Reversal learning of visual cues in Heliconiini butterflies
The mushroom bodies, an integrative region of the insect brain involved in learning and memory, have undergone volumetric increase in several independent lineages includes bees and ants, cockroaches and some beetles. However, the selective pressures driving these expansion events are not fully understood. One promising system for investigating this question is the Neotropical butterfly genus Heliconius, which exhibits markedly enlarged mushroom bodies compared with other members of the Heliconiini tribe. Notably, this neural elaboration co-occurs with the evolution of trapline foraging behaviour and an improved capacity for learning complex visual cues and long-term memory. Here, we further investigate the behavioural consequences of this brain expansion by testing reversal learning ability, a commonly used measure of cognition and behavioural flexibility in both vertebrates and invertebrates, across three Heliconius and three closely-related Heliconiini species. We trained butterflies to associate a food reward with either purple or yellow flowers, before training them with the reversed associations, and then reversing the cues again. All six successfully learned the reversed cues, and, contrary to our expectations, we found no evidence that Heliconius performed better than the other Heliconiini species. These results are surprising, given previous evidence linking the mushroom bodies to reversal learning in other insects, and the enhanced performance of Heliconius in other cognitive tests. This serves as a reminder that the functional consequences of brain expansion can be multifaceted, and do not necessarily result in an overall increase in general cognitive ability, but rather enhanced performance in specific, ecologically-relevant tasks.