Modality-specific long-term memory enhancement in Heliconius butterflies

How animals perceive, process and respond to environmental cues is tightly tuned to species-specific ecological demands and reflected by the structure of neural systems. In the Neotropical butterflies, Heliconius, the mushroom bodies, insect learning and memory centres are significantly expanded compared with their closest relatives. This expansion coincided with the evolution of a novel diet of pollen and the ability to learn spatial foraging routes. Previous research has shown that Heliconius have more accurate long-term visual memory than other Heliconiini. Here, we test whether this enhanced memory stability is specific to visual contexts by conducting a long-term olfactory memory assay in two Heliconius species and two outgroup species. We found no difference in long-term olfactory memory between Heliconius and outgroup Heliconiini, and combining data from olfactory and visual memory trials confirmed a modality-specific improvement in memory recall in Heliconius. Tests of how Heliconiini species prioritize olfactory and visual cues when presented in conflict show no consistent pattern, suggesting that variation in memory stability is not explained by inter-specific differences in attentiveness to sensory cues. Our data provide a rare case where memory performance has been compared across species and sensory modalities to identify evidence of a modality-specific shift.This article is part of the Theo Murphy meeting issue 'Selection shapes diverse animal minds'.