Capture tolerance: A neglected third component of aposematism?

Aposematism (considered here as an association between conspicuous colour patterns and the presence of a harmful secondary defence) has long been recognized as an anti-predator strategy, with salient traits serving as a warning signal to ward off would-be predators. Here we review evidence for a potentially widespread yet under-explored third component of this defensive syndrome, namely capture tolerance (the ability of the signaller to survive being captured and handled by would-be predators). We begin by collating the (largely anecdotal) available evidence that aposematic species do indeed have more robust bodies than cryptic species which lack harmful secondary defences, and that they are better able to survive being captured. We then present a series of explanations as to why aposematism and capture tolerance may be associated. One explanation is that a high degree of capture tolerance facilitates the evolution of post detection (“secondary”) defences and associated warning signals. However perhaps a more likely scenario is that a high capture tolerance is selected for in defended species, especially if conspicuous, because if they can survive for long enough to reveal their defences then they may be released unharmed. Alternatively, both capture tolerance and secondary defences may arise through independent or joint selection, with both traits subsequently facilitating the evolution of conspicuous warning signals. Whatever its ultimate cause, the three-way association appears widespread and has several key implications, including inhibiting the evolution of automimicry and shaping the evolution of tactile mimicry. Finally, we present a range of research questions and describe the challenges that must be overcome in developing a more critical understanding of the role of capture tolerance in the evolution of anti-predator defences.