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.
Highlights
Urbanization has dramatically changed natural landscapes and exposed wildlife to unique selection pressures including but not limited to increased impervious surface area, increased temperatures due to heat island effects, aerial and subterranean infrastructure, altered light–dark cycles via artificial lighting, increased noise, altered diets composed of highly processed human foods, exposure to toxins, introduction to novel predators and/or release from predation, and increased contact rates with humans
Brown rats (Rattus norvegicus) reared in cold temperatures (5°C) had smaller sinus and nasal cavity volumes than individuals reared at room temperature (22°C), where the latter trait was in the opposite direction of primate results, yet the authors noted that phenotypic plasticity in the lab may have produced results that differed from natural populations under selection (Rae, Viðarsdóttir, Jeffery, & Steegmann, 2006)
We observed significant differences in cranial shape for the main effects related to centroid size, time period, and sex that accounted for 21.2%, 13.8%, and 1.4% of the respective variation in the data (Table 1)
Summary
Urbanization has dramatically changed natural landscapes and exposed wildlife to unique selection pressures including but not limited to increased impervious surface area, increased temperatures due to heat island effects, aerial and subterranean infrastructure, altered light–dark cycles via artificial lighting, increased noise, altered diets composed of highly processed human foods, exposure to toxins, introduction to novel predators and/or release from predation, and increased contact rates with humans. Brown rats (Rattus norvegicus) reared in cold temperatures (5°C) had smaller sinus and nasal cavity volumes than individuals reared at room temperature (22°C), where the latter trait was in the opposite direction of primate results, yet the authors noted that phenotypic plasticity in the lab may have produced results that differed from natural populations under selection (Rae, Viðarsdóttir, Jeffery, & Steegmann, 2006) This experiment observed a reduction in cranial but not postcranial body size in the cold-reared rats (Rae et al, 2006). Digitizing error was minimal: the dorsal, ventral, and mandible error values were, respectively, 1.8%, 1.9%, and 4.2%
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
