Decomposition
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Pallor mortis |
Decomposition or rotting is the process by which tissues of a dead organism break down into simpler forms of matter. The process is essential for new growth and development of living organisms because it recycles the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death. It is a cascade of processes that go through distinct phases. It may be categorized in two stages by the types of end products. The first stage is characterized by the formation of liquid materials; flesh or plant matter begin to decompose. The second stage is limited to the production of vapors. The science which studies such decomposition generally is called taphonomy from the Greek word taphos, which means grave. Besides the two stages mentioned above, historically the progression of decomposition of the flesh of dead organisms has been viewed also as four phases:
- fresh (autolysis);
- bloat (putrefaction);
- decay (putrefaction and carnivores);
- dry (diagenesis).
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Ambient conditions
Exposure to the elements
A dead body that is exposed to the open elements, such as water and air, will decompose more quickly and attract much more insect activity than a body that is buried or confined in special protective gear or artifacts. This is due, in part, to the limited number of insects that can penetrate a coffin and the lower temperatures under soil.
Media for bacterial growth
Similarly, a submerged body is shielded from air-living organisms; however, it is exposed to a new set of waterborne breakdown agents such as the bountiful water life and marine bacteria.
Ambient temperature
Ultimately, the rate of bacterial decomposition acting on the tissue will depend upon the temperature of the surroundings. Colder temperatures decrease the rate of decomposition while warmer temperatures increase it.
Plant decomposition
Decomposition of plant matter occurs in many stages. It begins with leaching by water; the most easily lost and soluble carbon compounds are liberated in this process. Another early process is physical breakup or fragmentation of the plant material into smaller bits which have greater surface area for microbial colonization and attack. In smaller dead plants, this process is largely carried out by the soil invertebrate fauna, whereas in the larger plants, primarily parasitic life-forms such as insects and fungi play a major breakdown role and are not assisted by numerous detritivore species. Following this, the plant detritus (consisting of cellulose, hemicellulose, microbial products, and lignin) undergoes chemical alteration by microbes. Different types of compounds decompose at different rates. This is dependent on their chemical structure. For instance, lignin is a component of wood, which is relatively resistant to decomposition and can in fact only be decomposed by certain fungi, such as the black-rot fungi. Said fungi are thought to be seeking the nitrogen content of lignin rather than its carbon content. Lignin is one such remaining product of decomposing plants with a very complex chemical structure causing the rate of microbial breakdown to slow. Warmth determines the speed of plant decay, with the rate of decay increasing as heat increases, e.g. a plant in a warm environment will decay over a shorter period of time.
In most grassland ecosystems, natural damage from fire, insects that feed on decaying matter, termites, grazing mammals, and the physical movement of animals through the grass are the primary agents of breakdown and nutrient cycling, while bacteria and fungi play the main roles in further decomposition.
The chemical aspects of plant decomposition always involve the release of carbon dioxide.
Animal decomposition
Decomposition begins at the moment of death, caused by two factors: autolysis, the breaking down of tissues by the body's own internal chemicals and enzymes, and putrefaction, the breakdown of tissues by bacteria. These processes release gases that are the chief source of the unmistakably putrid odor of decaying animal tissue.
Most decomposers are bacteria or fungi, though scavengers also play an important role in decomposition if the body is accessible to insects and other animals. The most important insects that are involved in the process include the flesh-flies (Sarcophagidae) and blow-flies (Calliphoridae), such as the green-bottle fly seen in the summer. The most important non-insect animals that are typically involved in the process include larger scavengers, such as: coyotes, dogs, wolves, foxes, rats, crows and vultures. Some of these scavengers also remove and scatter bones, which they ingest at a later time.
Food decomposition
The decomposition of food, called spoilage in this context, is an important field of study within food science.
Human decomposition
Stages
Once death occurs, human decomposition takes place in stages. The process of tissue breakdown may take from several days up to years. At all stages of decomposition, insect activity occurs on the body as detailed below.
Fresh
The fresh stage of decomposition occurs during the first few days following death. There are no physical signs of decomposition during this time. However, homeostasis of the body has ceased, allowing cellular and soft tissue changes to occur because of the process of autolysis, the destruction of cells and organs due to an aseptic chemical process. At this point, the body enters algor mortis, the cooling of the body's temperature to that of its surroundings. When the body’s cells reach the final stage of autolysis, an anaerobic environment is created, that is, an environment wherein oxygen is not present. This allows the body’s normal bacteria to break down the remaining carbohydrates, proteins, and lipids. The products from the breakdown create acids, gases, and other products which cause volatile organic compounds (VOCs) and putrefactive effects. VOCs are produced during the early stages of human decomposition.[1]
Substances produced during the fresh stage of decomposition attract a variety of insects. Insects belonging to the order Diptera begin to lay their eggs on the body during this stage, especially members of the Calliphoridae family.[2] There is also considerable activity by soil dwelling insects around a body that it is on the ground or buried in the soil. The reasoning for this is simple: a dead human body serves as an excellent source of decaying matter to feed on in a very hospitable environment.
Putrefaction
Odor, color changes, and bloating of the body during decomposition are the results of putrefaction. The lower part of the abdomen turns green due to bacteria activity in the cecum. Bacteria break down hemoglobin into sulfhemoglobin, which causes the green color. A formation of gases enters the abdomen, which forces liquids and feces out of the body. The gases also enter the neck and face, causing swelling of the mouth, lips, and tongue. Due to this swelling and misconfiguration of the face, identification of the body can be difficult. Bacteria also enter the venous system causing blood to hemolyze. This leads to the formation of red streaks along the veins. This color soon changes to green through a process known as marbelization. It can be seen on the chest and shoulder area and on the thighs. The skin can develop blisters containing serous fluid. The skin also becomes fragile, leading to skin slippage, making it difficult to move a body. Body hair comes off easily. The change of the green discoloration to brown marks the transition of the early stage of putrefaction to the advanced decomposition stages.
During the putrefaction stage of decomposition the majority of insect activity again comes from members of the Calliphoridae family, and includes: Formicidae, Muscidae, Sphaeroceridae, Silphidae, Lepidoptera, Hymenoptera, Sarcophagidae, Histeridae, Staphylinidae, Phalangida, Piophilidae, Araneae, Sepsidae, and Phoridae. As with the fresh stage of decomposition if the body is on the ground or buried in soil there is also considerable insect activity by the soil-inhabiting arthropods.
Black putrefaction
After the body goes through the bloating stage it begins the black putrefaction stage. At this point the body cavity ruptures, the abdominal gases escape and the body darkens from its greenish color. These activities allow for a greater invasion of scavengers, and insect activity increases greatly. This stage ends as the bones become apparent, which can take anywhere from 10 to 20 days after death depending on the surrounding environmental conditions. This period is also dependent on the degree to which the body is exposed to each and any of the varying elements and conditions.
During the black putrefaction stage of decomposition, insects that can be found living in the body are: Calliphoridae larvae, Staphylinidae, Histeridae, Gamasid mites, Ptomaphila, Trichopterygidae, Piophilid larvae, Parasitic wasps, Staphylinid larvae, Trichopterygid larvae, Histerid larvae, Ptomaphila larvae, Dermestes, Tyroglyphid mites, Tineid larvae, and the Dermestes larvae. Some insects can also be found living in the soil around the body such as: Isopoda, Collembola, Dermaptera, Formicidae, Pseudoscorpiones, Araneae, Plectochetos, Acari, Pauropoda, Symphyla, Geophilidae, and Protura. The types of insects will differ based on where the body is, although Diptera larvae can be found feeding on the body in almost all cases.
Butyric fermentation
After the early putrefaction and black putrefaction phases have taken place, the body begins mummification, in which the body begins to dry out. Once the human carcass has mummified it goes through saponification, the formation of adipocere (grave wax), referring to the loss of body odor and the formation of a cheesy appearance on the cadaver. Mummification is considered a post-active stage because there is less definite distinction between changes as indicated by reduced skin, cartilage, and bone. Mummification is also indicated when all of the internal organs are lost due to insect activity.
Insects that can be found on the body during mummification include most of the same insects as in the putrefaction stage, but also include: Acarina, Nitidulidae, Cleridae, Dermestes caninus, and Trogidae. The main soil-inhabiting arthropods include Dermaptera and Formicidae.
Dry decay
When the last of the soft-tissue has been removed from the body, the final stage of decomposition, skeletonization, occurs. This stage encompasses the deterioration of skeletal remains, and is the longest of the decomposition processes. Skeletonization differs markedly from the previous stages, not only in length, but in the deterioration process itself.
The strength and durability of bone stems from the unique protein-mineral bond present in skeletal formation. Consequently, changes to skeletal remains, known as bone diagenesis, occur at a substantially slower rate than stages of soft-tissue breakdown. As the protein-mineral bond weakens after death, however, the organic protein begins to leach away, leaving behind only the mineral composition. Unlike soft-tissue decomposition, which is influenced mainly by temperature and oxygen levels, the process of bone breakdown is more highly dependent on soil type and pH, along with presence of groundwater. However, temperature can be a contributing factor, as higher temperature leads the protein in bones to break down more rapidly. If buried, remains decay faster in acidic-based soils rather than alkaline. Bones left in areas of high moisture content also decay at a faster rate. The water leaches out skeletal minerals, which corrodes the bone, and leads to bone disintegration.[3]
At the dry decay stage commonly found insects include: Sphaeroceridae, Acarina, Nitidulidae, Cleridae, Dermestes caninus, Trogidae, Tyroglyphid mites, and the Tineid larvae. The soil-inhabiting arthropods are: Collembola, Dermaptera, Heteroptera, Coleoptera and their larvae, parasitic Hymenoptera, Formicidae, Diptera larvae, Pseudoscorpiones, Aranae, Plectochetos, Acari, Pauropoda, Symphyla, Geophilidae, Protura, and Aphididae.
Importance to forensics
Various sciences study the decomposition of bodies under the general rubric of forensics because the usual motive for such studies is to determine the time and cause of death for legal purposes:
- Forensic pathology studies the clues to the cause of death found in the corpse as a medical phenomenon.
- Forensic entomology studies the insects and other vermin found in corpses; the sequence in which they appear, the kinds of insects, and where they are found in their life cycle are clues that can shed light on the time of death, the length of a corpse's exposure, and whether the corpse was moved.[4][5]
- Forensic anthropology is the branch of physical anthropology that studies skeletons and human remains, usually to seek clues as to the identity, race, and sex of their former owner.[6][7]
The University of Tennessee Anthropological Research Facility (better known as the Body Farm) in Knoxville, Tennessee has a number of bodies laid out in various situations in a fenced-in plot near the medical center. Scientists at the Body Farm study how the human body decays in various circumstances to gain a better understanding into decomposition.
Factors affecting decomposition
The rate and manner of decomposition in an animal body is strongly affected by a number of factors. In roughly descending degrees of importance, they are:
- Temperature;
- The availability of oxygen;
- Prior embalming;
- Cause of death;
- Burial, and depth of burial;
- Access by scavengers;
- Trauma, including wounds and crushing blows;
- Humidity, or wetness;
- Rainfall;
- Body size and weight;
- Clothing;
- The surface on which the body rests;
- Foods/objects inside the specimen's digestive tract (bacon compared to lettuce).
The speed at which decomposition occurs varies greatly. Factors such as temperature, humidity, and the season of death all determine how fast a fresh body will skeletonize or mummify. A basic guide for the effect of environment on decomposition is given as Casper's Law (or Ratio): if all other factors are equal, then, when there is free access of air a body decomposes twice as fast than if immersed in water and eight times faster than if buried in earth.
The most important variable is a body's accessibility to insects, particularly flies. On the surface in tropical areas, invertebrates alone can easily reduce a fully fleshed corpse to clean bones in under two weeks. The skeleton itself is not permanent; acids in soils can reduce it to unrecognizable components. This is one reason given for the lack of human remains found in the wreckage of the Titanic, even in parts of the ship considered inaccessible to scavengers. Freshly skeletonized bone is often called "green" bone and has a characteristic greasy feel. Under certain conditions (normally cool, damp soil), bodies may undergo saponification and develop a waxy substance called adipocere, caused by the action of soil chemicals on the body's proteins and fats. The formation of adipocere slows decomposition by inhibiting the bacteria that cause putrefaction.
In extremely dry or cold conditions, the normal process of decomposition is halted – by either lack of moisture or temperature controls on bacterial and enzymatic action – causing the body to be preserved as a mummy. Frozen mummies commonly restart the decomposition process when thawed, whilst heat-desiccated mummies remain so unless exposed to moisture.
The bodies of newborns who never ingested food are an important exception to the normal process of decomposition. They lack the internal microbial flora that produce much of decomposition and quite commonly mummify if kept in even moderately dry conditions.
Embalming is the practice of delaying decomposition of human and animal remains. Embalming slows decomposition somewhat, but does not forestall it indefinitely. Embalmers typically pay great attention to parts of the body seen by mourners, such as the face and hands. The chemicals used in embalming repel most insects, and slow down bacterial putrefaction by either killing existing bacteria in or on the body themselves or by "fixing" cellular proteins, which means that they cannot act as a nutrient source for subsequent bacterial infections. In sufficiently dry environments, an embalmed body may end up mummified and it is not uncommon for bodies to remain preserved to a viewable extent after decades. Notable viewable embalmed bodies include those of
- Vladimir Lenin, whose body was kept submerged in a special tank of fluid for decades and is on public display in Lenin's Mausoleum.
- Padre Pio, whose body was injected with formalin prior to burial in a dry vault from which he was later removed and placed on public display at the San Giovanni Rotondo.
- Evita Peron whose body was injected with paraffin and was kept perfectly preserved for many years, and still is as far as is known (her body is no longer on public display).
However even without embalming a body buried in a sufficiently dry environment may be well preserved for decades, such as the body of the murdered civil rights activist Medgar Evers.[8]
Bodies submerged in peat bog may become naturally "embalmed", arresting decomposition and resulting in a preserved specimen known as a bog body. The time for an embalmed body to be reduced to a skeleton varies greatly. Even when a body is decomposed, embalming treatment can still be achieved (the arterial system decays more slowly) but would not restore a natural appearance without extensive reconstruction and cosmetic work, and is largely used to control the foul odors due to decomposition.
In some cases, bodies are inexplicably preserved (with no technique used) for decades or centuries and appear almost the same as when they died. This is known as incorruptibility. Though, it is not known how long a body can stay free of decay.[9][10]
See also
- Cadaverine
- Decompiculture
- Embalming
- Putrescine
- Staling
References
- ↑ Statheropoulos M, Agapiou A. et al. (2007). "Environmental aspects of VOCs evolved in the early stages of human decomposition". Sci. Total Environ. 385 (1–3): 221–227. doi:10.1016/j.scitotenv.2007.07.003. PMID 17669473.
- ↑ Eberhardt TL, Elliot DA. (2008). "A preliminary investigation of insect colonization and succession on remains in New Zealand". Forensic Sci. Int. 176 (2–3): 217–223. doi:10.1016/j.forsciint.2007.09.010. PMID 17997065.
- ↑ Buckberry, Jo. "Missing, Presumed Buried? Bone Diagenesis and the Under-Representation of Anglo-Saxon Children"
- ↑ Smith, KGV. (1987). A Manual of Forensic Entomology. Cornell Univ. Pr.. pp. 464. ISBN 0801419271.
- ↑ Kulshrestha P, Satpathy DK. (2001). "Use of beetles in forensic entomology". Forensic Sci. Int. 120 (1–2): 15–17. doi:10.1016/S0379-0738(01)00410-8. PMID 11457603.
- ↑ Schmitt, A.; Cunha, E., Pinheiro, J. (2006). Forensic Anthropology and Medicine: Complementary Sciences From Recovery to Cause of Death. Humana Press. pp. 464. ISBN 1588298248.
- ↑ Haglund, WD.; Sorg, MH. (1996). Forensic Taphonomy: The Postmortem Fate of Human Remains. CRC Press. pp. 636. ISBN 0849394341.
- ↑ Quigley, C. (1998). Modern Mummies: The Preservation of the Human Body in the Twentieth Century. McFarland. pp. 213–214. ISBN 0786404922.
- ↑ http://people.howstuffworks.com/incorruptible.htm
- ↑ http://www.overcomeproblems.com/incorruptables.htm
External links
- 1Lecture.com, Food Decomposition (a Flash animation)
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