Urbanisation homogenises the functional trait space and reduces body size of an arboreal ant species (Hymenoptera: Formicidae)

Urban environments are known to significantly influence the ecological dynamics of ant communities, since some ant groups are strongly associated with cities. Although there is an increasing interest in identifying “urban phenotypes” in insects, the impact of urbanisation on morphological functional traits of ants remains poorly studied. This study investigates intraspecific variation in the morphology of the generalist arboreal ant Crematogaster scutellaris (OLIVIER, 1792) along a local urbanisation gradient within a metropolitan city in northern Italy. More than 500 workers were sampled, photographed, and functionally relevant morphological traits related to body size (pronotal and head width), defensive function (propodeal spines), and sensory abilities (antennae and eyes) were measured. The work aimed to show whether a shift in intraspecific functional traits is underway according to the urban functional trait space homogenisation hypothesis and / or the urban ecological filter hypothesis. The overall multi-dimensional trait space of ants from highly urbanised areas was reduced compared with those from less urbanised areas, indicating that morphological homogenisation is associated with increasing urbanisation. In addition, highly urbanised areas hosted smaller ants, with a higher density of ommatidia in the compound eyes, and with shorter antennae, suggesting that an ecological filter may be acting on the species. Possible urban environmental factors linked to these differences include increased temperatures (reduced body size), proportion of trees (positively correlated with antenna length), and proportion of cemented areas (positively correlated with ommatidia density). These findings highlight urbanisation as a significant driver of morphological variation in C. scutellaris.