Aposematism

The bright colours of this granular poison frog serve as a warning to predators of its noxious taste.

Aposematism (from apo- away, and semantic sign/meaning), 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 colours of flowers which lure pollinators. The warning signal may take the form of conspicuous colours, sounds, odours or other perceivable characteristics. Aposematic signals are beneficial for both the predator and prey, both of which 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.

Defence mechanism

Flamboyant cuttlefish colours warn of toxicity

Aposematism is a primary defence mechanism that warns potential predators of the existence of another, secondary defensive mechanism. "By definition, primary defences operate before a predator initiates any prey-catching behavior (Robinson, 1969; Edmunds, 1974) and their function is to prevent pursuit". The organism's secondary means of defence may include:

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.

Hycleus lugens, an aposematically coloured beetle

In these 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, or by warning postures which expose brightly coloured body parts (see Unkenreflex), or exposing eyespots. Velvet ants have both bright colours and produce audible noises when grabbed (via stridulation), which serve to reinforce the warning.

Aposematic signals are primarily visual and involve bright and contrasting colours. Research indicates that more often than not, warning signals are honest indications of noxious prey because conspicuousness evolves in tandem with noxiousness. Thus, the brighter and more conspicuous the organism, the more toxic it is. The most common and effective colours are red, yellow, and black. These colours provide heavy contrast against green foliage. Red, yellow, and black are also resistant to changes in shadows and luminescence, have luminescence contrast, and are highly chromatic. The final advantage of these colours is to provide distance dependent camouflage. This strategy utilizes the fact that although very conspicuous, these colours are not as visible at very large distances. This is an advantage because predators will not decide to investigate the unknown, conspicuous object and therefore will not give the organism any unnecessary attention to begin with. There is no "perfect" warning coloration though because signals are dependent on the environment. The background, light conditions, and predator vision all play a role in shaping a well adapted warning coloration. They may be accompanied by one or more signals other than colour. These may be specific odours, sounds or behaviour. Together, the predator encounters a multi-modal signal which is more effectively detected.

Prevalence

The skunk is an example of mammalian aposematism.

Aposematism is widespread in invertebrates, particularly insects, but less so in vertebrates, being mostly confined to a smaller number of reptile, amphibian, and fish species. Some plants, such as Polygonum sagittatum, a species of knotweed, are thought to employ aposematism to warn herbivores of chemical (such as unpalatability) or physical defences (such as prickled leaves or thorns). Sharply contrasting black-and-white skunks and zorillas are examples within mammals. Some brightly coloured birds with contrasting patterns may also be aposematic.

Aposematism in human evolution

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, to make hominids and early humans more intimidating (to look bigger and more colourful, and to sound louder).

Behaviour

The defence 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. As a consequence, 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

Alfred Russel Wallace, in response to an 1866 letter from Charles Darwin, was the first to suggest that the conspicuous colour schemes of some insects might have evolved through natural selection as a warning to predators. Darwin had proposed that conspicuous colouring could be explained in many species by means of sexual selection, but had realised 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 coloured bands of a hornet warned of its defensive sting, so could the bright colours 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 coloured 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 colouration in animals. The term aposematism was introduced by Wallace's friend Edward Bagnall Poulton in The Colours of Animals (1890).

Mimicry

A venomous coral snake

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

Further information: Mimicry

Aposematism is a sufficiently successful strategy that other organisms lacking the same secondary 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 which 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 19th 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 antipredation 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 19th 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

• Animal coloration
• Handicap principle
• Rattlesnake
• Redback spider
• Unkenreflex

References

• Capinera, John L. (Editor). (2008). Encyclopedia of Entomology, (2nd Ed). Springer Reference. ISBN 1-4020-6242-7, ISBN 978-1-4020-6242-1. Ltd preview in Google Books (12 Jan 2010).
• Eisner, T. & Grant, R.P. (1981). Eisner, T; Grant, RP (1981). "Toxicity, Odor Aversion, and ``Olfactory Aposematism". Science 213 (4506): 476. doi:10.1126/science.7244647. PMID 7244647. 
• Hristov, Nickolay I. & Conner, William E. (2005). Hristov, NI; Conner, WE (April 2005). "Sound strategy: acoustic aposematism in the bat–tiger moth arms race". Naturwissenschaften 92 (4): 164–169. doi:10.1007/s00114-005-0611-7. PMID 15772807.  Accessed on 13 Jan 2010.
• Kassarov, L. (2004). Kassarov, L (2004). "Is aposematism a valid concept in predator-prey relationships between birds and butterflies? A different point of view". Tropical Lepidoptera 12 (1-2): 1–15. 
• Komarek, S. (1998). Mimicry, Aposematism and Related Phenomena in Animals & Plants. Vesmir. ISBN 80-85977-15-X. 
• Poulton, Edward Bagnall, Sir. (1890). The Colours of Animals, their meaning and use, especially considered in the case of insects. London: Kegan Paul, Trench & Trübner. 
• Rubino, D. & McCarthy, B. (2004). Rubino, Darrin L; McCarthy, Brian C (2004). "Presence of aposematic (warning) coloration in vascular plants of southeastern Ohio". Journal of the Torrey Botanical Society (Torrey Botanical Society) 131 (3): 252–256. doi:10.2307/4126955. JSTOR 4126955. 
• Santos, J.C.; Coloma, Luis A. & Cannatella, D.C. (2003). Multiple, recurring origins of aposematism and diet specialization in poison frogs. PNAS October 28, 2003. doi:10.1073/pnas.100.22.12792. 
• Slotten, Ross. (2004). The Heretic in Darwin's Court:The Life of Alfred Russel Wallace. New York: Columbia University Press. ISBN 0-231-13010-4 
• Temple, S.A."Foul Fowl". Living Bird 13 (2): 11–14. 1994. 

Online

• 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 0-19-852860-4