomics of the brown rat (Rattus norvegicus) in New York City. Molecular Ecology, 27(1), 83–98. https://doi.org/10.1111/MEC.14437
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Guiry, E., & Buckley, M. (2018). Urban rats have less variable, higher protein diets. Proceedings. Biological Sciences, 285(1889). https://doi.org/10.1098/RSPB.2018.1441
Harpak, A., Garud, N., Rosenberg, N. A., Petrov, D. A., Combs, M., Pennings, P. S., & Munshi-South, J. (2021). Genetic Adaptation in New York City Rats. Genome Biology and Evolution, 13(1). https://doi.org/10.1093/GBE/EVAA247
Puckett, E. E., Sherratt, E., Combs, M., Carlen, E. J., Harcourt-Smith, W., & Munshi-South, J. (2020). Variation in brown rat cranial shape shows directional selection over 120 years in New York City. Ecology and Evolution, 10(11), 4739–4748. https://doi.org/10.1002/ECE3.6228
Yaski, O., Portugali, J., & Eilam, D. (2011). City rats: Insight from rat spatial behavior into human cognition in urban environments. Animal Cognition, 14(5), 655–663. https://doi.org/10.1007/S10071-011-0400-Y/METRICS
Featured photo: © Priya Sandhu, some rights reserved (CC-BY)
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