Functional trait interactions in a human-dominated world: Urbanization and reproduction in Eurasian red squirrels

Ecological traits such as behaviour, physiology and morphology mediate an organism's interaction with its environment, and understanding their joint contribution to reproductive fitness is essential for predicting biological responses to global change. We addressed how urbanization shapes trait-function relationships across natural, suburban or urban areas to advance the functional and mechanistic understanding of ecological processes in changing environments. We investigated how interactions among ecological (urbanization, season) and phenotypic (body mass, personality) traits influence reproductive fitness in female Eurasian red squirrels (Sciurus vulgaris) across a natural-urban gradient in Italy. Using 204 records of 72 female squirrels across 12 sites, we found that body mass, mediating the effect of the spring season, was the main factor affecting the probability to reproduce in female red squirrels. Further, our results suggest a direct effect of urbanization on squirrels' reproduction: The probability of reproducing was higher in natural habitats than in the urban and suburban sites. Female red squirrels in natural sites also engaged more frequently in explorative behaviours than their conspecifics in urban sites, but this did not translate into either direct or significant indirect effects of personality on reproductive success. Finally, body mass of females tended to be slightly higher in urban and suburban than in natural habitats, but this did not result in a higher reproductive rate. While body mass emerged as the dominant predictor of reproduction, its functional role was context-dependent, shaped by seasonality, environmental resource distribution and potentially by altered energetic dynamics in urban environments. In contrast, personality traits varied with habitat but did not mediate reproduction, highlighting how trait expression does not equate to ecological function unless matched with the appropriate environmental context. This work underscores the importance of integrating multiple levels of biological organization, behaviour, body mass, life history and environmental context to understand how global change alters ecological and evolutionary trajectories. As urbanization continues to transform natural habitats, studies that disentangle direct and indirect pathways from phenotype to fitness will be essential not only for predicting wildlife persistence but also for linking individual-level processes to population and community dynamics.