Scavengers are animals that feed on dead and decayingorganic matter.[1][2] Often the term is used to describe the consumption ofcarrion, the bodies of animals that have died from causes other thanpredation or the bodies of animals that have been killed by other predators.[3] However, the term is also used to describe animals that feed on rotting plant matter[2][4] orrefuse.[1]
Scavengers play an important role inecosystems by preventing the accumulation of decaying matter and helping to recycle nutrients.[2][10] The process and rate at which dead plant and animal material is scavenged is affected by bothbiotic andabiotic factors, such as plant species, carcass size, habitat, temperature, moisture levels, and seasons.[11][12][13]Detritivores anddecomposers complete this process by consuming the remains left by scavengers.[14][15]
Scavenger is an alteration ofscavager, from Middle Englishskawager meaning "customs collector", fromskawage meaning "customs", from Old North Frenchescauwage meaning "inspection", fromschauwer meaning "to inspect", ofGermanic origin; akin toOld Englishscēawian and Germanschauen meaning "to look at", and modern English "show" (withsemantic drift).[16]
Animals that subsist entirely or mainly on decaying biomass (e.g. dead animals, dead plants) are called obligate scavengers, while those capable of obtaining food via other methods are termed facultative scavengers.[17] Animals that rely specifically on carrion as a food source are called obligate necrophages.[5][18] Animals that feed on particulate plant or animal matter (e.g.humus,marine snow) are typically categorized asdetritivores rather than scavengers.[14] The midge flyPropsilocerus akamusi, which feeds ondetritus in the sediment of freshwaterlakes, is an example of a detritivore.[19]
Most of the vertebrates that eat carrion are facultative scavengers, capable of obtaining food viapredation or other methods, and eating carrion opportunistically.[5] Many largecarnivores that hunt regularly, such ashyenas andjackals, but also animals rarely thought of as scavengers, such as Africanlions,leopards, andwolves will scavenge if given the chance.[2] They may also use their size and ferocity to intimidate the original hunters into abandoning their kills (thecheetah is a notable victim, rather than a perpetrator).[21]Gulls,crows andmagpies frequently scavengeroadkill.[22] Other vertebrates, for exampleEgyptian mastigures, scavenge to survive during times of food scarcity.[23]
The stag beetle,Lucanus cervus, feeding on dead wood during the larval stage of its life cycle
Somestag beetles are obligate scavengers of dead plant material. For example,Lucanus cervus is dependent on dead wood during the larval stages of its life cycle.[28] AdultLucanus cervus beetles lay their eggs near the stumps of dead trees, and the larvae then spend the next 4 to 7 years feeding and growing in size. Types of wood eaten include oak, ash, elm, sycamore, lime and hornbeam.[28]
Pink bud moth larvae (also known as pink scavenger caterpillars) are facultative scavengers of dead plant material, feeding on rotting fruits, decaying flowers and leaves, but also the fruits and grains of live plants.[6] Termites are facultative scavengers too. Termites feed on dead trees and wood, but also live plants anddetritus such ashumus andexcrement.[29][30] Additional examples of facultative scavengers in this category aredarkling beetles (also known as tenebrionids) andbanana moth larvae.[31][32]
In the prehistoric eras, the speciesTyrannosaurus rex may have been anapex predator, preying uponhadrosaurs,ceratopsians, and possibly juvenile sauropods,[35] although some experts have suggested the dinosaur was primarily a scavenger. The debate about whetherTyrannosaurus was an apex predator or scavenger was among the longest ongoing feuds inpaleontology; however, most scientists now agree thatTyrannosaurus was an opportunistic carnivore, acting mostly as a predator but also scavenging when it could sense it.[36] Recent research also shows that while an adultT. rex would energetically gain little through scavenging, smaller theropods of approximately 500 kg (1,100 lb) might have gained levels similar to those of hyenas, though not enough for them to rely on scavenging.[37]
Other research suggests that carcasses of giant sauropods may have made scavenging much more profitable to carnivores than it is now. For example, a single 40 tonneApatosaurus carcass would have been worth roughly 6 years of calories for an average allosaur. As a result of this resource oversupply, it is possible that some theropods evolved to get most of their calories by scavenging giant sauropod carcasses, and may not have needed to consistently hunt in order to survive.[38][39] The same study suggested that theropods in relatively sauropod-free environments, such as tyrannosaurs, were not exposed to the same type of carrion oversupply, and were therefore forced to hunt in order to survive.
Scavengers play a fundamental role in the environment through the removal of decaying organisms, serving as a natural sanitation service.[40] While microscopic and invertebratedecomposers break down dead organisms into simple organic matter which are used by nearbyautotrophs, scavengers help conserve energy and nutrients obtained from carrion within the uppertrophic levels, and are able to disperse the energy and nutrients farther away from the site of the carrion than decomposers.[41]
Scavenging unites animals which normally would not come into contact,[42] and results in the formation of highly structured and complex communities which engage in nonrandom interactions.[43] Scavenging communities function in the redistribution of energy obtained from carcasses and reducing diseases associated with decomposition. Oftentimes, scavenger communities differ in consistency due to carcass size and carcass types, as well as by seasonal effects as consequence of differing invertebrate and microbial activity.[11]
Competition for carrion results in the inclusion or exclusion of certain scavengers from access to carrion, shaping the scavenger community. When carrion decomposes at a slower rate during cooler seasons, competitions between scavengers decrease, while the number of scavenger species present increases.[11]
Alterations in scavenging communities may result in drastic changes to the scavenging community in general, reduceecosystem services and have detrimental effects on animal and humans.[43] The reintroduction of gray wolves (Canis lupus) into Yellowstone National Park in the United States caused drastic changes to the prevalent scavenging community, resulting in the provision of carrion to many mammalian and avian species.[11] Likewise, the reduction of vulture species in India lead to the increase of opportunistic species such as feral dogs and rats. The presence of both species at carcasses resulted in the increase of diseases such as rabies and bubonic plague in wildlife and livestock, as feral dogs and rats are transmitters of such diseases. Furthermore, the decline of vulture populations in India has been linked to the increased rates of anthrax in humans due to the handling and ingestion of infected livestock carcasses. An increase of disease transmission has been observed in mammalian scavengers in Kenya due to the decrease in vulture populations in the area, as the decrease in vulture populations resulted in an increase of the number of mammalian scavengers at a given carcass along with the time spent at a carcass.[40]
Scavenging may provide a direct and indirect method for transmitting disease between animals.[44] Scavengers of infected carcasses may become hosts for certain pathogens and consequently vectors of disease themselves.[44] An example of this phenomenon is the increased transmission oftuberculosis observed when scavengers engage in eating infected carcasses.[45] Likewise, the ingestion of bat carcasses infected withrabies by striped skunks (Mephitis mephitis) resulted in increased infection of these organisms with the virus.
A major vector of transmission of diseases are various bird species, with outbreak being influenced by such carrier birds and their environment. Anavian cholera outbreak from 2006 to 2007 off the coast Newfoundland, Canada resulted in the mortality of many marine bird species. The transmission, perpetuation and spread of the outbreak was mainly restricted to gull species who scavenge for food in the area.[46] Similarly, an increase of transmission of avian influenza virus to chickens by domestic ducks from Indonesian farms permitted to scavenge surrounding areas was observed in 2007. The scavenging of ducks in rice paddy fields in particular resulted in increased contact with other bird species feeding on leftover rice, which may have contributed to increased infection and transmission of the avian influenza virus. The domestic ducks may not have demonstrated symptoms of infection themselves, though were observed to excrete high concentrations of the avian influenza virus.[47]
Many species that scavenge face persecution globally.[citation needed] Vultures, in particular, have faced incredible persecution and threats by humans. Before its ban by regional governments in 2006, the veterinary drugDiclofenac has resulted in at least a 95% decline ofGyps vultures in Asia. Habitat loss and food shortage have contributed to the decline of vulture species in West Africa due to the growing human population and over-hunting of vulture food sources, as well as changes in livestock husbandry. Poisoning certain predators to increase the number ofgame animals is still a common hunting practice in Europe and contributes to the poisoning of vultures when they consume the carcasses of poisoned predators.[40]
Highly efficient scavengers, also known as dominant or apex-scavengers, can have benefits to humans. Increases in dominant scavenger populations, such as vultures, can reduce populations of smaller opportunistic scavengers, such as rats.[48] These smaller scavengers are often pests and disease vectors.
InTibetan Buddhism, the practice ofexcarnation—that is, theexposure of dead human bodies to carrion birds and/or other scavenging animals—is the distinctive characteristic ofsky burial, which involves the dismemberment of humancadavers of whom the remains are fed tovultures, and traditionally the mainfunerary rite (alongsidecremation) used to dispose of the human body.[51] A similar funerary practice that features excarnation can be found inZoroastrianism; in order to prevent the pollution of the sacred elements (fire, earth, and water) from contact withdecomposing bodies, human cadavers are exposed on theTowers of Silence to be eaten by vultures and wild dogs.[52]
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^Schwartz, ALW; Williams, HF; Chadwick, E; Thomas, RJ; Perkins, SE (2018). "Roadkill scavenging behaviour in an urban environment".Journal of Urban Ecology.4 (1):1–7.doi:10.1093/jue/juy006.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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^Jankowiak, Ł; Malecha, AW; Krawczyk, AJ (2016). "Garbage in the diet of carnivores in an agricultural area".European Journal of Ecology.2 (1):81–86.doi:10.1515/eje-2016-0009.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Yirga, G; Leirs, H; De Longh, HH; Asmelash, T; Gebrehiwot, K; Deckers, J; Bauer, H (2015). "Spotted hyena (Crocuta crocuta) concentrate around urban waste dumps across Tigray, northern Ethiopia".Wildlife Research.42 (7):563–569.Bibcode:2015WildR..42..563Y.doi:10.1071/WR14228.ISSN1448-5494.S2CID86458636.
^Lieberman, Daniel; Bramble, Dennis (2007).The Evolution of Marathon Running: Capabilities in Humans. Adis Data Information BV. p. 288.doi:10.2165/00007256-200737040-00004. Retrieved2017-03-15.Human endurance running performance capabilities compare favourably with those of other mammals and probably emerged sometime around 2 million years ago in order to help meat-eating hominids compete with other carnivores. [...] [S]mall teeth, larger bodies and archaeological remains suggest that hominids started to incorporate meat and other animal tissues in the diet at least 2.5Ma, probably by hunting as well as scavenging. [...] [Endurance running] might have enabled hominids to scavenge carcasses from lions after they were abandoned but before hyenas arrived, as modern hunter-gatherers still do in East Africa.
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