We found a higher incidence of FA in the midsection of the skull in urban house mice compared to rural house mice. However, we did not detect an increase in FA in rural populations of several rodent species over the last two centuries, which supports the common practice in urban ecology studies of using rural populations as a control in rural vs urban comparisons when assessing the effects of urbanisation.
Several studies have found increased levels of asymmetry in response to different types of anthropogenic disturbance, such as high levels of urbanisation, in many organisms, including plants, invertebrates and vertebrates (Banaszak-Cibicka et al. 2018; Cuevas-Reyes et al. 2013; Eeva et al. 2000; Elek et al. 2014; Lazić et al. 2013; Teixeira et al. 2006; Weller & Ganzhorn 2004). Our finding of higher asymmetry in the skulls of urban house mice in Vienna compared to rural specimens adds to this literature, and offers further support to the use of FA as an useful indicator of stress experienced by organisms in urban environments. It must be noted however that there are some conflicting results in the use of FA to assess the effect of urban stressors. For example, in carabid beetles, FA increased with urbanisation in species considered to be negatively affected by urbanisation, whereas such a relationship was not found in species that are more tolerant to urbanisation (Weller & Ganzhorn 2004); however, in another study FA did not change along rural-urban gradients (Elek et al. 2014). Similarly, different studies in lizards have reported an increase in FA in response to urbanisation (Lazić et al. 2013), no changes in FA in response to urbanisation (Sacchi et al. 2018), and even a decrease in FA in response to urbanisation, the authors interpreting this last result as natural selection being stronger in urban populations and asymmetrical individuals being less likely to survive to adulthood (Winchell et al. 2019).
Our results on rural populations can be interpreted as rural animals from some species not being particularly affected by the levels of anthropogenic disturbance that have taken place in rural landscapes over the last decades (O’Donnell & delBarco-Trillo 2020). It must be noted however that our study was not intended to assess how fitness of rural animals may have changed over the decades in response to human activity, nor what are the different factors from such human activity more directly responsible for any potential effects on fitness. Our data cannot possibly address these questions nor contribute to the controversial relationship between symmetry and fitness (Dongen 2006; Lens et al. 2002; Møller 1997). More detailed studies may provide more nuanced results (Sacchi et al. 2018). For example, FA was higher in bank voles (Myodes glareolus) in disturbed rural areas in the northern coast of France than in undisturbed rural areas (Marchand et al. 2003).
Traits that have a direct impact on survival or fitness, e.g. extremities and traits involved in foraging, are under strong selection and may be developmentally more stable (De Coster et al. 2013) and thus not reflect the effects of environmental impacts particularly well (Clarke 1995; Dongen 2006; Winchell et al. 2019). In contrast, traits for which some level of asymmetry may not necessarily result in a decrease in fitness may be better indicators of the environmental stress experienced by individuals during development (Clarke 1995). Although it is unclear whether small skull asymmetries may directly decrease fitness, there are nonetheless several studies reporting that poor environmental conditions increase skull asymmetry (Maestri et al. 2015; Marchand et al. 2003; Oleksyk et al. 2004; Zakharov & Yablokov 1990).
In our study, the lack of an increase in FA over the last decades in rural areas was consistent across different species with different ecological requirements and thus possibly affected differently by any given anthropogenic activity. This validates our main conclusion that rural populations have been unaffected by anthropogenic activities in an increasing manner over the last centuries. For example, rodent species closely associated to bodies of water (e.g. Arvicola terrestris, the European water vole) may be more affected by fertilisers and other contaminants reaching and accumulating in those water bodies. We selected a broad array of species with different ecological requirements, including commensal species (Mus musculus, house mouse; Rattus norvegicus, brown rat; and Apodemus sylvaticus, wood mouse), species associated to water bodies (e.g. Arvicola terrestris and Rattus norvegicus), species associated with grasslands (e.g. Micromys minutus, Eurasian harvest mouse; and Microtus arvalis, common vole), species mainly associated to woodlands (e.g. Myodes glareolus; Apodemus flavicollis, yellow-necked mouse; Glis glis, edible dormouse; Muscardinus avellanarius, hazel dormouse; and Sciurus vulgaris, Eurasian red squirrel), species restricted to low-land areas where they are likely to be exposed to diverse anthropogenic activities (e.g. Cricetus cricetus, the common or European hamster), or species restricted to areas with soft turf where they can dig burrow systems (Spermophilus citellus, European ground squirrel). Despite these ecological differences, we did not find any increase in FA over time in any individual species. We found, however, that two species (Arvicola terrestris and Cricetus cricetus) had significantly higher FA than many other species, regardless of year of capture. These two species are closely exposed to pollution (e.g. pesticides) given their habitat preferences, but so are other species included in our analyses, so it is unclear why FA values were higher in these particular species. Similarly, the species differences that we observed cannot be explained through a distinction between specialist and generalist species (Coda et al. 2016; Teixeira et al. 2006).
Our study emphasises the importance of museum collections to address questions that may not be easily tackled otherwise. By using museum specimens collected over the last centuries we can investigate how populations respond over time to changing environmental conditions, e.g. the impact of anthropogenic disturbance (Pergams & Lacy 2008; Pergams & Lawler 2009; Snell-Rood & Wick 2013).
In conclusion, our study offers some general validation to the use of rural populations as an appropriate control when investigating the effect of urbanisation on urban populations, with the caveats that this may not be the case for all species, and that there may be abiotic and ecological factors uniquely affecting rural habitats. That is, rural habitats cannot be defined by the absence of human disturbance. Ultimately, any comparison of habitats should quantify the factors affecting each habitat and the impact that those factors may have on the populations under study.