Phenotypic Flexibility in the City: A Meta-Analysis on Variation.

Among global changes, urbanisation entangles a variety of human-induced rapid environmental changes, such as habitat fragmentation, temperature change, introduction of human food sources and pollution. Urban environments in contrast to non-urban ones are often assumed to be more heterogeneous and variable in space and time. A key feature of animals coping with high environmental variability ought to be phenotypic flexibility, that is, the capacity of individuals to express reversible variation in labile traits. However, this 'phenotypic flexibility hypothesis' has not been tested rigorously. We compiled available raw data and used a meta-analysis to estimate overall differences in among- and within-individual variation between urban and non-urban population pairs of wild animals. We considered within-individual variation as a proxy of phenotypic flexibility. Across all taxa, among-individual variation did not differ between urban and non-urban populations. Within-individual variation was marginally lower in urban populations compared to non-urban ones. The potential decrease of phenotypic flexibility in urban individuals could result from the multidimensionality and complexity of urban environmental conditions. Overall, the effects of urbanisation on phenotypic variation are not generalisable and depend on the taxa, species, and traits. Future studies should increase efforts to directly link temporal and spatial environmental variability with phenotypic individual variation.