The natural history of a grave

Heene Cemetery is designated a Sussex Site of Nature Conservation Importance, and is managed as a nature reserve, with consideration for the need to maintain access to the graves and to treat them with due respect. Read why neatness and tidiness do not encourage wildlife, or provide for its needs…

Six feet under

The phrase ‘six feet under’ is a popular euphemism for burial, and there was an old folk belief that one’s grave should be as deep as one is long. However, digging a grave to a specific depth of six feet is a practice that goes back to the London Plague of 1665, when an order was issued by the Lord Mayor and Aldermen of the City of London that plague victims were to be buried ‘at least six foot deep’. Although commonly believed to be bubonic plague, the symptoms described in many witness accounts certainly do not match those of this disease, but do match the symptoms of haemorrhagic fever. When London was eventually overwhelmed by plague victims, an estimated minimum of 100,000, plague pits 20 feet deep were dug until the outbreak dissipated in 1666 after the Great London Fire.

Many reasons have been published since to explain why six feet continued to be the preferred depth. It was felt unlikely at this depth that the odour of decay would penetrate to the surface, that any residual disease would spread to the living above ground, and that scavenging wild animals like foxes and crows would try to dig up corpses. Another reason given was that soil conditions were such that the sides of a deeper grave would cave in unless supports were used to brace the walls, particularly in areas of sandy soil. Gravediggers were generally short, poorly nourished individuals, and in a grave deeper than six feet would find it difficult to heave the soil out of the grave, and difficult to get in or out without a ladder.

Grave robbery, or ‘body snatching’ was a lucrative business in the early 1800s for those with unscrupulous contacts in medical schools willing to pay for recently exhumed cadavers for anatomical study and dissection.  In areas where grave robbers operated elaborate precautions were taken to protect the coffin and its contents, by covering with heavy stone slabs, enclosure in a stone box or vault, or by interment in an iron cage called a mortsafe.

A grave as an external habitat

A grave may be covered with granite, marble, concrete, or wood, with a headstone from similar materials, plus a kerb of stone, cast iron or timber.  The body within a casket may be embalmed.  All of these materials may attract living growth, from microorganisms, fungi and lichens, mosses, algae, and various invertebrates, depending on the prevailing conditions.  Headstones are prime perching places for seekers of food, and for growths of lichen.  Rain runoff from the structure creates moist crevices for sedges, mosses, woodlice, and many other plants and animals that require a damp environment.  A grave therefore is a different habitat from the grassy surroundings and contributes to the wildlife variety in the cemetery.

– The following paragraphs may be potentially upsetting as the scientific process of body decomposition is specified for educational purposes.

A corpse as a wildlife habitat

Soil creatures such as worms, carnivorous slugs, beetles and their larvae, ants and microorganisms will eventually get to a buried body however well embalmed and enclosed. Many different insects feed on cadavers, which is what forensic entomologists rely on to determine whether a body has been buried or not, and for how long. Blow Flies (members of the family Calliphoridae), Flesh Flies (Sarcophagidae) and House Flies (Muscidae) will push their way through cracks in the soil to the body or as near as they can get and then lay their eggs. The hatched larvae then continue the journey until the body is reached. Species of these three families are regularly seen in Heene cemetery. So determined are the members of the diminutive insect family Phoridae to reach cadavers that they are known as ‘coffin flies’. Various species of beetle will also burrow down to buried bodies.

The onset of corpse decomposition

Throughout our lives we live with internal populations of bacteria, fungi, and other microorganisms, that co-exist in a mutually beneficial relationship. When a body dies these populations continue to thrive on the nutrients they have been used to taking, but the body no longer takes in nutrients itself as it did when alive. The food supply for the resident populations is now limited and gradually decays along with the tissues and organs, with only the resilient parts, such as bones and teeth, resisting. The rate of bodily decomposition depends on a number of factors, particularly the environment in which the body has been kept.

A body left above ground in a sky burial decomposes approximately twice as fast as a body in water, and four times as fast as a body buried without enclosure underground.  Decomposition is faster in water-saturated ground and can take a long time in very dry well-drained conditions.  An embalmed body in a funeral parlour with an ambient temperature of 20 C and humidity level 50% will reveal the onset of noticeable decay in three days or less.  The use of embalming chemicals slows the rate of decomposition, more so if the air is dry.

Light, heat, moisture, and wildlife populations all affect the rate of decomposition of an exposed body.  Putting a body in a casket isolates it from many of the agents of putrefaction, at least for a time, especially if the burial is deep and in cool, unchanging conditions.  Embalming chemicals eventually evaporate or otherwise disperse, so can no longer hinder putrefaction.  Tissues vary in their rate of decay, with tendons and ligaments being the most resistant, but after a year only the skeleton and teeth will generally remain.  An unenclosed body will decay at a rate dependant on the soil conditions, the fastest rate of decay being in damp, warm acidic soil.  In dry, neutral soil bones can survive for several hundred years, with brittleness beginning after about forty years.

Corpse autodigestion

Grieving relatives may look upon the corpse of their loved one as ‘dead’, but in fact it is teeming with life, the resident organisms present in every living body of every organism on the Earth. They continue to feed upon the body while the tissues provide nourishment, and this process is called autodigestion or autolysis. When the heart stops beating cells are deprived of oxygen, and the products of biochemical reactions begin to accumulate, increasing the acidity of cellular fluids. As the cells break down through enzyme activity, especially in organs like the liver and brain that are rich in enzymes and aqueous fluids, their by-products spill into surrounding tissues. These processes cause discolouration, visible through the skin, and eventually the skin takes on a grey-blue mottled colour before darkening through greenish-black to black.

As the body temperature falls, and body parts are no longer refreshed by the blood stream and active chemical processes, their cells become depleted of their substrates and energy source. This causes their protein fibres to become locked in place, whereas in a living body muscle cells would be constantly contracting and relaxing. The onset of protein rigidity is called rigor mortis (the ‘stiffness of death’), and it normally starts in the eyelids, jaw, and neck muscles, before working its way down into the torso and limbs.

The parts of the body that normally have a microbial community, such as the gut, are now seething with activity, and will continue to do so while the consumable tissues last. The immune system of a living body prevents these microbe communities from spreading to other parts of the body, but when the body dies this restriction no longer applies, and intestinal bacteria spread to surrounding tissues, the stomach and the rest of the digestive system, the lymph nodes, the liver and spleen, the heart and brain, and finally the reproductive organs. The progress of this sequence can assist in determining the time of death. In quite a remarkably consistent way, it takes gut bacteria about 20 hours to reach the liver, and 58-60 hours to reach every organ in the body.

Corpse putrefaction

Once the oxygen in the area surrounding the body is used up anaerobic bacteria and other microorganisms multiply as they feed on the remaining bodily tissues. Carbohydrates are decomposed by a process akin to fermentation, producing gaseous by-products such as methane, hydrogen sulphide, and ammonia. Such a volume of gas causes bloating of the abdomen and other body parts, with discolouration by leakage from disintegrating blood cells. The decomposition of haemoglobin gives rise to the greenish-black colouration of the skin. As the gas pressure inside the body rises the skin begins to blister and becomes detached from the tissues and skeleton. Gases leak out from the bodily orifices, but if the pressure rises rapidly the abdomen can burst. The increasingly strong odour emanating from the body at this stage attracts more and more necrophagous (‘feeders on the dead’) insect species, and the body enters the final stage of putrefaction.

Necrophagous colonisation of a corpse

Once a decomposing body starts to purge the specialised receptors on insect antennae soon pick up the odours of putrefaction, and eggs are laid in orifices and open wounds. Typically a female fly will lay over 200 eggs, and they will hatch within 24 hours to produce first stage maggots (first instar larvae). These feed on the rotting flesh and then moult into second instar larvae, and afterwards into third instar larvae, before pupating and undergoing transformation into adult flies. This cycle will be repeated while there is edible flesh. A seething mass of maggots generates heat, and the temperature of the cadaver may rise by more than 10 C. This scenario attracts predators that feed on or parasitise the flies’ eggs and larvae, such as mites, spiders, beetles, ants, and wasps, creating a temporary mini-ecosystem.

All visitors to the body will leave their own chemical and microbiological signatures in the form of some of their own bodily fluids and microorganisms, and this can be of forensic value in deducing the parameters that appertain to the circumstances of the interment. The bacterial composition of an individual is unique, and this applies to humans as well as other species, and currently microbiologists are researching how to determine and document this unique personal microbiological signature.

The fluids that are purged from a decomposing body, once they have seeped into the surrounding soil, alter the chemical composition of the soil in recognisable ways, and these changes also have forensic value.  Any alteration in soil chemistry will attract specific wildlife, such as nematode worms, whose presence is a sign that a decomposing body has at some time been present.  Typically, a human body contains from 50 to 75% water, and once the body is dehydrated every kilogram of dry body mass eventually releases 32g nitrogen, 10g phosphorus, 4g potassium, and 1g magnesium into the soil.  Purging thus creates an organically rich surrounding soil, and as the nutrient leakage occurs at a measurable rate this provides forensic information about the likely time of interment.

The practice of embalming involves the treatment of a body with chemicals in an attempt to slow down the decomposition, but one way or another nature’s recyclers will not be denied.

Corpse mummification

In hot, dry conditions a body left exposed will mummify rather than decompose, because the skin quickly loses all its moisture and remains clinging to the bones. Generally, at a temperature of 25 C a corpse will be in an advanced stage of decomposition after 16 days, but this period will be extended as the ambient temperature rises, provided the conditions are dry. This is because the activity of microorganisms is hindered in very hot, dry circumstances. Burying in hot, dry sand has a similar effect. Under the British climate mummification is extremely rare, because our warm, damp climate favours rapid decomposition. The practice of embalming involves the treatment of a body with chemicals in an attempt to slow down the decomposition, but one way or another nature’s recyclers will not be denied.

Written by Brian Day