Time of death and decompositionEdexcel A-Level Biology A: Revision notes

The post-mortem interval is the time since death. Forensic scientists estimate it by combining several lines of evidence, because each method works best over a different time range and none is exact:

• body temperature (first 24 hours)
• degree of muscle contraction (rigor mortis, about 2 to 36 hours)
• extent of decomposition and stage of succession (days to months)
• forensic entomology, the study of insects on a body (days to weeks)

Estimates are given as a range, not a single time.

After death, metabolism stops and no heat is generated, so the body cools towards the temperature of its surroundings. The rectal temperature is measured with a thermometer, as the core cools more slowly than the skin.

A simple estimate assumes cooling of about 1.5 °C per hour from 37 °C. Worked example: a rectal temperature of 31.0 °C gives (37.0 − 31.0) ÷ 1.5 = 4.0 hours.

The rate is affected by:

• air temperature and wind (convection)
• clothing and covering, which insulate
• body mass and surface area to volume ratio
• water (cooling is much faster than in air)

The method is useful only for about the first 24 hours, as the temperature then reaches that of the surroundings.

After death the muscles first relax, then become stiff in rigor mortis.

In life, ATP is needed to detach myosin heads from actin and to pump calcium ions back by active transport. After death, respiration stops and ATP is no longer made. Cross-bridges between myosin and actin stay attached, so the muscles are fixed in a contracted state.

Rigor starts in small muscles of the jaw and face after about 2 to 4 hours, spreads to the limbs and is complete after about 12 hours. After about 24 to 36 hours the muscle proteins are broken down by enzymes and by microorganisms, so the muscles relax again. Warm conditions speed up the process and cold slows it. It is only a rough guide.

After death, autolysis (self-digestion by the body's own enzymes) is followed by putrefaction by bacteria, with gas production and bloating. Stages are fresh, bloat, active decay, advanced decay and dry remains.

Organisms colonise in a predictable order, called succession, because the body provides different food at each stage: blowflies first, then flesh flies, then carrion beetles, and later hide beetles and mites. The species present are matched with known sequences for the habitat and season to give a time range. The rate depends on temperature, moisture, oxygen, burial and access by insects.

Blowflies are attracted by the smell of decomposition and lay eggs within hours of death. The life cycle is egg, larva (maggot), pupa and adult.

The age of the oldest larvae gives a minimum time since death. It is estimated from the length of larvae, the stage reached (larva or pupa) and the species. The rate of development depends on temperature because it is controlled by enzymes, so the temperature record for the site is needed. Larvae in the body can also warm it, so that temperature measurements must be treated with care.

Limits: egg-laying can be delayed if the body is buried, wrapped or found at night or in cold weather.

Saprotrophic bacteria and fungi decompose dead organic matter. They secrete digestive enzymes onto the material (extracellular digestion), which hydrolyse proteins, lipids and carbohydrates to soluble amino acids, fatty acids and glucose. The microorganisms absorb these products.

They use some in respiration, which releases carbon dioxide into the atmosphere, and some to build their own biomass. Carbon dioxide is taken up by plants in photosynthesis, so carbon is recycled through the carbon cycle. Decomposition is faster in warm, moist, aerobic conditions. Detritivores, such as maggots, break the material into smaller pieces and increase the surface area for the microorganisms.