Aposematism

poison dart frog

Aposematism [app-uh-sem-uh-tizm], perhaps most commonly known in the context of warning coloration, describes a family of antipredator adaptations where a warning signal is associated with the unprofitability of a prey item to potential predators. It is one form of ‘advertising’ signal, with many others existing such as the bright colors of flowers which lure pollinators. The warning signal may take the form of conspicuous colors, sounds, odors, or other perceivable characteristics. Aposematic signals are beneficial for both the predator and prey, who both avoid potential harm.

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.

The organism advertises its capabilities via either bright coloration in the case of the ladybug, 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, or by warning postures which expose brightly-colored body parts (unkenreflex). Velvet ants have both bright colors and produce audible noises when grabbed (via stridulation), which serve to reinforce the warning.

It has been recently suggested that early hominids employed aposematims to intimidate predators and to obtain protein rich food via competitive scavenging. According to this suggestion, human habitual bipedalism, long legs, long head hair, as well as tradition of group singing, body painting and use of clothes evolved primarily as aposematic displays, in order to make hominids and early humans more intimidating (to look bigger and more colorful, and to sound louder).

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 colored 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.

Alfred Russel Wallace, in response to an 1866 letter from Charles Darwin, was the first to suggest that the conspicuous color schemes of some insects might have evolved through natural selection as a warning to predators. Darwin had proposed that conspicuous coloring could be explained in many species by means of sexual selection practices, but had realized that this could not explain the bright colouring of some species of caterpillar since they were not sexually active.

Wallace responded with the suggestion that as the contrasting colored bands of a hornet warned of its defensive sting, so could the bright colors of the caterpillar warn of its unpalatability. He also pointed out that John Jenner Weir had observed that birds in his aviary would not attempt to catch or eat a certain common white moth, and that a white moth at dusk would be as conspicuous as a brightly colored caterpillar during the day. After Darwin responded enthusiastically to the suggestion, Wallace made a request at a meeting of the Entomological Society of London for data that could be used to test the hypothesis. In response, John Jenner Weir conducted experiments with caterpillars and birds in his aviary for two years. The results he reported in 1869 provided the first experimental evidence for warning coloration in animals.

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 Amazon in the late nineteenth century. For example, a yellow jacket wasp and a honeybee are Müllerian mimics; their similar coloring 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.

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