Data from: Body size affects the evolution of hidden colour signals in moths

Many cryptic prey have also evolved hidden contrasting colour signals
which are displayed to would-be predators. Given that these hidden
contrasting signals may confer additional survival benefits to the prey by
startling/intimidating predators, it is unclear why they have evolved in
some species, but not in others. Here, we have conducted a comparative
phylogenetic analysis of the evolution of colour traits in the family
Erebidae (Lepidoptera), and found that the hidden contrasting colour
signals are more likely to be found in larger species. To understand why
this relationship occurs, we present a general mathematical model,
demonstrating that selection for a secondary defence such as deimatic
display will be stronger in large species when (i) the primary defence
(crypsis) is likely to fail as its body size increases and/or (ii) the
secondary defence is more effective in large prey. To test the model
assumptions, we conducted behavioural experiments using a robotic moth
which revealed that survivorship advantages were higher against wild birds
when the moth has contrasting hindwings and large size. Collectively, our
results suggest that the evolutionary association between large size and
hidden contrasting signals has been driven by a combination of the need
for a back-up defence and its efficacy.