The ongoing global urban expansion presents resident wildlife with many changes to their environmental structure. A few changes created by anthropogenic growth include habitat fragmentation, roads and buildings, pollution, and increased human presence. These consequences of urbanization have caused many species to become locally extinct, while others appear to thrive from the effects of urban growth. For these species, the decrease in natural predators and access to anthropogenic food sources must outweigh the perceived costs of urban living.
In urban environments, novel selective forces can cause divergence among conspecifics that live in areas of varying urbanization. Researchers often present two explanations of this divergence. Urban colonizers could have had a preexisting exaptation that made it easier for those individuals to transition to city living. Alternatively, urban populations may, over time, develop phenotypic changes in response to selective forces that diverge them from rural conspecifics.
A common area of study is whether a desensitized hypothalamic-pituitary-adrenal (HPA) axis in the vertebrate endocrine system can lower barriers to urban colonization. In response to a stressor, the HPA axis releases glucocorticoids to jumpstart physiological and/or behavioral redirection. However, an organism with consistently high levels of circulating glucocorticoids can have adverse health effects, such as stunted growth or impaired immune function. Thus, individuals with less sensitive HPA responses could have an advantage in urban environments, assuming that there are more introduced stressors in urban areas. There have been several studies on stress in urban birds and reptiles, from which the conclusions are often species-specific and age- and sex-dependent. But there is less literature describing the mammalian endocrine system, and this study aims to lessen that knowledge gap.
Evaluating squirrel stress levels in urban and forested sites
Rimbach et al. (2022) investigated the impacts of urbanization on stress in various Eastern Gray Squirrel (Sciurus carolinensis) populations in Durham, NC, U.S.A., and Guelph, ON, Canada. They used hair cortisol concentration (HCC) to assess the stress of individuals from urban and forest habitats. HCC is a standard stress metric because it can be used to determine long-term stress levels and is less invasive than blood sampling. The two metrics used to assess the degree of urbanization were the Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Built-up Index (NDBI), which measure vegetation growth and human-made surfaces, respectively.
Researchers collected over 150 hair samples from individuals in rural and urban locations from the sampled cities. During hair collection, they also took note of the individual’s approximate age class, sex, and reproductive status.
Squirrel sampling location from Guelph, ON, Canada (A) and Durham, NC, USA (B)
The color variation represents NDBI, where the darker colors indicate higher levels of urbanization and the lighter areas are less urbanized
Rimbach et al. found that HCC varied between the sampling areas. The Guelph, ON, samples had quadratic relationships with NDVI and NDBI and were not sex-dependent. Whereas the HCC samples from Durham, NC, did not correlate with NDVI or NDBI. These results were sex-dependent, males having higher HCC relative to females.
Squirrels from the sampling site in Guelph had lower HCC in areas with less vegetation (low NDVI) and more human-made structures (high NDBI), indicating lower stress levels in the urban areas. These results suggest that these urban squirrels benefit from their close proximity to humans, possibly enjoying the constant food access and lack of predation pressure. Rimbach et al. speculated that the lack of relationship between HCC and the indices in Durham, NC, was due to differences in urban characteristics relative to the Canadian sites. The Durham sites had higher NDVI and lower NDBI in their forest and urban sites compared to the Guelph locations. They posited that the results might have reflected the Canadian output if they sampled squirrels from more dense urban areas in Durham.
Shows the quadratic relationship between the indices of urbanization and HCC from the Canadian samples (A+B) and the lack of relationship to the USA samples (C+D)
Future studies should monitor long-term stress and determine the role of plasticity
In their discussion, Rimbach et al. state some limitations of their study, which might help guide future directions in this line of research. The sampling of squirrels in this study was opportunistic, and the individuals were not tracked to identify their ranges. The authors cannot be sure that each individual permanently resides in the site they were sampled. If an individual were simply passing through, the environment that influenced their HCC levels likely had different NDVI and NDBI to the sampling site. They also mainly used university campuses rather than dense city centers as their urban sites.
Future studies could utilize long-term stress monitoring in sites of various urban densities to disentangle these contributing factors. Another question underlying divergence between urban and rural populations is whether these differences are driven by phenotypic plasticity or heritable genetic change. Rimbach et al. suggest performing common garden-rearing experiments to determine if squirrel HCC levels are plastic or hertible changes.
As urban growth continues into further undisturbed habitats, understanding the impact on resident wildlife becomes increasingly vital. We must make every effort to understand how different species respond, positively or negatively, to anthropogenic changes. This knowledge will allow more tailored conservation and management techniques to protect vulnerable species. Perhaps, with further understanding, we could even develop urban planning approaches that are inclusive of various species and to benefit all who call our cities home.
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Featured image: © Cricket Raspet, some rights reserved (CC-BY)
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