<b>Nectar robber-mediated selection on calyx length in </b><b><i>Salvia castanea</i></b>

Calyx length may serve as a defensive trait that influences interactions with antagonists such as nectar robbers, but it is still unclear whether these agents are drivers of its polymorphic evolution. This study examines whether nectar robbers exert selection on calyx length and other floral traits (corolla tube length, floral mouth size, and flower production) of Salvia castanea by experimentally quantifying their effects on floral traits and plant reproductive success over two years. All floral traits differed significantly between years: in 2024, calyx length, corolla tube length, and flower production were significantly smaller, while floral opening size was larger than in 2023. Nectar robbing and seed set rates showed significantly opposite trends between years. Piecewise structural equation modeling revealed that although other floral traits exhibited varying effects on nectar robbing and seed set across years, only calyx length consistently negatively affected nectar robbing rate and positively affected seed set, while nectar robbing exerted a consistent negative effect on seed set. Calyx clipping significantly increased the probability of nectar robbing. Our results demonstrate that nectar robbers exerted selection for longer calyx length in S. castanea. More broadly, we highlight that nectar robbers can shape floral trait evolution by altering trait-fitness covariance.