Integrating morphological and molecular perspectives on foraging tactic constancy in a nectar-robbing bumble bee

Tactic constancy, where individuals consistently employ particular foraging tactics, provides a framework for linking ecologically relevant behaviors with morphology and neurogenomic state. Bumble bees ( Bombus ) can show tactic constancy to nectar robbing, a foraging tactic in which individuals bypass floral reproductive structures to access nectar through perforations. Although the ecological drivers of nectar robbing have been studied, its molecular and morphological underpinnings remain entirely unknown. Here, we investigated whether tactic constancy in Bombus bifarius foraging on Linaria vulgaris is associated with body size and brain transcriptomic differences. Workers were categorized as secondary nectar robbers, legitimate foragers, or tactic switchers based on detailed field observations, and were collected for body size measurements and whole brain RNA sequencing. Body size was associated with tactic: legitimate foragers were on average larger, whereas robbers were smaller, and switchers were intermediate. Differential expression analysis of 10,633 filtered genes revealed a small set of differentially expressed genes between nectar robbers and legitimate foragers, with no transcriptomic differences detected between switchers and any other foraging strategy. Together, these results suggest that foraging tactics are associated with body size, with a modest whole-brain transcriptomic signal observed between nectar robbers and legitimate foragers, providing new insights into the mechanistic basis of behavioral specialization in social insect colonies.