Why Would an Animal Want to Be Seen? Advertising That It Is a Bad Idea

An animal with a genuine defence faces a problem, namely that a predator which attacks and then rejects it has usually injured or killed it in the process, so the defence protects the species without protecting the individual. Conspicuous colouring solves that by making the animal unmistakable, so a predator that has had one bad experience avoids everything looking similar in future without needing to test. The strategy requires the signal to be distinctive, memorable and consistent, which is why the colours involved are a small and recurring set of combinations, principally black with red, yellow or orange, appearing in insects, amphibians, snakes and marine animals that are entirely unrelated.

The advertisement backs a range of unpleasant realities:

• •Toxins in the skin or tissues, as in poison frogs and several butterflies
• •Venom delivered by a sting or a bite
• •Foul taste or smell without serious toxicity
• •Irritant chemicals sprayed or released on contact
• •Physical defences including spines and hard shells
• •In several cases, no defence at all, which is the interesting exception

The strategy works only once predators have learned, which means some individuals must be attacked to supply the lesson, and that creates a genuine difficulty for explaining how it began. A first conspicuous individual among drab relatives is more likely to be attacked and gains nothing, since no predator has learned anything yet. Several resolutions have been proposed and are supported to different degrees. Kin selection helps if the individual's relatives live nearby and benefit from the lesson. Many defended animals survive an attack, so the teaching individual is not necessarily killed. Predators may avoid unfamiliar conspicuous prey innately rather than needing to learn. And the colouring may have begun as something else and been co-opted.

Several defended animals do not make their own chemistry and acquire it instead, which produces some of the more surprising relationships in the subject. Monarch butterflies accumulate toxic compounds from the milkweed their caterpillars eat, so an individual reared on a plant low in those compounds is harmless while looking identical to one that is not. Poison frogs lose their toxicity entirely in captivity, since the compounds come from the ants and mites they eat in the wild, which is why the species are safe to handle in a collection. Certain sea slugs eat stinging cnidarians and store the undischarged stinging cells in their own tissue. A bird in New Guinea carries a toxin in its feathers apparently obtained from beetles it eats.

Because the signal is learned, it can be exploited, and two distinct patterns result. Harmless species that resemble defended ones gain protection without paying for a defence, which works as long as the imitators remain rarer than the models, since predators learning from a majority of harmless encounters stop avoiding the pattern. Separately, several genuinely defended species have converged on the same appearance as each other, which is not cheating at all but a shared investment, since every predator lesson learned from one species protects all of them and the cost of teaching is spread. Distinguishing these two situations in the field requires knowing whether each species is actually defended, which is frequently harder than it sounds.

A defended animal that is attacked and rejected is usually already injured, so conspicuous colouring pays by making it unmistakable and letting one bad experience protect it thereafter. A small set of colour combinations recurs across unrelated groups. The difficulty is that teaching predators requires some individuals to be attacked, which is addressed by kin selection, survivable attacks and innate wariness. Harmless mimics work only while they stay rare.