Centris pallida (Hymenoptera: Apidae) male body size decreases across five decades

Historical data suggest that many bee species have declined in body size. Larger-bodied bees with narrow phenological and dietary breadth are most prone to declines in body size over time. This may be especially true in solitary, desert-adapted species that are vulnerable to climate change – such as Centris pallida (Hymenoptera: Apidae). In addition, body size changes in species with size-linked behaviors could threaten the prevalence of certain behavioral phenotypes long-term. C. pallida solitary bees are found in the Sonoran Desert. Males use alternative reproductive tactics (ARTs) and are dimorphic in both morphology and behavior. C. pallida male body size has been studied since the 1970s in the same population. We collected body size data in 2022 and combined it with published records from 1974-2022. We find a persistent decline in the mean head width of patrolling males, and shifts towards smaller body sizes in the populations of males found foraging and hovering. Both morphs declined in average body size, and the proportion of large-morph males in the population decreased by 8%. Mating males did not decline in mean body size over the last five decades. We discuss hypotheses related to the decline in C. pallida male head width. Finally, we advocate for C. pallida as an excellent study system for understanding the stability of ARTs with size-linked behavioral phenotypes. ### Competing Interest Statement The authors have declared no competing interest.