Long-term spatial memory across large spatial scales in Heliconius butterflies

Locating food in heterogeneous environments is a core survival challenge. The distribution of resources shapes foraging strategies, imposing demands on perception, learning and memory, and associated brain structures. Indeed, selection for foraging efficiency is linked to brain expansion in diverse taxa, from primates1 to Hymenopterans2. Among butterflies, Heliconius have a unique dietary adaptation, actively collecting and feeding on pollen, providing a source of essential amino acids as adults, negating reproductive senescence and facilitating an extended longevity3. Several lines of evidence suggest that Heliconius learn the spatial location of pollen resources within an individual's home range4, and spatial learning may be more pronounced at these large spatial scales. However, experimental evidence of spatial learning in Heliconius, or any other butterfly, is so far absent. We therefore tested the ability of Heliconius to learn the spatial location of food rewards at three ecologically-relevant spatial scales, representing multiple flowers on a single plant, multiple plants within a locality, and multiple localities. Heliconius were able to learn spatial information at all three scales, consistent with this ability being an important component of their natural foraging behaviour.

Rapid expansion and visual specialisation of learning and memory centres in the brains of Heliconiini butterflies

Changes in the abundance and diversity of neural cell types, and their connectivity, shape brain composition and provide the substrate for behavioral evolution. Although investment in sensory brain regions is understood to be largely driven by the relative ecological importance of particular sensory modalities, how selective pressures impact the elaboration of integrative brain centers has been more difficult to pinpoint. Here, we provide evidence of extensive, mosaic expansion of an integration brain center among closely related species, which is not explained by changes in sites of primary sensory input. By building new datasets of neural traits among a tribe of diverse Neotropical butterflies, the Heliconiini, we detected several major evolutionary expansions of the mushroom bodies, central brain structures pivotal for insect learning and memory. The genus Heliconius, which exhibits a unique dietary innovation, pollen-feeding, and derived foraging behaviors reliant on spatial memory, shows the most extreme enlargement. This expansion is primarily associated with increased visual processing areas and coincides with increased precision of visual processing, and enhanced long term memory. These results demonstrate that selection for behavioral innovation and enhanced cognitive ability occurred through expansion and localized specialization in integrative brain centers.

No evidence of social learning in a socially roosting butterfly in an associative learning task

Insects may acquire social information by active communication and through inadvertent social cues. In a foraging setting, the latter may indicate the presence and quality of resources. Although social learning in foraging contexts is prevalent in eusocial species, this behaviour has been hypothesized to also exist between conspecifics in non-social species with sophisticated behaviours, including Heliconius butterflies. Heliconius are the only butterfly genus with active pollen feeding, a dietary innovation associated with a specialized, spatially faithful foraging behaviour known as trap-lining. Long-standing hypotheses suggest that Heliconius may acquire trap-line information by following experienced individuals. Indeed, Heliconius often aggregate in social roosts, which could act as 'information centres', and present conspecific following behaviour, enhancing opportunities for social learning. Here, we provide a direct test of social learning ability in Heliconius using an associative learning task in which naive individuals completed a colour preference test in the presence of demonstrators trained to feed randomly or with a strong colour preference. We found no evidence that Heliconius erato, which roost socially, used social information in this task. Combined with existing field studies, our results add to data which contradict the hypothesized role of social learning in Heliconius foraging behaviour.