Quantifying visual acuity in Heliconius butterflies

Heliconius butterflies are well-known for their colourful wing patterns,
which advertise distastefulness to potential predators and are used during
mate choice. However, the relative importance of different aspects of
these signals will depend on the visual abilities of Heliconius and their
predators. Previous studies have investigated colour sensitivity and
neural anatomy, but visual acuity (the ability to perceive detail) has not
been studied in these butterflies. Here, we provide the first estimate of
visual acuity in Heliconius: from a behavioural optomotor assay, we found
that mean visual acuity = 0.49 cycles-per-degree (cpd), with higher acuity
in males than females. We also examined eye morphology and report more
ommatidia in male eyes. Finally, we estimated how visual acuity affects
Heliconius visual perception compared to a potential avian predator.
Whereas the bird predator maintained high resolving power, Heliconius lost
the ability to resolve detail at greater distances, though colours may
remain salient. These results will inform future studies of Heliconius
wing pattern evolution, as well as other aspects in these highly visual
butterflies, which have emerged as an important system in studies of
adaptation and speciation.