Scavenger
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Scavengers are animals that feed on dead and decaying organic matter.<ref name=":01">Template:Cite web</ref><ref name=":02">Template:Cite web</ref> Often the term is used to describe the consumption of carrion, the bodies of animals that have died from causes other than predation or the bodies of animals that have been killed by other predators.<ref name=":03">Template:Cite journal</ref> However, the term is also used to describe animals that feed on rotting plant matter<ref name=":02"/><ref name=":04">Template:Cite journal</ref> or refuse.<ref name=":01"/>
Vultures and burying beetles are examples of scavengers that feed on carrion,<ref name=":05">Template:Cite journal</ref> pink bud moth and stag beetle larvae are examples of scavengers that feed on rotting plant matter,<ref name=":13">Template:Cite journal</ref><ref name=":07">Template:Cite journal</ref> and raccoons and squirrels are examples of scavengers that feed on refuse.<ref name=":06">Template:Cite web</ref> Carrion-eating scavengers are called necrophages.<ref name=":14">Template:Cite book See also: Template:Cite web</ref>
Scavengers play an important role in ecosystems by preventing the accumulation of decaying matter and helping to recycle nutrients.<ref name=":02"/><ref name=":19">Template:Cite journal</ref> The process and rate at which dead plant and animal material is scavenged is affected by both biotic and abiotic factors, such as plant species, carcass size, habitat, temperature, moisture levels, and seasons.<ref name="Ecology2017">Template:Cite journal</ref><ref name=":09">Template:Cite journal</ref><ref name=":10">Template:Cite journal</ref> Detritivores and decomposers complete this process by consuming the remains left by scavengers.<ref name=":15"/><ref name=":08">Template:Cite journal</ref>
Etymology
Scavenger is an alteration of scavager, from Middle English skawager meaning "customs collector", from skawage meaning "customs", from Old North French escauwage meaning "inspection", from schauwer meaning "to inspect", of Germanic origin; akin to Old English scēawian and German schauen meaning "to look at", and modern English "show" (with semantic drift).<ref name=":27">Template:Cite book</ref>
Related terminology
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.<ref name=":24">Template:Cite web</ref> Animals that rely specifically on carrion as a food source are called obligate necrophages.<ref name=":05"/><ref name=":25">Template:Cite journal</ref> Animals that feed on particulate plant or animal matter (e.g. humus, marine snow) are typically categorized as detritivores rather than scavengers.<ref name=":15">Template:Cite web</ref> The midge fly Propsilocerus akamusi, which feeds on detritus in the sediment of freshwater lakes, is an example of a detritivore.<ref name=":26">Template:Cite journal</ref>
Types of scavengers
Scavengers that feed on carrion
Obligate scavenging of carrion (obligate necrophagy) is rare among vertebrates, due to the difficulty of finding enough carrion without expending too much energy.<ref name=":12">Template:Cite journal</ref> New World vultures such as the black vulture, and Old World vultures such as the griffon vulture, white-backed vulture and lappet-faced vulture, are examples of obligate carrion scavengers.<ref name=":05"/>
Most of the vertebrates that eat carrion are facultative scavengers, capable of obtaining food via predation or other methods, and eating carrion opportunistically.<ref name=":05"/> Many large carnivores that hunt regularly, such as hyenas and jackals, but also animals rarely thought of as scavengers, such as African lions, leopards, and wolves will scavenge if given the chance.<ref name=":02"/> They may also use their size and ferocity to intimidate the original hunters into abandoning their kills (the cheetah is a notable victim, rather than a perpetrator).<ref name=":22">Template:Cite book</ref> Gulls, crows and magpies frequently scavenge roadkill.<ref name=":23">Template:Cite journal</ref> Other vertebrates, for example Egyptian mastigures, scavenge to survive during times of food scarcity.<ref>Template:Cite journal</ref>
Aquatic and semi-aquatic vertebrates feed on carrion too. Carrion-eating scavengers found in marine settings include hagfish, great white sharks, northern wolffish and abyssal grenadiers, and carrion-eating scavengers found in freshwater settings include American alligators, Eurasian otters and common midwife toads.<ref name=":05"/>
Burying beetles, vulture bees and bone skipper flies are examples of obligately necrophagous invertebrates. They are all dependent on carrion during the larval stages of their life cycles. Adult burying beetles and vulture bees feed on carrion too.<ref name=":05"/><ref name=":11">Template:Cite journal</ref> Other invertebrates, such as blow flies, flesh flies and yellowjackets, also feed on carrion but are not reliant on it for survival.<ref name=":05"/> Blow fly and flesh fly larvae can feed on excrement, and some species, for example, Chrysomya putoria and Sarcophaga crassipalpis, can feed on living tissue.<ref name=":05"/><ref name=":16">Template:Cite book</ref> Also, yellowjackets can hunt caterpillars and other insects and feed on nectar, sap and fruit.<ref name=":17">Template:Cite journal</ref><ref name=":18">Template:Cite journal</ref>
In addition to the terrestrial examples above, many aquatic invertebrates consume carrion. The common octopus, European green crab and seven-armed starfish are all marine invertebrates that feed on carrion, and the ribbon leech Erpobdella obscura and red swamp crayfish are freshwater invertebrates that feed on carrion.<ref name=":05"/>
Scavengers that feed on dead plants
Some stag 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.<ref name=":21">Template:Cite web</ref> Adult Lucanus 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.<ref name=":21"/>
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.<ref name=":13"/> Termites are facultative scavengers too. Termites feed on dead trees and wood, but also live plants and detritus such as humus and excrement.<ref name=":29">Template:Cite web</ref><ref name=":30">Template:Cite journal</ref> Additional examples of facultative scavengers in this category are darkling beetles (also known as tenebrionids) and banana moth larvae.<ref name=":31">Template:Cite journal</ref><ref name=":32">Template:Cite journal</ref>
Scavengers that feed on discarded food
In urban settings, some animals regularly explore public parks and garbage cans for discarded food items that they can eat. Vertebrate examples of this type of scavenger include gulls, crows, feral pigeons, raccoons, opossums, brown rats and squirrels.<ref name=":02"/><ref name=":06"/><ref name=":19"/> Invertebrate examples include ants and blow flies.<ref name=":02"/><ref name=":19"/> In areas where there are municipal dumps, polar bears, raccoon dogs, red foxes, martens and polecats sometimes scavenge for food.<ref name=":02"/><ref name=":20">Template:Cite journal</ref> Hyenas also scavenge from municipal dumps in some prey-depleted districts of East Africa.<ref name=":28">Template:Cite journal</ref>
Prehistoric scavengers
In the prehistoric eras, the species Tyrannosaurus rex may have been an apex predator, preying upon hadrosaurs, ceratopsians, and possibly juvenile sauropods,<ref>Template:Cite journal</ref> although some experts have suggested the dinosaur was primarily a scavenger. The debate about whether Tyrannosaurus was an apex predator or scavenger was among the longest ongoing feuds in paleontology; however, most scientists now agree that Tyrannosaurus was an opportunistic carnivore, acting mostly as a predator but also scavenging when it could sense it.<ref>Template:Cite web</ref> Recent research also shows that while an adult T. rex would energetically gain little through scavenging, smaller theropods of approximately Template:Cvt might have gained levels similar to those of hyenas, though not enough for them to rely on scavenging.<ref>Template:Cite journal</ref>
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 tonne Apatosaurus 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.
Ecological function
Scavengers play a fundamental role in the environment through the removal of decaying organisms, serving as a natural sanitation service.<ref name="DroppingDead2011">Template:Cite journal</ref> While microscopic and invertebrate decomposers break down dead organisms into simple organic matter which are used by nearby autotrophs, scavengers help conserve energy and nutrients obtained from carrion within the upper trophic levels, and are able to disperse the energy and nutrients farther away from the site of the carrion than decomposers.<ref>Template:Cite journal</ref>
Scavenging unites animals which normally would not come into contact,<ref>Template:Cite journal</ref> and results in the formation of highly structured and complex communities which engage in nonrandom interactions.<ref name="Oikos2011">Template:Cite journal</ref> 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.<ref name="Ecology2017" />
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.<ref name="Ecology2017" />
Alterations in scavenging communities may result in drastic changes to the scavenging community in general, reduce ecosystem services and have detrimental effects on animal and humans.<ref name="Oikos2011" /> 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.<ref name="Ecology2017" /> 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.<ref name="DroppingDead2011" />
Disease transmission
Scavenging may provide a direct and indirect method for transmitting disease between animals.<ref name="Sci. Rep.">Template:Cite journal</ref> Scavengers of infected carcasses may become hosts for certain pathogens and consequently vectors of disease themselves.<ref name="Sci. Rep."/> An example of this phenomenon is the increased transmission of tuberculosis observed when scavengers engage in eating infected carcasses.<ref>Template:Cite journal</ref> Likewise, the ingestion of bat carcasses infected with rabies 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. An avian 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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
Threats
Many species that scavenge face persecution globally.Template:Citation needed Vultures, in particular, have faced incredible persecution and threats by humans. Before its ban by regional governments in 2006, the veterinary drug Diclofenac has resulted in at least a 95% decline of Gyps 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 of game animals is still a common hunting practice in Europe and contributes to the poisoning of vultures when they consume the carcasses of poisoned predators.<ref name="DroppingDead2011" />
Benefits to humans
Template:Further 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.<ref>Template:Cite journal</ref> These smaller scavengers are often pests and disease vectors.
In humans
In the 1980s, Lewis Binford suggested that early humans primarily obtained meat via scavenging, not through hunting.<ref>Template:Cite journal</ref> In 2010, Dennis Bramble and Daniel Lieberman proposed that early carnivorous human ancestors subsequently developed long-distance running behaviors which improved the ability to scavenge and hunt: they could reach scavenging sites more quickly and also pursue a single animal until it could be safely killed at close range due to exhaustion and hyperthermia.<ref>Template:Cite conference</ref>
In Tibetan Buddhism, the practice of excarnation—that is, the exposure of dead human bodies to carrion birds and/or other scavenging animals—is the distinctive characteristic of sky burial, which involves the dismemberment of human cadavers of whom the remains are fed to vultures, and traditionally the main funerary rite (alongside cremation) used to dispose of the human body.<ref>Template:Cite book</ref> A similar funerary practice that features excarnation can be found in Zoroastrianism; in order to prevent the pollution of the sacred elements (fire, earth, and water) from contact with decomposing bodies, human cadavers are exposed on the Towers of Silence to be eaten by vultures and wild dogs.<ref>Template:Cite book</ref>
Studies in behavioral ecology and ecological epidemiology have shown that cannibalistic necrophagy, although rare, has been observed as a survival behavior in several social species, including anatomically modern humans;<ref name="Sci. Rep."/> however, episodes of human cannibalism occur rarely in most human societies.<ref name="Sci. Rep."/><ref group="Note">For further informations about cannibalistic necrophagy among humans, see the articles Homo antecessor and List of incidents of cannibalism.</ref> Many instances have occurred in human history, especially in times of war and famine, where necrophagy and human cannibalism emerged as a survival behavior, although anthropologists report the usage of ritual cannibalism among funerary practices and as the preferred means of disposal of the dead in some tribal societies.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Gallery
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White-backed vultures feeding on a carcass of a wildebeest -
A jungle crow feeding on a small dead shark
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A polar bear scavenging on a narwhal carcass
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An Ibiza wall lizard scavenging on fish scraps left over from another predator
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Red weaver ants feeding on a dead giant African snail
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_scavenging_on_fish_scraps_leftover_from_another_predator_-_journal.pone.0060797.g001-B.png){kind=link}
_feeding_on_a_dead_African_giant_snail_(Achatina_fulica)_-_journal.pone.0060797.g001-F.png){kind=link}
See also
Notes
References
Further reading
External links
- Stitching a Life From the Scraps of Others – slideshow by The New York Times