The bright colours of this Yellow-winged Darter dragonfly serve as a warning to predators of its noxious taste.

This tendency to become highly noticeable and distinct from harmless organisms is the antithesis of crypsis, or avoidance of detection. Aposematism has been such a successful adaptation that harmless organisms have repeatedly evolved to mimic aposematic species, a pattern known as Batesian mimicry. Another related pattern is Müllerian mimicry, where aposematic species come to resemble one another.

Defense mechanism

Flamboyant cuttlefish colours warning of toxicity

Unpalatability

such as from the bitter taste arising from some insects such as the ladybird or tiger moth, or the noxious odour produced by the skunk, or:

Other danger

such as the poison glands of the poison dart frog, the sting of a velvet ant or neurotoxin in a black widow spider.

In these particular examples, the organism advertises its capabilities via either bright colouration in the case of the ladybird, frog and spider; or by conspicuous stripes in the case of the skunk. Various types of tiger moths advertise their unpalatability by either producing ultrasonic noises which warn bats to avoid them^[3], or by warning postures which expose brightly-coloured body parts (see Unkenreflex). Velvet ants have both bright colours and produce audible noises when grabbed (via stridulation), which serve to reinforce the warning.

Prevalence

The skunk is an example of mammalian aposematism.

Behaviour

The defense mechanism relies on the memory of the would-be predator; a bird that has once experienced a foul-tasting grasshopper will endeavour to avoid a repetition of the experience. One consequence of this is that aposematic species are often gregarious. Before the memory of a bad experience attenuates, the predator may have the experience reinforced through repetition, or else leave all the remaining and similarly coloured prey alone and safe. Aposematic organisms often move in a languid fashion as they have little need for speed and agility. Instead, their morphology is frequently tough and resistant to injury thereby allowing them to escape once the predator gets a bad taste or sting before the kill.

Origins of the theory

Gregarious nymphs of an aposematic milkweed bug, Lygaeus kalmii

Mimicry

A venomous coral snake

The harmless red milk snake mimics the bright colours of the venomous coral snake

Aposematism is a sufficiently successful strategy that other organisms lacking the same primary defence means may come to mimic the conspicuous markings of their genuinely aposematic counterparts. For example, the Aegeria moth is a mimic of the yellow jacket wasp; it resembles the wasp, but is not capable of stinging. A predator who would thus avoid the wasp would similarly avoid the Aegeria.

This form of mimicry, where the mimic lacks the defensive capabilities of its 'model', is known as Batesian mimicry, after Henry Walter Bates, a British naturalist who studied Amazonian butterflies in the second half of the nineteenth century. Batesian mimicry finds greatest success when the ratio of mimic to mimicked is low; otherwise predators learn to recognise the imposters. Batesian mimics are known to adapt their mimicry to match the prevalence of aposematic organisms in their environment.

A second form of aposematism mimicry occurs when two organisms share the same anti-predation defence and mimic each other, to the benefit of both species. This form of mimicry is known as Müllerian mimicry, after Fritz Müller, a German naturalist who studied the phenomenon in the Amazonian in the late nineteenth century. For example, a yellow jacket wasp and a honeybee are Müllerian mimics; their similar colouring teaches predators that a striped pattern is the pattern of a stinging insect. Therefore, a predator who has come into contact with either a wasp or a honeybee will likely avoid both in the future.

There are other forms of mimicry not related to aposematism, though these two forms are among the best known and most studied.

See also

• Handicap principle
• Rattlesnake
• Unkenreflex

Notes

References

• The arts of Deception and Camouflage, Mongabay.com
• The Functionality and Evolution of Aposematic Coloration, Sterling, T.

Further reading

• Ruxton, G. D.; Speed, M. P.; Sherratt, T. N. (2004). Avoiding Attack. The Evolutionary Ecology of Crypsis, Warning Signals and Mimicry. Oxford: Oxford University Press. ISBN 0198528604

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