Trends in Ecology & Evolution
OpinionAposematism: Unpacking the Defences
Section snippets
The Concept of Aposematism
Ever since Alfred Russel Wallace penned his response to Charles Darwin’s 1867 question about why nonreproductive caterpillars are brightly coloured – a letter in which Wallace recognized that conspicuous coloration advertises defences [1] – biologists have viewed warning coloration as principally signalling unpalatability. This is likely to derive from the reference to caterpillars, where unpalatability originally referred to being poisonous when ingested. Nonetheless, when Poulton [2] formally
Types of Defence
The myriad aposematic defences seen in nature, be they morphological, physiological, or behavioural 11, 12, can be divided into those constituting the least to the most intimate contact with the prey. Viewing defences through the lens of different stages in the predator–prey sequence is instructive 10, 13 because defences that operate earlier in the predatory sequence tend to be triggered and deployed more often than later ones and may be less costly (i.e., vigilance is less costly than flight
Defences Employed at a Distance before Contact
Odoriferous defences can be discerned by some predators well before they contact prey. For example, volatile irritants are produced by brightly coloured Heteroptera [20], hydrocyanide gas by gaudy polydesmid millipedes [21], and pyrazine odour by many garish insect groups 22, 23. The reek of some mephitids is unpleasant in itself and can advertise anal secretions that can be sprayed at a distance in some species (Figure 1A; [24]). Malodour is necessarily honest as the defence is the signal
Defences Employed at a Distance before Contact
Certain species advertise their ability to escape from predators by showing conspicuous colours that can be seen at a distance. For example, black flank stripes of artiodactyls may amplify the high jumps seen in stotting – a behaviour that signals an individuals’ ability to outrun a predator [31]. Such advertisements of flight speed may be open to a modicum of cheating. For example, pursuit-deterrent signals might be fakeable by low-quality individuals and between species: slow-flying beetles
Implications
Some aposematic signals are intimately tied to the defence itself, such as the echidna’s spines or the pyrazine odour of caterpillars. These signals are qualitatively honest (see [18]) and tend to be employed towards the beginning of the predatory sequence; only earlier defences can be inherently honest (Table 2). Other defences, particularly those that operate later, are not easy for a predator to assess unless it presses its attack. For example, bright-red coloration does not indicate
Concluding Remarks
Antipredator defences are extraordinarily diverse. They may be morphological, behavioural, or physiological, sometimes combined together in the same individual (see Outstanding Questions). Some of these defences, often morphological, can be seen at a distance. Colour signals associated with such defences are likely to be honest, although this requires empirical verification. Other defences, such as being poisonous, are difficult to discern accurately, raising the possibility that advertisements
Acknowledgments
We thank Kevin Arbuckle, Mike Speed, and two anonymous reviewers for helpful comments.
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