Forensic estimation of time of deathEdexcel International A Level Biology: Revision notes

The post-mortem interval is the time between death and the discovery of the body. Estimating it helps investigators decide who could have been present and whether an alibi is true.

No single method is exact, so forensic scientists combine several kinds of evidence:

• body temperature (first day)
• degree of muscle contraction, or rigor mortis (first day or two)
• extent of decomposition (days to weeks)
• stage of succession of organisms colonising the body
• forensic entomology, using the insects' life cycles

Early changes work best soon after death. Decomposition and insects are used later.

A living body keeps a constant core temperature of about 37 °C because respiration releases heat. After death respiration stops, so no heat is produced and the body cools towards the temperature of its surroundings.

A thermometer is placed in the rectum to measure the core temperature. After an initial plateau, a body cools at roughly 1.5 °C per hour.

Worked example: a core temperature of 31.5 °C: fall = 37.0 − 31.5 = 5.5 °C; time = 5.5 ÷ 1.5 ≈ 3.7 hours.

The rate of cooling is affected by:

• air or water temperature and wind
• clothing and coverings, which slow cooling
• body mass and fat, since larger bodies cool more slowly
• whether the body is wet or lying in water

After death respiration stops, so no ATP is made. Muscle contraction relies on actin and myosin, and ATP is needed for myosin heads to detach from actin. Without ATP the cross-bridges stay in place, so the muscles become stiff, a condition called rigor mortis.

• First noticeable after about 2–4 hours, beginning in small muscles of the face and jaw.
• At its maximum at about 12 hours.
• Passes after about 24–36 hours, when muscle proteins are broken down by enzymes and the cross-bridges are lost.

Warm conditions speed up rigor mortis and cold slows it down. The degree of stiffness is therefore a rough guide to the time since death.

After death, cells decompose in stages:

• Autolysis: cells lack oxygen, membranes fail and lysosomal enzymes digest the cells from within.
• Putrefaction: bacteria, especially from the gut, spread and break down tissues. They release gases such as methane and hydrogen sulfide, so the body bloats and later collapses, and colour changes occur.
• Feeding by insects and other decomposers removes the soft tissue.
• Eventually only dry remains (skin, hair, bone) are left.

The extent of decomposition can be compared with known timescales. It is useful for days to weeks after death, but the rate depends strongly on temperature, moisture, burial and access by insects.

Succession is the sequence in which different organisms colonise a body as it changes:

1. Blowflies arrive within minutes to hours and lay eggs on the fresh body.
2. Flesh flies and other flies arrive as the body bloats and decays.
3. Fly larvae feed on the soft tissue.
4. Beetles that feed on dried skin, hair and bone arrive in the later, drier stages.

Because each group is adapted to a particular stage of decay, the species present show how far decomposition has progressed and so roughly how long since death.

Forensic entomology uses insects to estimate time of death.

• Blowflies lay eggs soon after death. Each develops through egg, larva (maggot), pupa and adult.
• The entomologist identifies the species and the oldest stage present, which gives the age of the earliest colonisation.
• Each species has known life-cycle times at given temperatures. Development is faster when warmer because it depends on enzyme-controlled reactions, so the temperature at the scene must be recorded.

Worked example: at 22 °C, eggs hatch in 1 day, larvae last 5 days. Larvae at the end of larval stage mean 1 + 5 = 6 days since eggs were laid, so death was at least 6 days earlier.

The result is a minimum time, since eggs are laid after death.