What you need to know
- 6.20Time of death from body temperature and degree of muscle contraction, alongside the other signs in the next lesson.
No degrees-per-hour rule is official. Decomposition, succession and insects are 6C 3. These signs give a range, not a minute, and not a statistical test. Do not apply them to a real death.
By the end, you should be able to…
- Explain why a body cools toward the surroundings, and why the rate varies.
- Describe stiffening and later easing of muscles without a fixed timetable.
- Refuse a single time from one sign.
Build your understanding
Temperature falls toward the room or the field
After circulation stops, the body no longer replaces the heat it loses. Core temperature moves toward the temperature of the surroundings. A warmer room, clothing, a large body, or still air slows that fall. A cold, naked, small body in a draught cools faster. Measuring one temperature, without the ambient temperature and these factors, does not give a time.
Some classroom accounts say “about one degree an hour”. Treat that as a rough illustration of the idea of cooling, not a formula you may apply. The specification asks you to understand that temperature is evidence, not to memorise a constant. Say which factors would make your estimate earlier or later.
Muscles stiffen, then the stiffness passes
The degree of muscle contraction after death is rigor. While a little ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. remains, muscles stay relaxed. When respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. stops, ATP is no longer made. Cross-bridges that have formed cannot detach, because detachment needs a new ATP. The muscles become stiff. Stiffness often appears first in smaller muscles, such as the jaw, and later in larger ones. That order can be described. A clock time for it cannot.
Later, the tissues themselves are broken down by the body’s own enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. and by microorganisms. The stiffness then eases. Warm conditions speed both the onset and the later loss of stiffness. A cold body can still be stiff long after a warm body has begun to soften. “Stiff” therefore means “ATP has run out and proteins have not yet been destroyed”, not “this many hours”.
Several signs, one range
Temperature and rigor are the early signs in 6.20. Extent of decomposition, the stage of successionThe gradual change in the community of an area over time, as each set of species changes the conditions for the next. and forensic entomology come next, because they cover longer times. An estimate should say “consistent with a range” and name the assumptions. It should not say “death was at 02:10” from a thermometer.
This is biology of a mammal after death. It is not a method for you to use on an animal you find, and not a description of a real case. Nothing here is an official forensic protocol.
NOT: the body does not cool by one degree Celsius every hour, and stiffness is not a fixed number of hours.
- Clothing, body size, a warm room and still air slow cooling
- A cold, small, unclothed body in a draught cools faster
- Stiffness often starts in smaller muscles such as the jaw
- A cold body can stay stiff after a warm body has softened
- The estimate is a range, not a minute on a clock
Two traps this diagram blocks
- Do not use a fixed cooling rate of one degree per hour.
- Do not treat stiffness as proof of one particular hour.
From the diagram alone
Question
Explain why muscle stiffness develops after death and why it is later lost. [3]
Show a model answer
Respiration stops, so ATP is no longer made. Cross-bridges cannot detach without ATP, and the muscle becomes stiff. Later, enzymes and microorganisms break the tissues down, and the stiffness passes.
Make your reasoning visible
Name the factor
Clothing, size and ambient temperature change cooling. Temperature of the body alone is incomplete.
ATP, then breakdown
Stiffness is locked bridges. Easing is later destruction of the muscle, not a new supply of ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi..
Key explanations
- Temperature and rigor together support a range. They do not give a precise minute of death.
Avoid this misconception
The claim: A body cools by one degree Celsius every hour, so stiffness always means death was four hours ago.
The correction: Cooling depends on the surroundings and the body. Stiffness starts when ATP runs out and later passes as tissues break down. Both are ranges, and neither is a fixed hour count.
Can a thermometer tell you when someone died?
After death, the body cools toward its surroundings and muscles stiffen when ATP runs out, then ease later. Both depend on the conditions, so each gives a range of times.
What the model shows
A fictional body cooling in different rooms, the stiffening and easing of three muscle groups, and two cases that died at the same moment.
What to look for
The shape of the cooling curve, the order of stiffening, and how far the one-degree rule is from the truth.
What it tells us
Cooling depends on the temperature gap, clothing and size. Rigor is locked cross-bridges without ATP. A time of death is a range consistent with several signs.
Be careful what you claim
Fictional, simplified rates; not a forensic method.
Try it in the model: press Play, then drag the room temperature down to 8 °C.
Time of death: temperature and rigor
Cool a simplified, fictional body in different conditions, follow the stiffening and easing of its muscles, then estimate the time of death for two bodies that died at the same moment. Rates are fictional and this is not a forensic method.
What you see
- Cooling: a simplified figure shaded by core temperature, a thermometer beside the room temperature, and a graph of core temperature against hours with the “one degree an hour” rule dashed for comparison.
- Rigor: the jaw, arms and legs shaded by degree of contraction, a sarcomere showing ATP and locked myosin heads, and a graph of stiffness in each muscle group.
- Estimate: two fictional bodies that died at the same moment, with the time from the rule and the ranges from temperature and rigor on one timeline.
How to use it
- Cooling: set the room temperature, clothing, build and air, then play or drag Hours.
- Rigor: set the room temperature, then play or drag Hours up to 96.
- Estimate: choose the method, and show the fictional true time when you have an answer.
What it shows
- Why body temperature falls toward the surroundings and why the rate varies (6.20).
- Why muscles stiffen when ATP runs out and ease again as proteins are broken down (6.20).
- Why a time of death is a range from several signs, not a single time from one.
Limits of the model
- All rates are fictional and simplified. Real cooling often has a short plateau first, and the core is not one uniform temperature.
- Rigor in this model depends only on room temperature. Real onset also depends on activity before death, body temperature and muscle mass.
- Decomposition, succession and insects (6C 3) are not included. This is not a forensic method and must not be applied to a real death.
Guided tour · step 1 of 12
In Cooling, just below these steps, press Play (or drag Hours past 12).
Circulation and metabolism have stopped, so heat lost is no longer replaced. The core falls fastest at first, when the gap to the room is largest, and the curve flattens toward room temperature. It is not a straight line of one degree an hour.
Cooling
Questions
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Why does core temperature fall after death?
Show suggested answer
Circulation and metabolism have stopped, so heat is no longer replaced and the body cools toward the temperature of the surroundings.
Practice question
Retrieval and explanation · suggested answer
Give two factors that slow that cooling.
Show suggested answer
Any two of: warm surroundings, clothing or covering, a larger body, still air.
Practice question
Retrieval and explanation · suggested answer
Why do muscles stiffen when ATP is no longer made?
Show suggested answer
Myosin cannot detach from actin without ATP, so the cross-bridges stay locked.
Practice question
Retrieval and explanation · suggested answer
Why might a warm body have lost its stiffness while a cold body is still stiff?
Show suggested answer
Warm conditions speed the breakdown of muscle proteins that ends rigor. Cold slows that breakdown, so stiffness lasts longer.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two mammals of the same species die at the same moment. One is clothed in a warm room. One is unclothed in a cold room. A student uses the same cooling table for both and gets the same time. What is wrong, and which 6.20 sign might still differ even if you corrected the table? Assume no insects have arrived yet.
Show a hint
The table ignored the conditions. Then think about rigor.
Explore the reasoning
Cooling rate is not the same in the two places, so one table cannot give one time. Muscle stiffening and its later loss also run faster when it is warm, so the degree of contraction can differ too. Insects are excluded by the question. You still only have a range, not a minute.
Recall this lesson
Lesson recall questions
Question 1 of 5: List the five specified sources of evidence used to determine time of death of a mammal.
Time of death
Practice question
Retrieval and explanation · suggested answer
List the five specified sources of evidence used to determine time of death of a mammal.
Lesson: 6C 3: The Process of Decay
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Forensic work is measurement with a method
6C · Forensic Science and the Time of DeathNIST describes forensic science as applying scientific methods to evidence. The page discusses careful analysis, including DNA reference materials. It does not give a body-cooling formula or a…Specification 6.20Read & use the scienceClose article
Why this matters
NIST describes forensic science as applying scientific methods to evidence. The page discusses careful analysis, including DNA reference materials. It does not give a body-cooling formula or a rigor timetable. That absence is the point: a biology lesson can explain the signs, and it still is not a case method. Real investigations combine several imperfect indicators. Cooling depends on surroundings, and rigor varies with conditions, so a professional reports a supported interval rather than a clock-like certainty.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.20 understand how to determine the time of death of a mammal by examining the extent of decomposition, stage of succession, forensic entomology, body temperature and degree of muscle contraction
Use the science
Why would quoting “NIST says the body was dead for five hours” misuse both the page and the specification?
Explore a possible response
NIST does not state a five-hour rule on that page. 6.20 expects the biological reasons temperature and stiffness change, plus the factors that make any time uncertain.
Explore the source
A time-of-death estimate is a range
6C · Forensic Science and the Time of DeathA review of forensic entomology stresses that insect evidence is influenced by temperature, species and the time when insects first reached a body. Investigators may estimate a minimum…Specification 6.20Read & use the scienceClose article
Why this matters
A review of forensic entomology stresses that insect evidence is influenced by temperature, species and the time when insects first reached a body. Investigators may estimate a minimum interval since colonisation, which is not automatically the time since death. Cooling and rigor mortis offer other clues, but both depend on surroundings and individual variation. The forensic scientist’s job is to combine methods and state uncertainty honestly. Television often compresses this into a precise hour; a responsible report explains assumptions and a supported range. The specification names several indicators because no single biological clock is perfect.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.20 understand how to determine the time of death of a mammal by examining the extent of decomposition, stage of succession, forensic entomology, body temperature and degree of muscle contraction
Use the science
Why can the oldest larva provide a minimum interval since colonisation without proving the exact hour of death?
Explore a possible response
Insects may arrive after death, and their development rate depends on species and temperature; the larva dates its own growth, not the death itself.
Explore the source
- A review of postmortem intervals and forensic entomologyMedicine, Science and the Law · Abstract