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Automimicry

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Eyespots of foureye butterflyfish (Chaetodon capistratus) mimic its own eyes, which are camouflaged with a disruptive eye mask, deflecting attacks from the vulnerable head.

In zoology, automimicry, Browerian mimicry, or intraspecific mimicry, is a form of mimicry in which the same species of animal is imitated. There are two different forms.

In one form, first described by Lincoln Brower in 1967, weakly-defended members of a species with warning coloration are parasitic on more strongly-defended members of their species, mimicking them to provide the negative reinforcement learning required for warning signals to function. The mechanism, analogous to Batesian mimicry, is found in insects such as the monarch butterfly.

In another form, first noted by Edward B. Poulton in 1890, a less vulnerable part of an animal's body resembles a more vulnerable part, for example with deceptive eyespots or a false head that deflects attacks away from the real head, providing an immediate selective advantage. The mechanism is found in both vertebrates such as fishes and snakes, and insects such as hairstreak butterflies.

Automimicry has sometimes been put to military use. The A-10 Thunderbolt (Warthog) was often painted with a false canopy on its underside, imitating itself, while the armoured recovery vehicle variant of the Churchill tank had a dummy gun, imitating an armed variant of the same tank.

Mimicry of distasteful members of the same species

Further information: Signalling theory

Automimicry was first reported by the ecologist Lincoln Brower and colleagues, who found that monarch butterflies reared on cabbage were palatable to blue jays. However, monarchs raised on their natural host plant, milkweed, were noxious to jays - in fact, jays that ingested them vomited.[1][2] Subsequently, Brower proposed the hypothesis of automimicry involving a polymorphism or spectrum of palatability: some individuals might be defended, and others palatable.[3]

It turns out that many species of insects are toxic or distasteful when they have fed on plants that contain chemicals of particular classes, but not when they have fed on plants that lack those chemicals. For instance, some milkweed butterflies feed on milkweeds (Asclepias) which contain the cardiac glycoside oleandrin; this makes them poisonous to most predators. These insects are often aposematically coloured and patterned. When feeding on innocuous plants, they are harmless and nutritious, but a bird that has sampled a toxic specimen even once is unlikely to risk tasting harmless specimens with the same aposematic coloration.[2][4] Such acquired toxicity is not limited to insects: many groups of animals have since been shown to obtain toxic compounds through their diets, making automimicry potentially widespread. Even if toxic compounds are produced by metabolic processes with an animal, there may still be variability in the amount that animals invest in them, so scope for automimicry remains even when dietary plasticity is not involved. Whatever the mechanism, palatability may vary with age, sex, or how recently they used their supply of toxin.[2]

If insect-eating birds, like this wagtail eating a moth, tend to avoid, or to taste and spit out, toxic insects, then mimicry of distasteful forms by harmless morphs of the same species should be favoured.

The existence of automimicry in the form of non-toxic mimics of toxic members of the same species (analogous to Batesian mimicry[5]) poses two challenges to evolutionary theory: how can automimicry be maintained, and how can it evolve? For the first question, as long as prey of the species are, on average, unprofitable for predators to attack, automimicry can persist. If this condition is not met, then the population of the species rapidly crashes.[2] The second question is more difficult, and can also be rephrased as being about the mechanisms that keep warning signals honest. If signals were not honest, they would not be evolutionarily stable. If costs of using toxins for defence affects members of a species, then cheats might always have higher fitness than honest signallers defended by costly toxins. A variety of hypotheses have been put forth to explain signal honesty in aposematic species.[6] First, toxins may not be costly. There is evidence that in some cases there is no cost, and that toxic compounds may actually be beneficial for purposes other than defence. If so, then automimics may simply be unlucky enough not to have gathered enough toxins from their environment.[7] A second hypothesis for signal honesty is that there may be frequency-dependent advantages to automimicry. If predators switch between host plants that provide toxins and plants that do not, depending on the abundance of larvae on each type, then automimicry of toxic larvae by non-toxic larvae may be maintained in a balanced polymorphism.[8][9] A third hypothesis is that automimics are more likely to die or to be injured by a predator's attack. If predators carefully sample their prey and spit out any that taste bad before doing significant damage ("go-slow" behaviour), then honest signallers would have an advantage over automimics that cheat.[10]

False head

Many blue butterflies (Lycaenidae) such as this gray hairstreak (Strymon melinus) have a false head at the rear, held upwards at rest, deflecting attacks from the actual head.
Further information: Eyespot (mimicry) and Disruptive eye mask

Many insects have filamentous "tails" at the ends of their wings and patterns of markings on the wings themselves. These combine to create a "false head". This misdirects predators such as birds and jumping spiders (Salticidae). Spectacular examples occur in the hairstreak butterflies; when perching on a twig or flower, they commonly do so upside down and shift their rear wings repeatedly, causing antenna-like movements of the "tails" on their wings. Studies of rear-wing damage support the hypothesis that this strategy is effective in deflecting attacks from the insect's head.[11][12][13][14]

Natural selection in favour of features that deflect predators' attacks is straightforward to explain: variants of patterns that more effectively deflect attack are favoured, since animals with ineffective variants are likely to be killed. Naturalists[a] since Edward B. Poulton in his 1890 book The Colours of Animals[15] have noted that butterflies with eyespots or other false head markings can be expected to escape with minor wing damage while the predator gets only "a mouthful of hindwing" instead of an insect meal.[12] In Poulton's words:

Each hind wing in these [hairstreak] butterflies is furnished with a 'tail', which in certain species is long, thin, and apparently knobbed at the end. When the butterfly is resting on a flower the wings are closed and the hind wings are kept in constant motion ... This movement, together with their appearance, causes the 'tails' to bear the strongest likeness to the antennae of a butterfly; the real antennae being held [downwards] so as not to attract attention. Close to the base of the supposed antennae an eye-like mark, in the most appropriate position, exists in many species. The effect of the marking and movement is to produce the deceptive appearance of a head at the wrong end of the body. The body is short and does not extend as far as the supposed head, so that the insect is uninjured when it is seized.[15]

Pygmy owl (Glaucidium californicum) showing eyespots behind head

A 1981 experiment confirmed the expected correlation between deceptiveness and survival in butterflies.[12]

Among vertebrates, snakes such as the rubber boa and the coral snake coil up and hide their head, instead displaying their tail as a false head.[16] Some fishes such as the foureye butterflyfish have eyespots near their tails, and when mildly alarmed swim slowly backwards, presenting the tail as a head; however, various hypotheses for the function of such eyespots have been proposed.[17] Several species of pygmy owl bear false eyes (ocelli) on the back of the head, misleading predators into reacting as though they were the subject of an aggressive stare.[18]

Military usage

Further information: Aircraft camouflage

Automimicry has sometimes been used in military vehicles and aircraft. Among vehicles, specialised variants such as the British Second World War Churchill armoured recovery vehicle had no room for an actual gun, but was fitted with a dummy weapon, imitating the armed version of the same tank, to give it some protection.[19]

The ground attack A-10 Thunderbolt (Warthog) was sometimes painted with a camouflage scheme that included both disruptive coloration and automimicry in the form of a false canopy on the underside. This was intended to confuse the enemy about the aircraft's attitude and likely direction of travel.[20][21]

Notes

  1. ^ Including Swynnerton, 1926, and Blest, 1957.[12]

References

  1. ^ Brower, Lincoln Pierson; Cook, Laurence M.; Croze, Harvey J. (March 1967). "Predator Responses to Artificial Batesian Mimics Released in a Neotropical Environment". Evolution. 21 (1): 11–23. doi:10.2307/2406736. JSTOR 2406736. PMID 28556119.
  2. ^ a b c d Ruxton, Graeme D.; Sherratt, T. N.; Speed, M. P. (2004). Avoiding Attack: the Evolutionary Ecology of Crypsis, Warning Signals, and Mimicry. Oxford University Press. pp. 176–182. ISBN 9780198528593.
  3. ^ Brower, Lincoln Pierson; Ryerson, William N.; Coppinger, Lorna L.; Glazier, Susan C. (27 September 1968). "Ecological Chemistry and the Palatability Spectrum". Science. 161 (3848): 1349–1351. Bibcode:1968Sci...161.1349B. doi:10.1126/science.161.3848.1349. PMID 17831347. S2CID 45185502.
  4. ^ Svennungsen, Thomas Owens; Holen, Øistein Haugsten (2007). "The evolutionary stability of automimicry". Proc. R. Soc. B. 274 (1621): 2055–2063. doi:10.1098/rspb.2007.0456. PMC 2275178. PMID 17567561.
  5. ^ Brower, Lincoln P.; Pough, F. Harvey; Meck, H. R. (August 1970). "Theoretical Investigations of Automimicry, I. Single Trial Learning". Proceedings of the National Academy of Sciences. 66 (4): 1059–1066. Bibcode:1970PNAS...66.1059B. doi:10.1073/pnas.66.4.1059. PMC 335786. PMID 16591844.
  6. ^ Summers, K.; Speed, M. P.; Blount, J. D.; Stuckert, A. M. M. (2015). "Are aposematic signals honest? A review". Journal of Evolutionary Biology. 28 (9): 1583–1599. doi:10.1111/jeb.12676. PMID 26079980. S2CID 9920426.
  7. ^ Leimar, Olof; Enquist, Magnus; Sillen-Tullberg, Birgitta (1 January 1986). "Evolutionary Stability of Aposematic Coloration and Prey Unprofitability: A Theoretical Analysis". The American Naturalist. 128 (4): 469–490. doi:10.1086/284581. JSTOR 2461331. S2CID 84450857.
  8. ^ Ruxton, Graeme D.; Speed, M. P. (2006). "How can automimicry persist when predators can preferentially consume undefended mimics?". Proceedings of the Royal Society B: Biological Sciences. 273 (1584): 373–378. doi:10.1098/rspb.2005.3238. PMC 1560041. PMID 16543181.
  9. ^ Brower, Lincoln P.; Pough, F. Harvey; Meck, H. R. (15 August 1970). "Theoretical Investigations of Automimicry, I. Single Trial Learning". Proceedings of the National Academy of Sciences of the United States of America. 66 (4): 1059–1066. Bibcode:1970PNAS...66.1059B. doi:10.1073/pnas.66.4.1059. JSTOR 59920. PMC 335786. PMID 16591844.
  10. ^ Guilford, Tim (October 1994). ""Go-slow" Signalling and the Problem of Automimicry". Journal of Theoretical Biology. 170 (3): 311–316. Bibcode:1994JThBi.170..311G. doi:10.1006/jtbi.1994.1192.
  11. ^ Sourakov, Andrei (2013). "Two heads are better than one: false head allows Calycopis cecrops (Lycaenidae) to escape predation by a Jumping Spider, Phidippus pulcherrimus (Salticidae)". Journal of Natural History. 47 (15–16): 1047–1054. doi:10.1080/00222933.2012.759288. S2CID 84454608.
  12. ^ a b c d Robbins, Robert K. (November 1981). "The "False Head" Hypothesis: Predation and Wing Pattern Variation of Lycaenid Butterflies". The American Naturalist. 118 (5): 770–775. doi:10.1086/283868. S2CID 34146954.
  13. ^ López-Palafox, Tania; Cordero, Carlos (2017-06-22). "Two-headed butterfly vs. mantis: do false antennae matter?". PeerJ. 5: e3493. doi:10.7717/peerj.3493. PMC 5483043. PMID 28652941.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  14. ^ Cordero, Carlos; López-Palafox, Tania G. (August 2015). "The movement of 'false antennae' in butterflies with 'false head' wing patterns". Current Zoology. 61 (4): 758–764. doi:10.1093/czoolo/61.4.758.
  15. ^ a b Poulton, Edward B. (1890). The Colours of Animals. Kegan Paul, Trench, Trübner. pp. 206–209.
  16. ^ Graham, Sean P. (18 February 2018). American Snakes. JHU Press. p. 319. ISBN 978-1-4214-2360-9.
  17. ^ Meadows, D. W. (11 February 1993). "Morphological variation in eyespots of the foureye butterflyfish (Chaetodon capistratus): Implications for eyespot function". Copeia. 1993 (1): 235–240. doi:10.2307/1446319. JSTOR 1446319.
  18. ^ "Northern Pygmy Owl (Glaucidium californicum)". Owl Research Institute. Archived from the original on 28 December 2015. Retrieved 23 August 2015.
  19. ^ Chamberlain, Peter; Ellis, Chris (1969). British and American Tanks of World War II. Arco Publishing. p. 70.
  20. ^ Shaw, Robert (1985). Fighter combat: tactics and maneuvring. Naval Institute Press. p. 382. ISBN 0-87021-059-9.
  21. ^ Neubeck, Ken (1999). A-10 Warthog Walk Around. Squadron/Signal Publications. pp. 72–77, 92. ISBN 0-89747-400-7.
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