Reward Tracking and Memory Decay in the Monarch Butterfly,Danaus plexippus L. (Lepidoptera: Nymphalidae)

Due to their long‐distance migration routes and high longevity, monarch butterflies (Danaus plexippus) are likely to benefit from learning how to discriminate and remember suitable feeding resources. In this study, we assessed monarchs’ abilities to track changing nectar sources over time and to retain learned information presented in two conditioning schedules. Non‐preferred (blue and red) and preferred (yellow) artificial flowers were concomitantly offered to monarchs in a three‐phase experiment. In each phase, flowers of only one color contained sucrose solution, while the others contained water. The rewarding color was changed in each phase. Instantaneous observations were made to assess butterfly visits to each color during each phase; continuous observations over the first 90 min of a new phase allowed us to look in more detail at the transition process. Overall, monarchs tracked sucrose availability, visiting the rewarding flowers more often than the unrewarding ones, regardless of innate preferences. However, butterflies reverted to innate color preferences when the newly rewarding color was different from the initial trained color. In a second experiment, memory decay was compared for butterflies trained according to two schedules: ‘single training’ (sucrose solution in red vs. water in blue artificial flowers in one 15‐min session per day) or ‘intermittent training’ (as above, but in two 7.5‐min sessions per day). Afterwards, butterflies were tested on alternate days for a week in arrays containing unrewarding models of both colors. Following either training schedule, memory persisted for at least 3 d after reinforcement ceased. Our findings reveal that monarchs are able to change their feeding responses according to the flowers’ reward status despite innate preferences, as well as to retain flower information for about half a week regardless of the conditioning dynamics.