Dopaminergic system as the mechanism underlying personality in a beetle.

Individuals in many animal species exhibit 'personality,' consistent differences in behaviour across time, situations and/or contexts. Previous work has revealed a negative genetic correlation between intensity of tonic immobility and walking activity levels in the confused flour beetle, Tribolium confusum, thus suggesting these beetles exhibit personality in activity-related traits. The present study investigated the mechanism underlying this correlation. We used individuals derived from two strains established via artificial divergent selection for duration of tonic immobility. "Long" (L) strains exhibited higher frequencies and longer durations of tonic immobility, and lower activity levels, while "Short" (S) strains exhibited lower frequencies and shorter durations of tonic immobility and higher activity levels. We found that the duration of tonic immobility, and activity level, could be altered by caffeine administration; L strains fed with caffeine exhibited decreased durations of tonic immobility and increased activity levels. We also found that brain dopamine levels were lower in L strains than in S strains. Consequently, this study demonstrates that the dopaminergic system plays an important part in controlling the genetic correlation between tonic immobility and activity levels in this species.