Variation in brown rat cranial shape shows directional selection over 120 years in New York City

Abstract

Urbanization exposes species to novel environments and selection pressures that may change morphological traits within a population. We investigated how the shape and size of crania and mandibles changed over time within a population of brown rats (Rattus norvegicus) living in Manhattan, New York, USA, a highly urbanized environment. We measured 3D landmarks on the cranium and mandible of 62 adult individuals sampled in the 1890s and 2010s. Static allometry explained approximately 22% of shape variation in crania and mandible datasets, while time accounted for approximately 14% of variation. We did not observe significant changes in skull size through time or between the sexes. Estimating the P‐matrix revealed that directional selection explained temporal change of the crania but not the mandible. Specifically, rats from the 2010s had longer noses and shorter upper molar tooth rows, traits identified as adaptive to colder environments and higher quality or softer diets, respectively. Our results highlight the continual evolution to selection pressures. We acknowledge that urban selection pressures impacting cranial shape likely began in Europe prior to the introduction of rats to Manhattan. Yet, our study period spanned changes in intensity of artificial lighting, human population density, and human diet, thereby altering various aspects of rat ecology and hence pressures on the skull.

Genetic Adaptation in New York City Rats

“…2010 ), may be common in NYC rats, but entirely missed in this study. For example, the putative adaptation of cranial morphology identified in NYC rats is likely polygenic ( Puckett et al. 2020 ).…”

“…Urban environments changed dramatically from the late 19th into the 20th century—a period that spans around 500 rat generations. A recent example is found in evidence for a significant change in rat cranial shape in New York City (NYC) over a 120-year period ( Puckett et al. 2020 ).…”

“…2018 ). Rats in NYC evolved longer noses—which have been interpreted as adaptations to cold, and shorter upper tooth rows—which were interpreted as adaptations to higher quality, softer diets ( Puckett et al. 2020 ).…”

The interplay of spatial scale and landscape transformation moderates the abundance and intraspecific variation in the ecomorphological traits of a phyllostomid bat

“…Species that are able to adapt—or even prosper—in anthropogenic landscapes, are subject to environmental filters that lead to changes in the frequency of phenotypes across populations (Alberti et al 2017). That is, human environments might promote directional selection processes that alter the ecomorphological traits of the species (e.g., Puckett et al 2020). This pattern has been increasingly documented in several vertebrates such as lizards (Lazić et al 2013, Hall & Warner 2017, Putman & Tippie 2020), small non-volant mammals (Snell-Rood & Wick 2013, Puckett et al 2020), bats (Tomassini et al 2014), birds (Liker et al 2008, Caizergues et al 2021), and middle-sized carnivores (Parsons et al 2020).…”

“…That is, human environments might promote directional selection processes that alter the ecomorphological traits of the species (e.g., Puckett et al 2020). This pattern has been increasingly documented in several vertebrates such as lizards (Lazić et al 2013, Hall & Warner 2017, Putman & Tippie 2020), small non-volant mammals (Snell-Rood & Wick 2013, Puckett et al 2020), bats (Tomassini et al 2014), birds (Liker et al 2008, Caizergues et al 2021), and middle-sized carnivores (Parsons et al 2020). Nevertheless, most studies focused on comparing populations between discrete categories of land-use intensification, e.g., urban vs. rural, ignoring the role of landscape emergent properties and the spatial scales that better correlate with phenotypic changes of fauna in human environments (Strubbe et al 2020).…”