Honest signalling in predator-prey interactions: testing the resource allocation hypothesis

Warning signals are honest if they reliably deliver information about prey
unprofitability to predators. One potential mechanism that may create and
maintain a positive relationship between the strength of signals and
defence is the resource allocation between these costly traits. Here, we
test this hypothesis using the wood tiger moth Arctia plantaginis, whose
females’ red hindwings are a warning signal to predators but show
considerable variation in colouration within populations. These moths also
produce a defensive chemical that is known to influence avian predator
attack risk. Using dietary manipulations, image and chemical analyses, and
experiments with ecologically relevant predators we demonstrate that
protein availability during development can influence the strength of both
the primary warning signal and the secondary defence. Our results show
that females raised on a high-protein or ad libitum natural diet produced
more distasteful defensive fluids than those raised on a low-protein diet
or subjected to periodic food deprivation. While the patterning of the
warning signal was unaffected by food deprivation, its efficacy was
diminished in moths raised on a low-protein diet. However, this change was
imperceptible to avian predators. Critically, resource availability
influenced the relationship between signal strength and defence: moths on
a high-protein diet displayed a positive correlation between warning
signal strength and unpalatability, whereas this correlation was absent in
moths raised on a natural diet. These findings show that resource
availability can weaken the reliability of warning signals as an indicator
of an individual’s defensive capabilities, highlighting the complex
interplay between ecological conditions and the evolution of honest
signalling.