Are cities shaping bee behavior? female-female interactions in the solitary megachilid Osmia cornuta in an urban matrix

Cities are hotter, more fragmented, and less vegetated habitats than surrounding semi-natural areas. While such urban conditions have been shown to affect a variety of morpho-physiological traits in wild bees, potential effects on behavior have not been investigated. Behavioral plasticity may be a predictor of species success in urban matrices as more aggressive individuals may be advantaged in competitive contexts. Here, the behavioral plasticity (frequency of aggressive, avoidance, and tolerant behaviors during female-female interactions) of the solitary bee Osmia cornuta was tested using circle-tube experiments at four sites in Milan (Italy) to test the hypothesis that higher levels of urbanization (as defined by shifts in temperature and land-use change) increase intra-specific aggression. The behavioral profile of O. cornuta was largely dominated by aggressive interactions towards conspecifics at all sites (likely due to nest usurpation). However, behavioral variation was observed across the studied sites. First, aggression increased with mean site temperature (likely due to the heat stress caused by the Urban Heat Island effect). Bees tested later in the morning (at hotter times of the day) were also more aggressive. Second, bees were more likely to avoid interacting in greener, less urbanized areas. Additionally, larger differences in body size between paired bees also increased aggression, possibly because larger individuals are more prone to attack smaller ones. Although a larger urban gradient is needed to confirm these findings, the results suggest a shift towards more aggressive behavior in solitary bees exposed to higher levels of urbanization, similar to what has previously been observed in some vertebrates and spiders in urban habitats.