Editing Camouflage (section)

== Mechanisms ==

Animals can camouflage themselves by one or more principles using a variety of mechanisms. For example, some animals achieve background matching by changing their skin coloration to resemble their current background.{{sfn|Forbes|2009|pages=52, 236}}

=== Changeable skin coloration ===

{{further|Active camouflage|Snow camouflage}}

Animals such as [[chameleon]], frog,{{sfn|Cott|1940|pages=30–31}} flatfish such as the [[peacock flounder]], squid, octopus and even the isopod [[idotea balthica]] actively change their skin patterns and colours using special [[chromatophore]] cells to resemble their current background, or, as in most chameleons, for [[Signalling theory|signalling]].{{sfn|Forbes|2009|pages=52, 236}} However, [[Smith's dwarf chameleon]] does use active colour change for camouflage.<ref>{{cite journal |last1=Stuart-Fox |first1=Devi |last2=Moussalli |first2=Adnan |last3=Whiting |first3=Martin J. |title=Predator-specific camouflage in chameleons |journal=Biology Letters |date=23 August 2008 |volume=4 |issue=4 |pages=326–9 |doi=10.1098/rsbl.2008.0173 |pmid=18492645 |pmc=2610148 }}</ref>

{{multiple image
|total_width=400px
|align=left
|image1=Peacock Flounder Bothus mancus in Kona.jpg
|caption1=Four frames of the same [[peacock flounder]] taken a few minutes apart, showing its ability to match its coloration to the environment
|image2=Melanophores with dispersed or aggregated melanosomes.svg
|caption2=Fish and frog melanophore cells change colour by moving pigment-containing bodies.
}}

Each chromatophore contains pigment of only one colour. In fish and frogs, colour change is mediated by a type of chromatophore known as [[melanophore]]s that contain dark pigment. A melanophore is star-shaped; it contains many small pigmented [[organelle]]s which can be dispersed throughout the cell, or aggregated near its centre. When the pigmented organelles are dispersed, the cell makes a patch of the animal's skin appear dark; when they are aggregated, most of the cell, and the animal's skin, appears light. In frogs, the change is controlled relatively slowly, mainly by [[hormone]]s. In fish, the change is controlled by the brain, which sends signals directly to the chromatophores, as well as producing hormones.<ref name="Wallin">{{cite journal |last=Wallin |first=M. |url=http://www.bioscience-explained.org/ENvol1_2/pdf/paletteEN.pdf |archive-url=https://web.archive.org/web/20110722083738/http://www.bioscience-explained.org/ENvol1_2/pdf/paletteEN.pdf |archive-date=2011-07-22 |url-status=live |title=Nature's Palette |journal=Bioscience Explained |year=2002 |access-date=17 November 2011 |volume=1 |issue=2 |pages=1–12}}</ref>

The skins of cephalopods such as the octopus contain complex units, each consisting of a chromatophore with surrounding muscle and nerve cells.{{sfn|Cott|1940|page=32}} The cephalopod chromatophore has all its pigment grains in a small elastic sac, which can be stretched or allowed to relax under the control of the brain to vary its opacity. By controlling chromatophores of different colours, cephalopods can rapidly change their skin patterns and colours.<ref name="Cloney">{{cite journal |title=Ultrastructure of Cephalopod Chromatophore Organs |author1=Cloney, R. A. |author2=Florey, E. |journal=Zeitschrift für Zellforschung und Mikroskopische Anatomie |volume=89 |issue=2 |pages=250–280 |year=1968 |pmid=5700268 |doi=10.1007/BF00347297|s2cid=26566732 }}</ref><ref>{{cite web |url=http://marinebio.org/species.asp?id=553 |title=Day Octopuses, Octopus cyanea |publisher=MarineBio Conservation Society |access-date=31 January 2013 |archive-url=https://web.archive.org/web/20160320205031/http://marinebio.org/species.asp?id=553 |archive-date=20 March 2016 |url-status=dead }}</ref>

On a longer timescale, animals like the [[Arctic hare]], [[Arctic fox]], [[stoat]], and [[rock ptarmigan]] have [[snow camouflage]], changing their coat colour (by moulting and growing new fur or feathers) from brown or grey in the summer to white in the winter; the Arctic fox is the only species in the [[Canidae|dog family]] to do so.<ref name="Churchill">{{cite web |url=http://churchillpolarbears.org/churchill/arctic-wildlife |title=Arctic Wildlife |publisher=Churchill Polar Bears |year=2011 |access-date=22 December 2011}}</ref> However, Arctic hares which live in the far north of [[Canada]], where summer is very short, remain white year-round.<ref name="Churchill"/><ref name="ParksCanada">{{cite book |title=The Status of Arctic Hare (Lepus arcticus bangsii) in Insular Newfoundland |url=http://www.env.gov.nl.ca/env/wildlife/endangeredspecies/ssac/arctic_hare.pdf |publisher=Newfoundland Labrador Department of Environment and Conservation |last=Hearn |first=Brian |date=20 February 2012 |access-date=3 February 2013 |page=7 |archive-url=https://web.archive.org/web/20160304200851/http://www.env.gov.nl.ca/env/wildlife/endangeredspecies/ssac/arctic_hare.pdf |archive-date=4 March 2016 |url-status=dead }}</ref>

The principle of varying coloration either rapidly or with the changing seasons has military applications. ''[[Active camouflage]]'' could in theory make use of both dynamic colour change and counterillumination. Simple methods such as changing uniforms and repainting vehicles for winter have been in use since World War II. In 2011, [[BAE Systems]] announced their [[Adaptiv]] infrared camouflage technology. It uses about 1,000 hexagonal panels to cover the sides of a tank. The [[Peltier plate]] panels are heated and cooled to match either the vehicle's surroundings (crypsis), or an object such as a car (mimesis), when viewed in infrared.<ref>{{Cite news |url=https://www.bbc.co.uk/news/technology-14788009 |journal=BBC News |title=Tanks test infrared invisibility cloak |date=5 September 2011 |access-date=13 June 2012}}</ref><ref name=Adaptiv-Cloak>{{cite web |url=http://www.baesystems.com/en/feature/adativ-cloak-of-invisibility |title=Adaptiv – A Cloak of Invisibility |publisher=BAE Systems |year=2011 |access-date=14 November 2015}}</ref><ref>{{cite web |url=http://www.baesystems.com/en/feature/adativ-cloak-of-invisibility#! |title=Innovation Adaptiv Car Signature |publisher=BAE Systems |year=2012 |access-date=14 November 2015}}</ref>

<gallery class="center" mode="nolines" heights="150px" widths="150px">
File:Rock Ptarmigan (Lagopus Muta).jpg|Rock ptarmigan, changing colour in springtime. The male is still mostly in winter plumage
File:Norwegian Winter War Volunteers.jpg|Norwegian volunteer soldiers in [[Winter War]], 1940, with white camouflage overalls over their uniforms
File:Arctic Hare.jpg|Arctic hares in the low arctic change from brown to white in winter
File:Bundesarchiv Bild 101III-Roth-173-01, Russland, Raum Charkow, Jagdpanzer.jpg|Snow-camouflaged German [[Marder III]] [[jagdpanzer]] and white-overalled crew and infantry in Russia, 1943
File:Yemen Chameleon (cropped).jpg|[[Veiled chameleon]], ''Chamaeleo calyptratus'', changes colour mainly in relation to mood and for signalling.
File:Adaptiv infrared camouflage demo hiding tank as car.jpg|[[Adaptiv]] infrared camouflage lets an armoured vehicle mimic a car.
</gallery>

=== Self-decoration ===

{{main|Self-decoration}}

Some animals actively seek to hide by decorating themselves with materials such as twigs, sand, or pieces of shell from their environment, to break up their outlines, to conceal the features of their bodies, and to match their backgrounds. For example, a [[caddisfly]] larva builds a decorated case and lives almost entirely inside it; a [[Naxia tumida|decorator crab]] covers its back with seaweed, sponges, and stones.{{sfn|Forbes|2009|pages=50–51 and passim}} The [[Nymph (biology)|nymph]] of the predatory [[Reduvius personatus|masked bug]] uses its hind legs and a '[[Tarsus (skeleton)|tarsal]] fan' to decorate its body with sand or dust. There are two layers of bristles ([[trichome]]s) over the body. On these, the nymph spreads an inner layer of fine particles and an outer layer of coarser particles. The camouflage may conceal the bug from both predators and prey.<ref>{{cite journal |last=Wierauch |first=C. |year=2006 |title=Anatomy of disguise: camouflaging structures in nymphs of Some Reduviidae (Heteroptera) |journal=American Museum Novitates |issue=3542 |pages=1–18 |doi=10.1206/0003-0082(2006)3542[1:AODCSI]2.0.CO;2 |hdl=2246/5820 |s2cid=7894145 |url=http://digitallibrary.amnh.org/bitstream/2246/5820/1//v3/dspace/updateIngest/pdfs/N3542.pdf |archive-url=https://web.archive.org/web/20170816190844/http://digitallibrary.amnh.org/bitstream/2246/5820/1//v3/dspace/updateIngest/pdfs/N3542.pdf |archive-date=2017-08-16 |url-status=live }}</ref><ref>{{cite magazine |last=Bates |first=Mary |title=Natural Bling: 6 Amazing Animals That Decorate Themselves |url=http://news.nationalgeographic.com/2015/06/150610-animals-camouflage-decoration-bugs-science |archive-url=https://web.archive.org/web/20150611230313/http://news.nationalgeographic.com/2015/06/150610-animals-camouflage-decoration-bugs-science/ |url-status=dead |archive-date=11 June 2015 |magazine=National Geographic |access-date=11 June 2015|date=2015-06-10 }}</ref>

Similar principles can be applied for military purposes, for instance when a [[sniper]] wears a [[ghillie suit]] designed to be further camouflaged by decoration with materials such as tufts of grass from the sniper's immediate environment. Such suits were used as early as 1916, the British army having adopted "coats of motley hue and stripes of paint" for snipers.{{sfn|Forbes|2009|pages=102–103}} Cott takes the example of the larva of the [[blotched emerald]] moth, which fixes a screen of fragments of leaves to its specially hooked bristles, to argue that military camouflage uses the same method, pointing out that the "device is ... essentially the same as one widely practised during the Great War for the concealment, not of caterpillars, but of caterpillar-tractors, [gun] battery positions, observation posts and so forth."{{sfn|Cott|1940|page=360}}<ref>{{Cite journal |last1=Ruxton |first1=Graeme D. |author1-link=Graeme Ruxton |last2=Stevens |first2=Martin |author2-link=Martin Stevens (biologist) |title=The evolutionary ecology of decorating behaviour |journal=Biology Letters |date=1 June 2015 |page=20150325 |volume=11 |issue=6 |doi=10.1098/rsbl.2015.0325 |pmid=26041868 |pmc=4528480 }}</ref>

<gallery class="center" mode="nolines" heights="150px" widths="150px">
File:Hyastenus elatus.jpg|This [[Hyastenus elatus|decorator crab]] has covered its body with sponges.
File:IDF-CombatEngineeringSniper001.jpg|Sniper in a [[Ghillie suit]] with plant materials
File:Reduvius personatus, Masked Hunter Bug nymph camouflaged with sand grains.JPG|''[[Reduvius personatus]]'', masked hunter bug nymph, camouflaged with sand grains
File:Battle of Lake Khasan-Camouflaged soviet tanks.jpg|Soviet tanks under netting dressed with vegetation, 1938
</gallery>

=== Transparency ===

{{further|Underwater camouflage}}

[[File:Expl0469 - Flickr - NOAA Photo Library.jpg|thumb|upright|left|Many animals of the open sea, like this ''[[Aurelia labiata]]'' jellyfish, are largely transparent.]]

Many [[Sea|marine]] animals that float near the surface are highly [[Transparency and translucency|transparent]], giving them almost perfect camouflage.{{sfn|Herring|2002|pages=190–191}} However, transparency is difficult for bodies made of materials that have different [[refractive index|refractive indices]] from seawater. Some marine animals such as [[jellyfish]] have gelatinous bodies, composed mainly of water; their thick [[Mesoglea|mesogloea]] is acellular and highly transparent. This conveniently makes them [[buoyancy|buoyant]], but it also makes them large for their muscle mass, so they cannot swim fast, making this form of camouflage a costly trade-off with mobility.{{sfn|Herring|2002|pages=190–191}} Gelatinous [[plankton]]ic animals are between 50 and 90 percent transparent. A transparency of 50 percent is enough to make an animal invisible to a predator such as [[cod]] at a depth of {{convert|650|m|ft}}; better transparency is required for invisibility in shallower water, where the light is brighter and predators can see better. For example, a cod can see prey that are 98 percent transparent in optimal lighting in shallow water. Therefore, sufficient transparency for camouflage is more easily achieved in deeper waters.{{sfn|Herring|2002|pages=190–191}}

[[File:Hyalinobatrachium uranoscopum01a.jpg|thumb|upright|[[Glass frogs]] like ''[[Hyalinobatrachium uranoscopum]]'' use partial transparency for camouflage in the dim light of the rainforest.]]

Some tissues such as [[muscle]]s can be made transparent, provided either they are very thin or organised as regular layers or fibrils that are small compared to the wavelength of visible light. A familiar example is the transparency of the lens of the vertebrate [[eye]], which is made of the protein [[crystallin]], and the vertebrate [[cornea]] which is made of the protein [[collagen]].{{sfn|Herring|2002|pages=190–191}} Other structures cannot be made transparent, notably the [[retina]]s or equivalent light-absorbing structures of eyes – they must absorb light to be able to function. The [[camera]]-type eye of vertebrates and cephalopods must be completely opaque.{{sfn|Herring|2002|pages=190–191}} Finally, some structures are visible for a reason, such as to lure prey. For example, the [[nematocyst]]s (stinging cells) of the transparent [[siphonophore]] ''[[Agalma okenii]]'' resemble small [[copepod]]s.{{sfn|Herring|2002|pages=190–191}} Examples of transparent marine animals include a wide variety of [[larva]]e, including [[radiata]] (coelenterates), siphonophores, [[salps]] (floating [[tunicate]]s), [[gastropoda|gastropod molluscs]], [[polychaete]] worms, many shrimplike [[crustacean]]s, and fish; whereas the adults of most of these are opaque and pigmented, resembling the seabed or shores where they live.{{sfn|Herring|2002|pages=190–191}}{{sfn|Cott|1940|page=6}} Adult [[comb jelly|comb jellies]] and jellyfish obey the rule, often being mainly transparent. Cott suggests this follows the more general rule that animals resemble their background: in a transparent medium like seawater, that means being transparent.{{sfn|Cott|1940|page=6}} The small [[Amazon River]] fish ''[[Microphilypnus amazonicus]]'' and the shrimps it associates with, ''[[Pseudopalaemon gouldingi]]'', are so transparent as to be "almost invisible"; further, these species appear to select whether to be transparent or more conventionally mottled (disruptively patterned) according to the local background in the environment.<ref>{{cite journal |title=The almost invisible league: crypsis and association between minute fishes and shrimps as a possible defence against visually hunting predators |author1=Carvalho, Lucélia Nobre |author2=Zuanon, Jansen |author3=Sazima, Ivan |journal=Neotropical Ichthyology |date=April–June 2006 |volume=4 |issue=2 |pages=219–224 |doi=10.1590/S1679-62252006000200008|doi-access=free }}</ref>

=== Silvering ===

[[File:Herringadultkils2.jpg|thumb|left|upright=1.15|The adult herring, ''[[Clupea harengus]]'', is a typical silvered fish of medium depths, camouflaged by reflection.]]
[[File:Herring Silvering.jpg|thumb|upright=0.7|The herring's reflectors are nearly vertical for camouflage from the side.]]

Where transparency cannot be achieved, it can be imitated effectively by silvering to make an animal's body highly reflective. At medium depths at sea, light comes from above, so a mirror oriented vertically makes animals such as fish invisible from the side. Most fish in the upper ocean such as [[sardine]] and [[herring]] are camouflaged by silvering.{{sfn|Herring|2002|pages=192–195}}

The [[marine hatchetfish]] is extremely flattened laterally, leaving the body just millimetres thick, and the body is so silvery as to resemble [[aluminium foil]]. The mirrors consist of microscopic structures similar to those used to provide [[structural coloration]]: stacks of between 5 and 10 crystals of [[guanine]] spaced about {{frac|1|4}} of a wavelength apart to interfere constructively and achieve nearly 100 per cent reflection. In the deep waters that the hatchetfish lives in, only blue light with a wavelength of 500 nanometres percolates down and needs to be reflected, so mirrors 125 nanometres apart provide good camouflage.{{sfn|Herring|2002|pages=192–195}}

In fish such as the herring which live in shallower water, the mirrors must reflect a mixture of wavelengths, and the fish accordingly has crystal stacks with a range of different spacings. A further complication for fish with bodies that are rounded in cross-section is that the mirrors would be ineffective if laid flat on the skin, as they would fail to reflect horizontally. The overall mirror effect is achieved with many small reflectors, all oriented vertically.{{sfn|Herring|2002|pages=192–195}} Silvering is found in other marine animals as well as fish. The [[cephalopods]], including squid, octopus and cuttlefish, have multilayer mirrors made of protein rather than guanine.{{sfn|Herring|2002|pages=192–195}}

=== Counter-illumination ===

{{main|Counter-illumination}}

[[File:Squid Counterillumination.png|thumb|Principle of [[counter-illumination]] in the [[Watasenia scintillans|firefly squid]]]]

Counter-illumination means producing light to match a background that is brighter than an animal's body or military vehicle; it is a form of active camouflage. It is notably used by some species of [[squid]], such as the [[Watasenia scintillans|firefly squid]] and the [[Abralia veranyi|midwater squid]]. The latter has light-producing organs ([[photophores]]) scattered all over its underside; these create a sparkling glow that prevents the animal from appearing as a dark shape when seen from below.<ref name="Abralia with photo">{{cite web |url=http://ocean.si.edu/ocean-photos/midwater-squid-abralia-veranyi |title=Midwater Squid, Abralia veranyi |publisher=Smithsonian National Museum of Natural History |access-date=28 November 2011}}</ref> Counterillumination camouflage is the likely function of the [[bioluminescence]] of many marine organisms, though light is also produced to attract<ref>{{cite journal |last=Young |first=Richard Edward |title=Oceanic Bioluminescence: an Overview of General Functions |journal=Bulletin of Marine Science |date=October 1983 |volume=33 |issue=4 |pages=829–845 |url=http://www.ingentaconnect.com/content/umrsmas/bullmar/1983/00000033/00000004/art00003}}</ref> or to detect prey<ref>{{cite journal |last1=Douglas |first1=R. H. |last2=Mullineaux |first2=C. W. |last3=Partridge |first3=J. C. |title=Long-wave sensitivity in deep-sea stomiid dragonfish with far-red bioluminescence: evidence for a dietary origin of the chlorophyll-derived retinal photosensitizer of Malacosteus niger |journal=Philosophical Transactions of the Royal Society B |date=September 2000 |volume=355 |issue=1401 |pages=1269–1272 |pmc=1692851 |doi=10.1098/rstb.2000.0681 |pmid=11079412}}</ref> and for signalling.

Counterillumination has rarely been used for military purposes. "[[Diffused lighting camouflage]]" was trialled by Canada's [[National Research Council (Canada)|National Research Council]] during the Second World War. It involved projecting light on to the sides of ships to match the faint glow of the night sky, requiring awkward external platforms to support the lamps.<ref name="NavalMuseumQuebec">{{cite web |title=Diffused Lighting and its use in the Chaleur Bay |url=http://www.navy.forces.gc.ca/navres/NMQ_MNQ/researches_recherches/diffusedLighting_camouflageLumineux/index-eng.asp |access-date=3 February 2013 |work=Naval Museum of Quebec |publisher=Royal Canadian Navy |archive-url=https://web.archive.org/web/20130522231113/http://www.navy.forces.gc.ca/navres/NMQ_MNQ/researches_recherches/diffusedLighting_camouflageLumineux/index-eng.asp |archive-date=22 May 2013}}</ref> The Canadian concept was refined in the American [[Yehudi lights]] project, and trialled in aircraft including [[Consolidated B-24 Liberator|B-24 Liberators]] and naval [[Grumman TBF Avenger|Avengers]].<ref name="Hambling">{{cite magazine |last=Hambling |first=David |title=Cloak of Light Makes Drone Invisible? |magazine=Wired |date=9 May 2008 |url=https://www.wired.com/dangerroom/2008/05/invisible-drone |access-date=17 June 2012}}</ref> The planes were fitted with forward-pointing lamps automatically adjusted to match the brightness of the night sky.<ref name="NavalMuseumQuebec"/> This enabled them to approach much closer to a target – within {{Convert|3000|yards}} – before being seen.<ref name="Hambling"/> Counterillumination was made obsolete by [[radar]], and neither diffused lighting camouflage nor Yehudi lights entered active service.<ref name="NavalMuseumQuebec"/>

<gallery class="center" mode="nolines" heights="150px" widths="150px">
File:HMS Largs by night with incomplete Diffused Lighting Camouflage 1942.jpg|[[HMS Largs|HMS ''Largs'']] by night with incomplete [[diffused lighting camouflage]], 1942, set to maximum brightness
File:HMS Largs bulwark with Diffused Lighting Camouflage fittings.jpg|Bulwark of HMS ''Largs'' showing 4 (of about 60) diffused lighting fittings, 2 lifted, 2 deployed
File:Principle of Yehudi Lights with Avenger head-on view.jpg|[[Yehudi Lights]] raise the average brightness of the plane from a dark shape to the same as the sky.
</gallery>

=== Ultra-blackness ===

{{further|Underwater camouflage}}

[[File:Humpback anglerfish.png|thumb|[[Humpback anglerfish|Blackdevil anglerfish]] is one of several deep-sea fishes camouflaged against very dark water with a black dermis.]]

Some deep sea fishes have very black skin, reflecting under 0.5% of ambient light. This can prevent detection by predators or prey fish which use bioluminescence for illumination. ''[[Oneirodes]]'' had a particularly black skin which reflected only 0.044% of 480&nbsp;nm wavelength light. The ultra-blackness is achieved with a thin but continuous layer of particles in the [[dermis]], [[melanosome]]s. These particles both absorb most of the light, and are sized and shaped so as to scatter rather than reflect most of the rest. Modelling suggests that this camouflage should reduce the distance at which such a fish can be seen by a factor of 6 compared to a fish with a nominal 2% reflectance. Species with this adaptation are widely dispersed in various orders of the [[phylogenetic tree]] of bony fishes ([[Actinopterygii]]), implying that [[natural selection]] has driven the [[convergent evolution]] of ultra-blackness camouflage independently many times.<ref name="Davis Thomas 2020">{{cite journal |last1=Davis |first1=Alexander L. |last2=Thomas |first2=Kate N. |last3=Goetz |first3=Freya E. |last4=Robison |first4=Bruce H. |last5=Johnsen |first5=Sönke |last6=Osborn |first6=Karen J. |display-authors=3 |title=Ultra-black Camouflage in Deep-Sea Fishes |journal=Current Biology |year=2020 |volume=30 |issue=17 |pages=3470–3476.e3 |issn=0960-9822 |doi=10.1016/j.cub.2020.06.044 |pmid=32679102 |doi-access=free |bibcode=2020CBio...30E3470D }}</ref>