All subjects · Biology · WBI14 (Unit 4)

Decomposition and estimating time of death

Decomposers recycle carbon from dead organisms. Forensic scientists use the predictable changes in a body after death to estimate when death happened.

Decomposition and carbon recycling

Decomposers (saprobionts, mainly bacteria and fungi) secrete enzymes onto dead organic matter and absorb the soluble products (extracellular digestion). They respire these products, releasing carbon dioxide back into the atmosphere, where producers can fix it again by photosynthesis. Without decomposers, carbon would stay locked in dead bodies and waste.

Body changes after death

Body temperature (algor mortis): after death the body cools towards the temperature of its surroundings. The rate depends on air temperature, body mass, clothing and air movement. If the cooling rate is known, time since death can be estimated: time = (normal body temperature - body temperature) / rate of cooling.
Degree of muscle contraction (rigor mortis): after death, respiration stops, ATP runs out and myosin heads stay attached to actin, so muscles stiffen. Rigor begins about 1 to 2 hours after death, is complete by about 12 hours, and passes off over roughly 24 to 48 hours as proteins break down. Warmer conditions speed it up.
Extent of decomposition: the body's own enzymes digest cells (autolysis); gut bacteria spread and produce gases that make the body bloat; tissues then liquefy. The stage reached gives a rough time scale.

Forensic entomology and succession

Insects such as blowflies often lay eggs on a body within hours of death. Their eggs hatch into larvae (maggots) that grow through stages at a known rate for a given temperature. Identifying the stage or measuring larval length gives a minimum time since death.

As the body decays, a predictable succession of species arrives: first flies, then beetles that feed on the larvae and on drier tissue, then others, so the community present indicates the stage of decay.

Worked example

A body is found with a core temperature of 31.0 °C. Assume normal body temperature is 37.0 °C and the body cools at 1.5 °C per hour. Estimate the time since death.

Temperature drop = 37.0 - 31.0 = 6.0 °C
Time = 6.0 / 1.5 = 4 hours

Answer: About 4 hours.

Key idea: Decomposers secrete enzymes, absorb products and respire, returning CO2. Time of death: body cooling, rigor mortis (starts about 1 to 2 h, full by about 12 h, gone by 24 to 48 h), stage of decomposition, insect larvae and succession.

Check yourself

A body has a temperature of 29.5 °C. Assuming normal body temperature of 37.0 °C and cooling at 1.5 °C per hour, estimate the time since death in hours.
Show answer
5 hours. Drop = 7.5 °C; 7.5 / 1.5 = 5 hours.
A body cools from 37.0 °C at 0.8 °C per hour. What would its temperature be after 6 hours, in °C?
Show answer
32.2 °C. 37.0 - 4.8 = 32.2 °C.
How do decomposers return carbon to the atmosphere?
- They photosynthesise
- They store all carbon as fossil fuel
- They release oxygen
- They respire the products of digestion, releasing CO2
Show answer
They respire the products of digestion, releasing CO2. Respiration releases CO2.
What is extracellular digestion?
- Secreting enzymes onto food outside the cell and absorbing the products
- Digesting food inside a vacuole
- Digesting food in a stomach
- Breaking down DNA in the nucleus
Show answer
Secreting enzymes onto food outside the cell and absorbing the products. Saprobionts digest outside their cells.
What causes rigor mortis?
- Muscles absorb water and swell
- Calcium ions are removed from muscles
- ATP runs out, so myosin heads stay bound to actin
- Bacteria make muscles contract
Show answer
ATP runs out, so myosin heads stay bound to actin. Detaching myosin from actin needs ATP.
How do blowfly larvae help estimate time of death?
- They develop through stages at a known rate at a given temperature
- They arrive only after several months
- They stop the body cooling
- They change the DNA of the body
Show answer
They develop through stages at a known rate at a given temperature. Larval stage indicates how long since eggs were laid.
Which factor makes estimates of time of death from body temperature less reliable?
- The colour of the hair
- The person's blood group
- Changes in air temperature and clothing
- The number of teeth
Show answer
Changes in air temperature and clothing. Environment and insulation affect cooling rate.

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Last updated 7 November 2026. Spotted a mistake? Tell us.