Automimicry

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Eyespots of foureye butterflyfish (Chaetodon capistratus) mimic its own eyes, deflecting attacks from the vulnerable head.

In zoology, Automimicry, Browerian mimicry, or intraspecific mimicry, occurs within a single species of animal. In one form, weakly-defended members of an aposematic species are parasitic on more strongly-defended members of their species, mimicking them to provide the negative reinforcement learning required for warning signals to function. In another form, a less vulnerable part of an animal's body resembles a more vulnerable part, for example with eyespots or a false head that helps to deflect attacks away from the real head.

Automimicry has rarely been put to military use, but the A-10 Thunderbolt (Warthog) was often painted with a false canopy on its underside.

History[edit]

Automimicry was first reported by the ecologist Lincoln Brower,[1] who found that monarch butterflies (Danaus plexippus) reared on cabbage (Brassica oleracea) were palatable to blue jays (Cyanocitta cristata bromia). However, monarchs raised on their natural host plant, milkweed (Asclepias curassavica) were noxious to jays - in fact, jays that ingested them vomited. Subsequently, Brower put forth the hypothesis of automimicry, in which a polymorphism within a species existed: some individuals might be defended, and others palatable. 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]

Evolution[edit]

The existence of automimicry poses two challenges to evolutionary theory: how can automimicry be maintained, and how can automimicry 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. 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.[3] If this hypothesis is true, then automimics may simply be unlucky enough not to have gathered enough toxins from their environment.

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 may be maintained in a balanced polymorphism.

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[4]), then honest signallers might still have an advantage over automimics that cheat.

In vertebrates[edit]

One form of such mimicry is where one part of an organism's body resembles another part. For example, the tails of some snakes resemble their heads; they move backwards when threatened and present the predator with the tail, improving their chances of escape without fatal harm. Some fishes have eyespots near their tails, and when mildly alarmed swim slowly backwards, presenting the tail as a head. Some insects such as some lycaenid butterflies have tail patterns and appendages of various degrees of sophistication that promote attacks at the rear rather than at the head. Several species of pygmy owl bear "false eyes" on the back of the head, misleading predators into reacting as though they were the subject of an aggressive stare.[5]

Pygmy owl (Glaucidium californicum) showing eyespots behind head

Some writers use the term "automimicry" when the mimic imitates other morphs within the same species. For example, in a species where males mimic females or vice versa, this may be an instance of sexual mimicry in evolutionary game theory. Examples are found in some species of birds, fishes, and lizards.[6] Quite elaborate strategies along these lines are known, such as the well-known "scissors, paper, rock" mimicry in Uta stansburiana,[7] but there are qualitatively different examples in many other species, such as some Platysaurus.[8]

In insects[edit]

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.

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.[9][10]

Many species of insects are toxic or distasteful when they have fed on certain plants that contain chemicals of particular classes, but not when they have fed on plants that lack those chemicals. For instance, some species of the subfamily Danainae feed on various species of the Asclepiadoideae in the family Apocynaceae, which render them poisonous and emetic to most predators. Such insects frequently are aposematically coloured and patterned. When feeding on innocuous plants however, they are harmless and nutritious, but a bird that once has sampled a toxic specimen is unlikely to eat harmless specimens that have the same aposematic coloration.[2][11]

Military usage[edit]

Underside of A-10 Thunderbolt II with false canopy painted in, as if the plane was the right way up

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.[12][13]

References[edit]

  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. doi:10.2307/2406736. 
  2. ^ a b c Ruxton, Graeme D.; T. N. Sherratt,; M. P. Speed (2004). Avoiding Attack: the Evolutionary Ecology of Crypsis, Warning Signals, and Mimicry. Oxford University Press. 
  3. ^ 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. 
  4. ^ Guilford, Tim (October 1994). ""Go-slow" Signalling and the Problem of Automimicry". Journal of Theoretical Biology. 170 (3): 311–316. doi:10.1006/jtbi.1994.1192. 
  5. ^ "NORTHERN PYGMY OWL (Glaucidium californicum)". Owl Research Institute. Retrieved 23 August 2015. 
  6. ^ Plaistow, Stewart J.; Johnstone, Rufus A.; Colegrave, Nick; Spencer, Matthew (2004). "Evolution of alternative mating tactics: conditional versus mixed strategies". Behavioral Ecology. 15 (4): 534–542. doi:10.1093/beheco/arh029. 
  7. ^ Schell, Robert & Dettman, Jessica. Ecology and breeding colors of the side-blotched lizard (Uta stansburiana) in the Grand Canyon.[1]
  8. ^ Lewis, Belinda Ann. Sexual Selection and Signalling in the Lizard Platysaurus minor. Thesis [2]
  9. ^ Andrei Sourakov (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
  10. ^ Robbins, Robert K. The "False Head" Hypothesis: Predation and Wing Pattern Variation of Lycaenid Butterflies. The American Naturalist Vol. 118, No. 5 (Nov., 1981), pp. 770-775
  11. ^ 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. 
  12. ^ Shaw, Robert (1985). Fighter combat: tactics and maneuvering. Naval Institute Press. p. 382. ISBN 0-87021-059-9. 
  13. ^ Neubeck, Ken (1999). A-10 Warthog Walk Around. Squadron/Signal Publications. pp. 72–77, 92. ISBN 0-89747-400-7.