Necrophages are animals that feed on decomposing dead animal biomass, such as the muscle and soft tissue of carcasses andcorpses (also known ascarrion).^[1][2] The term derives from Greeknekros, meaning 'dead', andphagein, meaning 'to eat'.^[2][3] Many hundreds of necrophagous species have been identified including invertebrates in theinsect,^[4]malacostracan^[5] andgastropod^[6]classes and vertebrates such asvultures,hyenas,quolls andwolves.^[5]

Necrophagous insects are important inforensic science^[4] as the presence of some species (e.g.Calliphora vomitoria) in abody, coupled with information on their development stage (e.g. egg, larva, pupa), can yield information ontime of death.^[7][8] Information on the insect species present can also be used as evidence that a body has been moved,^[7][9] and analysis of insect tissue can be used as evidence that drugs or other substances were in the body.^[7][10]

Necrophages are useful for other purposes too. Inhealthcare,green bottle fly larvae are sometimes used to removenecrotic (dead) tissue from non-healing wounds,^[11][12] and inwaste management,black soldier fly larvae are used to convert decomposing organic waste into animal feed.^[13][14] Biotechnological applications for necrophage-derivedgenes,molecules andmicrobes are also being explored.^[5][15]

Necrophages can be classified according to their nutritional reliance on carrion and also their level ofadaptation to carrion feeding. Animals are described as 'obligate necrophages' if they use carrion as their sole or main food source and depend on carrion for survival or reproduction.^[5] The term 'specialists' is also sometimes used in recognition that these animals havetraits favoring necrophagy and making other feeding behaviors difficult.^[18] For example, largewingspans facilitate the energy-efficientgliding vultures need to cover long distances in search of carrion,^[16] but reduce theagility needed to killprey.^[17] Animals that eat carrion opportunistically and retain the traits needed to find and consume other food sources are described as 'facultative necrophages' and 'generalists'.^[5][17]

Both obligate and facultative necrophages are sometimes sub-classified as 'wet' and 'dry' feeders.^[19] These terms differentiate animals feeding on moist, putrefying tissue from animals feeding on desiccated and keratinized tissues.^[19]

The European bone skipper,Thyreophora cynophila, is an obligately necrophagousfly. It relies on carrion bone marrow in the first stage of its life cycle.^[20] Many other types of fly are facultatively necrophagous. Examples commonly found on land includeblow flies,flesh flies,muscid flies,black soldier flies,ensign flies andthread-horns. Other necrophagous flies, for exampleblack flies andlake flies, are semi-aquatic.^[4][21] Types of carrion fed upon include wildlife,^[22][23][24] livestock and poultry carcasses, slaughterhouse and fishing discards, and human bodies.^[5]

Necrophagous flies have severaltraits andadaptations that facilitate their feeding behavior. For example, blow flies and flesh flies have a well-developed sense of smell^[26][27] and are highly mobile.^[7][19] This enables them to rapidly detect and locate carrion.^[7][25][26] Also, flesh flies and some blow flies laylarvae instead ofeggs.^[26][28] This gives these flies a competitive advantage over other slower-developing, egg-laying species.^[28] In addition, blow flies, flesh flies, muscid flies and black soldier flies have many defenses against the pathogens and toxins found in carrion. These include aprotective lining in their midgut,antibiotic-producingmicrobiota species, and a large number ofpattern recognition receptors,lysozymes,antimicrobial peptides anddetoxification enzymes.^[5]

The diversity and abundance of necrophagous fly species vary geographically and seasonally.^[7][20][27] For example,Chrysomya species are present in subtropical regions of the USA but are rare in most of Canada.^[7] This geographic variation is attributable to factors such as soil type and meteorological conditions, and the effects these have on carrion decomposition.^[7] Whether urbanization affects fly species richness is open to dispute.^[27][29] Seasonally, many necrophagous fly species are observed in higher abundance in summer,^[27] butThyreophora cynophila is more active in winter.^[20]

Flies play a critical role in forensic science as they are often the first insects to discover and colonize human remains.^[7][21] Blow flies can arrive within minutes and begin laying eggs in the nose, mouth and other openings. Because adult flies very rarely deposit eggs in live hosts, the age of the developingfly larvae can be used to estimatetime of death.^[7] Fly larvae can also provide information regardingcause of death because necrophagous flies deposit their eggs in any open wounds.^[7]

Vulture bees are a small group of obligately necrophagous bees in theTrigonagenus.^[30][31]Trigona worker bees play a similar role to worker bees in theApis genus; however, along with collecting pollen, nectar, and plant resins,Trigona workers also collect carrion.^[30][32] Although pollen is associated with higher energy value, carrion is preferred byTrigona bees because it is biochemically easier to extract energy from.^[31] This dead animal tissue is used as a source of amino acids too.^[33]

Cerumen pots are utilized by someTrigona species, such asT. necrophaga, as vesicles to store foodstuff.^[34] The foodstuff ofT. necrophaga consists of both honey and carrion from vertebrate carcasses.^[30] Ultimately, the stored food is utilized by developing larvae and the worker bee itself as a source of nutrition and energy. Due to the rapid decomposition of carrion, especially in warm temperatures, the bees must efficiently metabolize the carrion to avoid rotten carrion in their cerumen pots.^[30]

Trigona hypogea communicate the presence of a valuable carcass through olfactory signals.^[30] The bees create an odour trail between their nest and the prospective animal carcass; thus, the bees recruit the other nest members to respond and exploit the corpse's resources rapidly. Additionally, interspecific competition is observed inTrigona hypogea bees. The bees are observed to defend their colonized food item, including but not limited to a monkey, lizard, fish, or snake carcass, from competing necrophages, such as flies.

Numerous beetles in theNicrophorus genus are obligately necrophagous, for exampleNicrophorus americanus andN. vespilloides.^[5] Many other beetles are facultative necrophages includingcheckered beetles,^[22]dermestid beetles,^[7]diving beetles,^[21][35]scarab beetles,^[36]silphine beetles^[7] andwater scavenger beetles.^[35] Types of carrion eaten include wildlife, livestock and poultry carcasses, livestock viscera and human bodies.^[5]

Necrophagous beetles have evolved many diet-relatedadaptations. For example,Nicrophorus species have specializedolfactory sensors on theirantennae to help them detect carrion.^[37][38] These sensors are sensitive todimethyl sulphide and othersulfur-containing compounds emitted by bodies after death.^[39] Also,Nicrophorus,Necrodes and other necrophagoussilphine beetle species are flight-capable, making it easier for them to reach carrion.^[40][41][42]Nicrophorus andDermestes species have many defenses against dietarypathogens andtoxins too. These include physical traits such asprotective gut linings, antibacteriallectins andlysozymes,mutualistic relationships withmicrobiotabacteria, and behavioral traits such as preferentially selecting fresh carcasses and smearing carcasses with antibacterial and toxin-degrading exudates.^[5] Given the often-limited availability of carrion, the ability of these beetles toshare this resource with other beetles^[37][38] and defend it against flies and ants^[43][44] is also an advantage.

Regarding food preferences and the logistics of carrion use,N. vespilloides and otherburying beetles favor small carcasses (e.g. rodents and small birds)^[39] as these are easier to transport, clean and conceal from competitors.^[38][45] Diving beetles, scarab beetles and water scavenger beetles have all been observed feeding on amphibian carrion (e.g.granular toads andtree frogs).^[35][36] The scarab beetleScybalocanthon nigriceps uses its front legs andclypeus to shape frog carrion into pellets for eventual consumption.^[36] Other scarab beetles, for example,Coprophanaeus ensifer, build their burrows near carcasses for easier transportation of carrion pieces to offspring.^[46]

Beetles that feed on human remains are important inforensic science. Terrestrial beetles such ascheckered beetles anddermestid beetles colonize bodies in a predictable sequence and have well-characterized life cycles, so they can sometimes be used to estimatetime of death.^[7][47] Aquatic beetles are less useful for estimating time of death^[7] but can cause physical damage to submerged bodies that must be distinguished from inflicted injuries when determiningcause of death.^[21] For example, the facultatively necrophagous diving beetleMeridiorhantus validus createspostmortem channels and chambers in human bodies that must be differentiated fromantemortem piercing injuries.^[21]

TheNassa mud snails,Nassarius festivus andNassarius clarus, scavenge on dead and decaying animal matter in the intertidal zone ofeulittoral soft shores.^[6][48] AtShark Bay in Australia,Nassarius clarus feeds on thecarrion of fishes andbivalves.^[48] In the presence of carrion, the animal'sproboscis performs a search reaction followed by a quick onset of feeding.^[48] When faced with a competitor, such as ahermit crab, at the site of the carrion, theNassarius clarus attack the competition to defend their meal.^[48]Nassarius clarus are attracted to fish and bivalve carrion to a distance of 26meters and have a heightened interest in areas where the sand has been disturbed; thus, indicating the potential presence of organic detritus or damaged fauna.^[48]

Whelks such asBuccinum undatum,rock snails such asDicathais orbita,olive snails such asOliva sayana, and the bone-eatingRubyspira snails are also necrophagous.^[5]

Many marine invertebrates feed on carrion includingcephalopods (e.g.Octopus vulgaris),hermit crabs (e.g.Coenobita perlatus),squat lobsters (e.g.Munida tenuimana),star fish (e.g.Asterias rubens),sea anemones (e.g.Actinoscyphia aurelia),amphipods (e.g.Eurythenes gryllus),annelids (e.g.zombie worms), andribbon worms (e.g.Parborlasia corrugatus). Types of carrion consumed include deadseals,pilchards,jellyfish andtunicates, bones fromwhale falls, and fishery discards such aswhiting andlangoustine.^[5]

Marine necrophages are less useful inforensic science than terrestrial necrophages. This is partly because human deaths occur less frequently in marine settings than terrestrial settings, and partly becausehuman remains are less likely to be recovered in marine settings. In addition, in any aquatic system, there are a large number of environmental and biological factors that can confound calculation of minimumpost-mortem interval. These includecurrent andwave action, water temperature, oxygen concentration, and a greaterdiversity of necrophagous organisms colonizing the remains.^[7]

Many vulture species are obligately necrophagous including thebearded vulture,black vulture,cinereous vulture,Eurasian griffon,Himalayan vulture,king vulture andturkey vulture.^[5][16] Types of carrion fed upon include dead wildlife, livestock, poultry and companion animals, human remains (sky burial), hunting discards, slaughterhouse offal and roadkill.^[5] Typically, muscle tissue is consumed,^[50] but bearded vultures feed on bones and bone marrow.^[51] In addition to eating carrion,Egyptian vultures feed on small live animals such as turtles, eggs and rotting fruit.^[52][53][54]

Vultures have many adaptations that help them detect, locate and consume carrion. For example, all vultures have keen eyesight,^[16] andNew World vultures have a highly developed sense of smell.^[16][17]Hooded vultures also have excellent auditory perception, enabling them to hear distant predation-related noises and the distress calls of dying animals.^[55] In addition,gliding flight enables vultures to cover long distances to reach carrion,^[16][17] strongbeaks allow vultures to cut through thickanimal skin,^[56] and strongimmune defenses protect vultures frompathogens in carrion.^[5] Given the inherently unpredictable andephemeral nature of carrion as a food source, the ability of vultures to survive long periods between meals is also advantageous.^[57][58]

Somehuman activities have had an adverse impact on vultures inSicily,^[52] theAzerbaijan Republic^[59] and other countries.^[60] For example, changes infarming practices such as the indoor raising of cattle and incineration or burial of cattle carcasses have reduced food availability for Eurasian griffon vultures.^[52][59] Shootings of birds, removal of nestlings from nests,^[59] and drug pollution^[60] have also contributed to declines in vulture populations.

Although vultures are the only birds that depend on carrion for food, many other birds eat carrion opportunistically. Numerousbirds of prey, for examplewedge-tailed eagles,brown goshawks andbuzzards, feed on carrion.Long-eared owls andgreat horned owls also feed on carrion, as dowading birds such as themarabou stork andgreater adjutant,seabirds such as thenorthern giant petrel andivory gull, andpasserine birds such as thecarrion crow,rook andforest raven. Types of carrion eaten include dead wildlife, dead livestock,roadkill, and carrion stolen from vultures or raided fromcrocodilecaches.^[5]

Many species ofmonitor lizard are facultatively necrophagous. For example,Indian monitors,Komodo dragons andlace monitors feed on dead fish,heath monitors feed onmarsupial carrion, andyellow-spotted monitors feed on carrion raided fromcrocodilecaches. Komodo dragons also feed onhuman remains.^[5] On the island ofKomodo, villagers had to move their graves from sandy to clay ground, and pile rocks on top of them, to stop lizards digging up and eating the bodies there.^[61]

Manycanids feed opportunistically on carrion. Examples includefoxes such as theArctic fox,corsac fox,gray fox,kit fox andred fox,hyenas such as thebrown hyena,spotted hyena andstriped hyena,black-backed jackals andgolden jackals, as well ascoyotes,culpeos,dingoes,gray wolves,Pampas zorros, andraccoon dogs. Types of carrion fed upon ranges from dead wildlife and dead livestock to abandoned predator kills and hunting discards.^[5]

Anatomically and immunologically, canids are well-adapted to carrion feeding. For example,gray wolves hold food in theirstomachs for at least twice as long ashumans and otherprimates.^[5][15] This gives theirgastric acid longer to killpathogens before they enter thesmall intestine.^[15] Theintestinal tracts of canids are also much shorter than those of primates, so pathogens have less time tomultiply and causedisease prior to theirexpulsion.^[62] Anti-botulinum antibodies are an additional antibacterial defense detected in coyotes.^[5]

Possums, quolls and Tasmanian devils are facultatively necrophagous. For example,brushtail possums feed opportunistically onwild boar,kangaroo,pademelon anddeer carrion,spotted-tailed quolls feed on kangaroo and pademelon carrion, andTasmanian devils feed onpossum,wallaby, kangaroo,wombat and pademelon carrion.^[5] Historically, quolls have also fed on human remains.^[63]

Pademelon carrion is eaten frequently by Australian necrophages as pademelons are common in farming areas there, and often end up as roadkill.^[64]

Brushtail possums eat carrion mostly inwinter. This may be because there is less competition from necrophagous insects in winter, or because other food items such as leaves and fruit are more difficult to find.^[65]Quolls are also thought to scavenge mostly in winter, perhaps because they are hungriest in winter or because carrion is more readily available in winter.^[63]

Numerous species ofhagfish feed on carrion, includingAtlantic hagfish,black hagfish,blueband hagfish,Pacific hagfish,slender hagfish andwhiteface hagfish. Types of carrion eaten include deadwhales,hake,mackerel,pilchards andjellyfish, as well as fishery discards ofwhiting andlangoustine.^[5]

Hagfish are highly adapted to carrion feeding. For example, they are able to survive extended periods without eating^[66] and can detect carrion from long distances. Hagfish are also highly mobile and can arrive at carcasses within minutes or hours of an animal’s death. Immune defenses such aslysozyme-containing slime and antimicrobial proteins (e.g. myxinidin) are thought to protect hagfish from microbialpathogens.^[5]

In maggot debridement therapy,sterile, medical-grade larvae of the necrophagous flyLucilia sericata are used to eliminatenecrotic (dead) tissue from non-healing skin and soft-tissue wounds.^[68][69][70] This is important as dead tissue can facilitatebacterial growth, impedewound healing, and reduce the effectiveness oftopical medications.^[71] Physicians may administer the fly larvae directly to skin and soft tissue wounds^[72] or indirectly within a sealed mesh bag.^[67][73] Larvae then debride the wound by digesting, liquefying and consuming the dead tissue.^[68][69] The USFood and Drug Administration (FDA) have clearedLucilia sericata larvae for use as a "medical device" in the US todebride several types of wound including pressure ulcers, neuropathic foot ulcers, and nonhealing surgical wounds.^[72][73]

Necrophagous flies and beetles play an important role inforensic entomology due to theirpostmortem colonization of human remains.^[7][21] For example, inhomicide cases,forensic medical examiners can sometimes determine the minimumpost-mortem interval based on the fly and beetle species present in the body and their development stage.^[7][74] This is because these insects rarely deposit eggs in live hosts, they colonize bodies in a predictablesequence following death, and information is available on how long it takes different species to reach different stages of development.^[7] Because insect arrival and departure time and larval development time are affected by seasonal changes,^[75] temperature,^[76]moisture levels,^[77]air exposure,^[78]geographical region,^[79] and other factors, these must all be carefully considered when estimating minimum post-mortem interval.^[80]

In some countries, the black soldier fly,Hermetia illucens, is used to process food industry by-products and food waste.Hermetia illucens is a facultative necrophage and can grow on a wide range of decomposingorganic substrates including those of animal origin (e.g.abattoir waste),^[81] plant origin (e.g.almondhulls),^[82] and a mix of both (e.g. meat, fish and vegetablefood waste).^[83] Fly larvae are grown on this organic waste and then used aslivestock feed orfish feed. Thefrass generated by the larvae can be used as soilfertilizer too. This waste conversion process, known asbioconversion, has several advantages. It reduces thegreenhouse gas emissions caused by microbial decomposition of food waste inlandfills (e.g.methane), it generates high-qualityprotein for feeding livestock, and it generates low-cost fertilizer forcrop cultivation.^[84][85]

In natural history museums, researchers and curators sometimes wish to study or display animalskeletons. In theNatural History Museum in London, theAmerican Museum of Natural History in New York, and theAustralian National Wildlife Collection in Canberra,dermestid beetles are used for skeleton preparation. Dermestid beetles eathair,skin andflesh, but not bones, and are considered an excellent way of creating clean, professional-gradebone specimens.^[86][87]

InTibet and other parts ofAsia, necrophagous birds play a central role in a traditionalfuneral practice calledsky burial. Funerals begin with monks chanting mantra and burningjuniperincense, and the funeral party carrying thedeceased to a large fenced mountain meadow. In this meadow, the body is then cut apart by sky burial masters, andvultures move in to eat it. Other birds and animals, for examplemagpies andravens, eat any leftover tissue.^[88][89]

Antibiotic-producing bacteria have been discovered in thegut microbiotas ofburying beetles andflesh flies.^[5]

Necrophages and theirmicrobiotas ("friendlybacteria") produce severalmolecules of medical interest. These include molecules that can bind to bacterialpathogens (e.g.lectins), inhibit pathogen growth (e.g.chitin,cycliclipopeptides), and kill pathogens (e.g.antimicrobial peptides,lysozymes). In nature, these molecules are thought to block pathogen entry into theinteguments (e.g.skin,cuticle) andcirculatory systems (e.g.blood,hemolymph) of necrophages, and enable theimmune systems of necrophages todetect, inhibit and kill any pathogens that breach these barriers.^[5][15] Research is underway in Germany,^[90] China,^[91] the USA^[92] and other countries^[93][94] to develop these molecules for use in medicine. Possible applications include antimicrobialwounddressings,antibacterial drugs, anddrug delivery systems for bacterial infections.^[5][92][95]

Given the enormous damage thatcitrus canker,citrus greening,bacterial soft rot andwildfire disease cause tofood andcash crops each year, there is interest in developing new plant varieties that are more resistant to infection. One possible solution is to enhance the natural defenses of crops usingantimicrobial peptides. Necrophagous insects are a rich source of these peptides, andtransgenic research in Japan,^[96] the USA,^[96] and Brazil^[97][98] has shown thatsarcotoxin IA (fromSarcophaga peregrina) can help protectorange trees and other crops.^[5]

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  The American burying beetle,Nicrophorus americanus, about to take flight.
• The American carrion beetle,Necrophila americana, feeding on vole carrion.
• 
  The Ibiza wall lizard,Podarcis pityusensis, feeding on fish carrion.

• Thanatophage
• Detritivore
• Decomposer
• Saprotrophic nutrition
• Consumer-resource systems

• The dictionary definition ofnecrophage at Wiktionary

• Eating behaviors
• Zoology
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