What you'll learn
- What forensic scientists mean by time of death and post-mortem interval.
- How body cooling, rigor mortis, livor mortis and decomposition give clues.
- How insects can be used to estimate a minimum time since death.
- Why all estimates depend strongly on the environment, especially temperature.
The big idea: estimating time of death
In forensic biology, you are rarely finding an exact “time of death”. Instead, you gather evidence that narrows down a likely time window.
Post-mortem interval
The post-mortem interval (PMI) is the time that has passed since death. Forensic evidence usually estimates the PMI as a range, not a single exact value.
After death, the body no longer maintains homeostasis: control of internal conditions such as temperature, pH and water balance. Cells stop receiving oxygen and glucose, ATP production falls, membranes become leaky, and enzymes and microorganisms begin breaking tissues down.
Use several clues together
No single method is a perfect clock. A stronger PMI estimate comes from combining body temperature, muscle stiffness, blood pooling, decomposition stage, insect evidence and environmental information.
Early changes after death
Body cooling: algor mortis
Algor mortis
Algor mortis is the cooling of the body after death as heat is lost to the surroundings.
A living human has a core body temperature of about 37 °C. After death, metabolism no longer generates heat, so the body tends towards the ambient temperature. Heat loss happens by conduction, convection, radiation and evaporation.
Body temperature is most useful in the early PMI. It becomes much less helpful once the body temperature is close to the surroundings.
A simplified calculation may be possible if the question gives you a cooling rate:
PMI=Tnormal−Tmeasuredcooling rate\text{PMI} = \frac{T_\text{normal} - T_\text{measured}}{\text{cooling rate}}PMI=cooling rateTnormal−TmeasuredIn reality, cooling is not perfectly linear. It depends on body size, clothing, air movement, water, burial, fever before death, surrounding temperature and whether the body is indoors or outdoors.
Estimating time since death from cooling
A body has a measured core temperature of 30.2 °C. Assume normal body temperature was 37.0 °C and the body has cooled at 1.7 °C h⁻¹.
-
Calculate the temperature drop:
37.0 ∘C−30.2 ∘C=6.8 ∘C37.0\,^\circ\text{C} - 30.2\,^\circ\text{C} = 6.8\,^\circ\text{C}37.0∘C−30.2∘C=6.8∘C -
Substitute into the PMI equation:
PMI=6.8 ∘C1.7 ∘C h−1=4.0 h\text{PMI} = \frac{6.8\,^\circ\text{C}}{1.7\,^\circ\text{C h}^{-1}} = 4.0\,\text{h}PMI=1.7∘C h−16.8∘C=4.0h -
Convert to seconds if required:
4.0 h×3600 s h−1=1.44×104 s4.0\,\text{h} \times 3600\,\text{s h}^{-1} = 1.44 \times 10^4\,\text{s}4.0h×3600s h−1=1.44×104s
So the estimated PMI is about 4.0 h, assuming the cooling rate is valid.
Treating cooling as an exact clock
Body temperature gives an estimate, not a certainty. If the body has reached ambient temperature, cooling can no longer tell you how long it has been there.
Muscle stiffening: rigor mortis
Rigor mortis
Rigor mortis is the stiffening of muscles after death due to changes in actin and myosin interactions.
In living muscle, ATP is needed to detach myosin heads from actin after a contraction. After death, ATP production stops. Calcium ions leak from stores in muscle cells, allowing cross-bridges to form, but there is not enough ATP to detach them properly. The muscles become stiff.
Rigor mortis usually begins a few hours after death, becomes more obvious, then disappears as proteins are broken down during decomposition. The timing is affected by temperature, activity before death, muscle mass and environmental conditions.
Link rigor mortis to ATP
If you are asked to explain rigor mortis, mention ATP. Low ATP means actin-myosin cross-bridges cannot detach normally.
Blood pooling: livor mortis
Livor mortis
Livor mortis is the settling of blood in the lowest parts of the body after circulation stops, causing purple-red discolouration.
Once the heart stops pumping, gravity pulls blood into lower tissues. Early on, the discolouration may move if the body is repositioned. Later, it becomes “fixed” as blood cells break down and pigments diffuse into tissues.
Livor mortis can help with two forensic questions:
- how long the body has been dead, approximately
- whether the body may have been moved after death
Decomposition and succession
Decomposition
Decomposition is the breakdown of dead organic material by the body’s own enzymes, bacteria, fungi and animals such as insects.
There are two important biological processes:
- Autolysis: the body’s own enzymes digest cells and tissues.
- Putrefaction: microorganisms, especially bacteria, break down tissues and release gases and odorous compounds.
Over time, a dead body is colonised by a changing community of organisms. This is a form of succession, where one set of organisms changes the conditions and is gradually replaced by another set.
Early decomposition may involve bacteria and blowflies. Later stages may involve different fly species, beetles, mites, fungi and soil organisms. The exact sequence depends on temperature, humidity, access by insects, burial, clothing, water and season.
Decomposition is temperature-dependent
Warm conditions usually speed up enzyme activity, bacterial growth and insect development. Cold conditions slow these processes down.
Forensic entomology
Forensic entomology
Forensic entomology is the use of insect evidence in forensic investigations, including estimating the PMI.
Blowflies are often among the first insects to colonise a body. Adult flies may lay eggs in moist areas such as wounds, eyes, nostrils or the mouth. The eggs hatch into larvae, often called maggots, which pass through stages called instars before forming pupae and then adults.
The oldest insect stage found can estimate the minimum PMI. This is because the insects must have been developing for at least that long. However, death may have occurred earlier if insects could not access the body immediately.
Minimum PMI
A minimum PMI is the shortest likely time since death based on the evidence. Insect age usually gives a minimum because colonisation may be delayed.
Using temperature: accumulated degree-hours
Insect development is strongly controlled by temperature. A species may only develop normally above a lower developmental threshold, which is the temperature below which development is very slow or stops.
A simple thermal summation model is:
ADH=(Tmean−TLDT)t\text{ADH} = (T_\text{mean} - T_\text{LDT})tADH=(Tmean−TLDT)twhere:
- ADH means accumulated degree-hours
- TmeanT_\text{mean}Tmean is the mean environmental or maggot-mass temperature
- TLDTT_\text{LDT}TLDT is the lower developmental threshold
- ttt is time
For changing temperatures, investigators sum the thermal contribution from different time intervals:
ADHtotal=∑(Ti−TLDT)Δti\text{ADH}_\text{total} = \sum (T_i - T_\text{LDT})\Delta t_iADHtotal=∑(Ti−TLDT)ΔtiEstimating minimum PMI from insect development
A blowfly species needs 390 °C h above its lower developmental threshold to reach the larval stage found on a body. The lower developmental threshold is 10 °C. The mean temperature is 23 °C.
-
Calculate the effective temperature above the threshold:
23 ∘C−10 ∘C=13 ∘C23\,^\circ\text{C} - 10\,^\circ\text{C} = 13\,^\circ\text{C}23∘C−10∘C=13∘C -
Rearrange the ADH equation to find time:
t=ADHTmean−TLDTt = \frac{\text{ADH}}{T_\text{mean} - T_\text{LDT}}t=Tmean−TLDTADH -
Substitute the values:
t=390 ∘C h13 ∘C=30 ht = \frac{390\,^\circ\text{C h}}{13\,^\circ\text{C}} = 30\,\text{h}t=13∘C390∘C h=30h -
Interpret the result biologically:
30 h×3600 s h−1=1.08×105 s30\,\text{h} \times 3600\,\text{s h}^{-1} = 1.08 \times 10^5\,\text{s}30h×3600s h−1=1.08×105s
The insects suggest a minimum PMI of about 30 h. The actual PMI could be longer if egg-laying was delayed.
Insects estimate colonisation time
The oldest insects tell you when colonisation probably began, not necessarily the exact moment of death. Wrapping, burial, cold weather, indoor location or lack of access can delay egg-laying.
Practical evidence collection
Forensic entomology depends on careful sampling. Investigators may:
- collect the largest larvae, pupae and any empty pupal cases
- sample insects from different body regions to reduce bias
- record air, soil, body and maggot-mass temperatures
- preserve some insects to stop development at the collection stage
- rear some insects to adults for more accurate species identification
- compare findings with local weather data
The species matters because different insects develop at different rates. A temperature-development dataset for the wrong species would give the wrong PMI.
Ignoring the microclimate
The temperature experienced by larvae may differ from the general air temperature. A maggot mass can be warmer than the surroundings because larvae respire and release heat.
Pulling the evidence together
A forensic pathologist builds a timeline from all the evidence:
- Algor mortis: useful early, especially before the body reaches ambient temperature.
- Rigor mortis: linked to ATP depletion and muscle protein breakdown.
- Livor mortis: indicates blood settling and possible movement of the body.
- Decomposition stage: broad estimate, strongly affected by environment.
- Insect evidence: often powerful for later PMI, especially with temperature data.
Think in ranges
Good forensic conclusions usually sound like “the evidence is consistent with death occurring between…” rather than “death occurred at exactly…”.
In the exam
- Use the correct term: PMI is time since death; insect evidence often gives a minimum PMI.
- When doing calculations, write the formula, substitute values with units, and interpret the biological meaning.
- For evaluation questions, always mention environmental temperature and at least one factor that could delay or speed up decomposition or insect development.
Check yourself
- Why does a lack of ATP cause rigor mortis?
- Why does the oldest insect stage usually give a minimum PMI rather than an exact PMI?
- Name two environmental factors that could affect body cooling or decomposition rate.
