When a living organism dies—be it a fungus, a plant, an animal, or a human being—decay is the final event to occur. The individual parts of the body decompose at different rates. In various ways and in numerous stages, they are reduced to small pieces, decomposed, and finally broken down to low-molecular-weight, inorganic basic substances (carbon dioxide, water, etc.). This process can happen through enzymes of the body itself; through animals, fungi, and bacteria as biotic factors; and finally through abiotic (chemicalphysical) factors. The time required for the decomposition crucially depends on the temperature, humidity, oxygen content, pH value, and biological milieu.
Within an ecosystem, decomposition plays an important role. Not only do many organisms feed on the dead organic material, but through decomposition, nutrients that were once absorbed by living organisms can be released and used again by primary producers (most of all by plants). However, the decay process can also come to a standstill or it can slow down significantly—bodies can mummify, ice or amber can conserve them, bodies can form grave wax, or they can remain as skeletons or bog bodies for posterity.
Knowledge of the processes of decomposition and decay is important for forensics, archaeology, physical anthropology, ecology, and in the funeral and cemetery management industry.
Preliminary Remarks on Time
How long a body takes to decompose and what accelerates or decelerates this process depends on the nature and condition of the remains. At the death of a mammal, numerous circumstances have an impact upon the process of decomposition. Important factors can be whether the animal was slim or fat before death and also if it had a fever or any open wounds. In case of infection (sepsis), germs may have entered the body, thus accelerating the process of decomposition. Antibiotics in the body might have delayed the growth of bacteria and therefore may have slowed down the postmortal microbiotic decomposition. The time needed for the decomposition also depends on the ecological circumstances. It is therefore of utmost importance to factor in what animals, fungi, and bacteria had access to the cadaver as well as to consider the weather and soil conditions (moist/ dry, warm/cold, acidic/basic). Another issue to be taken into account is whether the cadaver lay close to the surface of the earth, deep within the ground, or in water. Acidic, warm, and humid milieus (with a permeable soil) accelerate decomposition, whereas basic, dry, and cold milieus are usually more preservative. Due to the wide variety of these factors, the time given as an estimate of the relationship between time and the degree of decomposition in Casper’s rule can therefore be used only as an approximation in a multifactorial process. Casper’s rule provides an estimation as follows: The degree of degradation of a corpse that has been lying on the soil surface for 1 week corresponds to that of a corpse that has been lying in water for 2 weeks or buried in soil for 8 weeks.
Death and Dying
The death of multicellular organisms is a gradual process. For example, if the cardiovascular system of a human being collapses, the individual organs, tissues, and cells can still be vital for several hours or days. The proliferation activity of the epidermis is an example of such “intermediate life,” which finishes at 20°C ambient temperature after 35 to 40 hours. It is possible that some cells and tissues are still viable, while other parts of the body have already started to display evidence of decomposition. Not until life on the cellular level has also been completely stopped can one speak of absolute death.
Processes of Decomposition
Autolysis is the self-digestion of cells and tissues caused by the body’s own enzymes. In cells of a dying body, the defect cell compartments set free enzymes such as proteases or lipases. They attack the proteins of the cell itself and dismember and denature them. The degrading enzymes can also enter the intercellular spaces and destroy the connection between the different cells (tissue necrosis).
The tissues, which are sensitive to failure of the oxygen supply, and cells, which have many hydrolytic enzymes, are strongly affected from the beginning. Many autolytic enzymes have their temperature optimum at 34°C to 40°C; therefore, a warm milieu enhances autolysis (warm surroundings or fever before death).
A vast number of animals of different species and classes feed on dead plant and/or animal material (necrophages) or on excrements of other animals (coprophages), and so they are all part of the chain of organic decomposition. On the one hand there are animals that only occasionally feed on dead material, such as omnivore animals eating a carcass (e.g., rats, gulls, ants). On the other hand there are animal species that feed only on dead organic material or that depend on it during a certain stage of development. In particular, several articulates belong to these species. The larvae of snow scorpion flies, for example, feed on dead plant material and also on carcasses. Other animals may also particularly favor a certain species (e.g., the pine sawyer beetle on the pine tree). Certain remains, like the keratin of feathers, horn, or hair, can be utilized by only a few animals such as the fur beetle.
Not all species that take part in the decomposition process appear simultaneously on the dead substrate. In each case the point of time of their appearance depends on their requirement profile (e.g., fresh cadavers, intermediate products of the decomposition, or feces as feed). Therefore one speaks of a succession.
Bacteria and Fungi
Among the organisms of decomposition processes are bacteria and fungi, whose demands on the substrate are very different. Often they excrete exoenzymes, which disintegrate high-molecular- weight substrates (e.g. lignin, cellulose, protein) outside the organism to form more basic products that can be reabsorbed. The substrate is reduced by several species through many stages and is finally assimilated and/or transferred into an inorganic compound (so-called remineralization). In nature, whole groups of different organisms often work together by ingesting and feeding on decomposition or waste products so that their feces can then be used by another species as a feeding substrate.
The chemical milieu, the temperature, and moisture all have an effect on the time needed for autolytic, microbiotic, and animal decomposition. Besides the aforementioned indirect effects, abiotic factors also have a direct effect on the animal or plant material. Mechanical damage can be caused by wind and water. Frost can disperse tissue, variations in temperature and moisture can cause cracks to form (e.g., in bones), and pressure can also have an impact on buried substrate. Substances can dissolve in a moist milieu and can be carried away, especially in permeable sediments. The pH value of the soil can be preservative (depending on the composition of the body parts) or destructive; calcareous compartments (shells) can be affected by an acidic milieu, whereas in an alkaline milieu they are preserved for centuries.
There are many examples of the decomposition of organic materials (organic decay in the sea, in lakes, of insects, of a dung heap, etc.). Two types are discussed here.
Litter and Wood
The litter (leaves, needles) on the forest soil is disintegrated by articulates, by bacteria, and most of all by fungi. The decomposition takes place in many stages: In the first step the fallen but almost undamaged leaf and needle material has to be made accessible for the destructive microorganisms (destruents) because it is almost impossible for the microorganisms to get through the cutinized layer. An entry is primarily made possible for them through the damage caused by the macro fauna (arthropods and snails that tear the epidermis). Leaves that have fallen to the ground are mostly disintegrated by earthworms, which also eat intact foliage that they have pulled into their tubes during the night. The reduced pieces of the litter and the droppings of the primary decomposers can further be reduced by secondary decomposers such as springtails, mites, roundworms, and potworms. In the third decomposition phase, soil microorganisms (such as fungi; bacteria, especially actinomy- cetes) can totally decompose the rehashed litter of animal feces up to its inorganic elements.
Decomposition proceeds best under well- aerated, warm-humid conditions (e.g., in the tropical rainforest) because of quick and direct microbial oxidation to carbon dioxide, water, nitrates, and the like. Under cold, acidic, and partially anaerobic conditions, the decomposition is slowed down and peat accumulation can occur. In temperate soil the decomposition depends on the interplay of soil animals and microorganism. The better the soil fauna is developed (e.g., in deciduous mixed forests), the higher the level of decomposition that will occur.
In nature the wood of dead trees is mainly decomposed by fungi. Animal and bacterial decomposition does not play such an important role, but the decomposition through fungi can then lead to insects being able to digest the wood. As far as fungi are concerned, basidiomycetes with a hymeneal fruiting body are far more often involved in decomposition of wood than are ascomycetes and fungi imperfecti. Their hyphae grow into the wood, and during their growth they disintegrate the polymeric carbohydrates and the lignin (creating wood decay). Normally not all substances of the wood are attacked equally, and in fact the secreted enzymes affect the substances of the cell wall differently. This allows a classification of the wood decay by fungi. It occurs as brown rot, white rot, or soft rot.
In the case of brown rot, the fungi (exclusively the fungi of the basidiomycetes) feed on cellulose or hemicellulose so that the lignin part of the wood remains. For example, in 6 months the weight of sapwood can be reduced about 43% by the house fungus. The wood becomes brown, is transverse cracked, and molders cubically. Brown rotten wood parts can last for centuries within the soil because of the high lignin proportion.
In the case of white rot, which is often a very fast decomposition process, lignin, cellulose, and hemicellulose are being decomposed by releasing oxidizing enzymes. The rotten wood becomes white and long-stranded because of the bleaching process. It is possible to distinguish between simultaneous rot and selective delignification. In the case of the former, the carbohydrates and the lignin are being degenerated almost instantaneously; in the case of the latter the lignin is the first to be degenerated, whereas the cellulose is left for later decomposition.
The third type is soft rot. First of all the cellulose is decomposed to hemicellulose, whereas the lignin is more repressive. The wood becomes very soft in texture and gray to dark brown in color. In contrast to the brown rot, the soft rot is not caused by basid- iomycetes but by ascomycetes and fungi imperfecti.
Approximately 4 minutes after death, the decay of the cadaver begins with the process of autolysis. This can initially be noticed in the organs, which have a huge energy requirement, or many lysosomes, such as the intestine, stomach, and accessory organs of digestion. Without clear boundaries between the stages, this autolysis moves to decomposition by putrefactive bacteria and finally ends with skeletization.
The anaerobic conditions that develop within the body, and the nutrient-rich fluids released due to the autolysis, stimulate bacterial growth (especially endosymbiotic gut bacteria). Their emigration out of the gut is supported by the pressure caused by gases produced during decomposition. Bacteria can also enter the body via the skin (especially through open wounds), the mouth, the nasal cavity, and so forth. The bacteria disintegrate released carbohydrates, proteins, and lipids to different acids, gases, and other substances. They cause a change of colors and odors. The organs soften and finally become fluid. The detachment of the epidermis and its attachments and also the bloating of tissue are evident.
After the fluid produced during the decomposition has run off, normally the oxidative processes intensify. In this case the decay occurs in a rather dry milieu; organs and tissues decompose into peat, which is often supported by fungi, especially by molds.
In addition to chemical and microbacterial factors, animals can also be involved in the decomposition of a corpse. An example of a typical series of insects that would interact with a human corpse on the soil surface is as follows:
Shortly after death, attracted by hydrogen sulfide and ammoniac substances, blowflies begin to fly around the orifices and wounds. After oviposi- tion, maggots hatch out of the eggs, they shed their skin twice, and after the termination of their feed intake, they usually leave the corpse to pupate. The speed of the development cycle depends mainly on the fly species and the ambient temperature. If these parameters are known, the time of the first colonization can be calculated (that is the job of the forensic entomologist). The dominant blowflies are accompanied by the fleshflies and the houseflies. As the decomposition progresses, cheese-skippers and latrine flies can be found, and later, fruit flies and humpbacked flies also appear. As the dehydration of the cadaver takes place, larder beetles and caterpillars of the clothes moths appear; these are responsible for the decomposition of the skin. The last remains are removed by the spider beetles and meal worm beetles.
Under “normal” conditions, that means in water-permeable and aerated soil and in 1- to 3-meter depth, the skeletization takes 5 to 7 years. The decomposition proceeds in order of anatomic strength and condition. Joint connections, which are held by a strong ligament and tendon apparatus, are preserved the longest, in particular, the pelvis girdle and the spine.
Concerning the decomposition of bones, once again biological, chemical, and physical factors contribute. The fragmentation of the bone can be caused by pressure of the soil or frost wedging. In acidic soil the hydrolysis of the mineral components (above all, hydroxyapatite) is accelerated. The big inner surface of the bone provides a large area for microorganisms, which are able to produce acidic metabolites and therefore to hydrolyze the apatite and finally to decompose the then unmasked proteins of the bone (above all, collagen). In this way the skeleton can finally decompose completely. Sandy soil indirectly promotes the decomposition of the bone because soluble components can dissolve, while loess soil, on the other hand, promotes the preservation of the bone.
Normally a certain type of physical or chemical milieu is the reason that small animals and microbes are unable to decompose organic substances: high aridity, temperatures under the freezing point, exclusion of light and oxygen, high salt concentrations, or a low pH value. Some instances where organic decomposition under normal conditions is interrupted are described next.
Plants and animals that die in a cold season and cannot be reached by warm-blooded animals or articulata that produce warmth are either preserved or decompose more slowly than usual. Under the right conditions, dead creatures can be preserved for centuries within the ice as the low temperatures avert the microbial decomposition, just like a freezer. Prominent examples are the carcasses of mammoths that were enclosed in the Siberian permafrost (c. 10,000 years old) and the glacier mummy (called Ötzi by researchers) who died in the 4th century BCE in the Alps.
In an acidic, raised bog the soft tissue of animals can be preserved, too, because of the oxygen deficit and the high concentration of antibacterial humic acid. The humic acid also causes the decalcification of the bones and gives a tan color to skin and hair. Sometimes even inner organs are preserved. The most prominent examples for such preservation are the bog bodies of Northern Europe, which go back to the last ice age.
Preservation can also take place in resin that encloses plant parts or small animals. The hardened resin, which can turn to amber when pressurized, conserves the organic substances. Famous examples are the fossils found in amber from the Baltic Sea (c. 50 million years old) or in Dominican amber (c. 30 million years old).
Under anaerobic conditions in a moist milieu, grave wax (adipocere) can be formed. Bacterial activity transforms the unsaturated fatty acids of the corpse into saturated fatty acids (above all, palmitic and staritic acids)—a process that can take months or even years. Adipocere can be preserved for centuries. To a large extent the corpus of the dead body is preserved in its exterior layers (the fat tissue). The inner organs can also be well preserved if they are soaked with fatty acids. The parts of the body that contain less fat (often head and limbs) are usually skeletized. Grave wax corpses can form, especially if a fat corpse remains underwater or if water cannot drain from an earthen grave.
Mummification most commonly occurs where there is extreme dryness, heat, and especially draught. The tissues rapidly dry out and are therefore protected from microbial decomposition. It can happen at different stages during decomposition and can involve the whole corpse or only some limbs.
Finally, the hard parts of an animal body (shell, bones) can be preserved in basic and neutral milieus for centuries, even thousands of years. If secondary mineralization takes place, skeletons can finally form fossils.
See also Chemistry; Diseases, Degenerative; Dying and
Death; Ecology; Mummies; Thanatochemistry
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