Wondering how a forensic witness determines the time of death? Let our clear and simply article tell you all about it.
When determining time of death following a crime, accident or other unexplained circumstances, there is a range of evidence which a forensic pathologist might draw on. This includes body temperature, stomach contents, progress of rigor mortis and other physical changes, blood pooling, condition of traumatic injuries, and insect activity. Unless someone died with a doctor and a reliable clock present, time of death will likely be an estimate.
There are many considerations in determining the time of death. As well as looking at electronic records and witness statements, an expert forensic witness is needed to analysis and explain what the condition of the corpse might indicate about the time passed since death. To learn more about what a dead body can tell a forensic pathologist, carry on reading.
Body temperature can be an extremely important piece of evidence in determining time of death.
Average adult body temperature is around 98.6 degrees Fahrenheit. After death, a corpse will gradually lose or gain heat until it reaches the temperature of its environment and then remain stable. Ideally the temperature of both the body and its immediate surroundings (air, water, snow, earth etc…) should be taken.
While there are many slightly different formulae for estimating the rate of corpse heat loss, a general rule of 1.5 degree per hour works well. This gives a formula is for estimating hours since death:
Hours since death = 98.6 – corpse core temperature / 1.5
The sooner a body is examined after death, the more accurate an estimate can be made. Once the corpse has reached the same temperature as the surrounding environment, taking its temperature may become useless. Even when taken quickly and correctly, there are several possible factors which could influence the accuracy of the result, for example:
– Variation in normal body temperature
Not every human being has a normal body temperature of exactly 98.6 degrees Fahrenheit. Some people will have have higher or lower standard temperatures than others.
Infections could lead to a high temperature at death, while dehydration or shock could lower someone’s body temperature.
– Time of day
Normal body temperature fluctuates slightly over the course of the day, often being lower in the morning and higher later in the day.
Thick, insulating clothing could slow loss of heat from the body. Light, skimpy clothing could allow greater loss of heat.
– Body fat level
Like heavy clothing, fat is a good insulator and the body of an obese person will cool more slowly than a thin person without much body fat.
– Age and health status
The bodies of children and the elderly can lose heat faster. Heat loss may also be quicker from those whose bodies were affected by chronic illness or emaciation.
Heat loss may be quicker when the body is in contact with colder materials (e.g. marble, metal) compared to warmer materials (e.g. textiles, carpet). Full sunlight will slow cooling, while cold wind or rain will speed it up.
Examining the contents of a corpse’s stomach and intestines can show approximately when someone ate their last meal, based on the state of digestion of the food and how far it has passed through the digestive system.
In a normal healthy individual, research indicates that the stomach will have emptied 4-6 hours after eating a meal. Knowing this and examining stomach contents can help to work out a time of death.
So, if a corpse’s stomach is full and there is little evidence of digestion, it is likely that death occurred soon after eating. Similarly, an empty stomach indicates that death probably happened 4-6 hours after that person last ate, and an empty small intestine means it probably happened 12+ hours after last eating.
The stiffening and contraction of body muscles called rigor mortis generally begins 2-4 hours after death and affects the whole body within 8-12 hours after death. By this point every part of the body will be entirely stiff and resistant to movement, potentially fixing the corpse in its position at death (unless it was moved subsequently).
Decomposition occurs through the two processes of autolysis (a breakdown of cells and internal organs) and bacterial action. Autolysis is a chemical process which can be accelerated by heat, slowed down by cold or halted by freezing.
Greenish discoloration due to decomposition is generally evident within 36 hours, accompanied by the odor of putrefaction. The overall rate at which decomposition proceeds is linked to temperature, environment, body size etc.. but it is hard to draw more precise conclusions based on this factor alone.
Research shows that different microorganism activity will be present in the mouth of a corpse compared to a living person. This is a relatively new area of forensic science research.
There are at least two ways in which eyes can help estimate a time of death.
One is through observing corneal clouding, which occurs around two hours after death, with the cornea then continuing to become more opaque over the next 1-2 days.
Another is through a measurement of potassium levels in the eye’s vitreous humor (the jelly-like substance which fills the space between the lens and the retina).
Throughout the body potassium will be released after death as blood cells break down. In the eye, this happens more slowly and predictably than in the blood, and is not temperature-dependent. A sample from the vitreous humor can therefore be used to estimate a time of death.
After death there are a number of changes to the appearance of skin which can help in determining time of death.
Livor mortis is a purple-red coloration that appears on body areas not exposed to any pressure once the heart has stopped beating, as blood settles and pools due to gravity. It begins with reddish patches appearing 20 – 30 minutes after death and progresses over 2-4 hours with these patches coming together in larger areas of purple discoloration.
At this stage of livor mortis, the skin is ‘blanchable’, which means that when pressure is applied it will go white as blood is diverted from the area being pressed, but will then become purple-is again once pressure is removed and blood returns. This can last for 8-12 hours after death. After 12 hours, the color of the skin will normally become fixed and is no longer blanchable.
When a corpse is submerged in water for a prolonged time, the skin dissolves at a known rate and this may also be used in estimating a time of death.
The status of traumatic injuries can give many clues about time of death. Evidence might include the level of blood clotting around a wound, or signs of healing of a broken bone. Fresh injuries may indicate a more recent time of death.
Forensic entomology – insect activity
Forensic entomology is a relatively new field in forensic science which allows forensic pathologists to determine time of death through interpretation of insect colonization levels and activity on a corpse. The different stages and quantities of maggots present, for example, can indicate a specific period of time since death if we know the life-cycle of that particular fly.
Limitations in determining time of death
If someone dies in a room with a qualified doctor, several witnesses, CCTV and an accurate clock present, establishing time of death might be straightforward. In many cases, however, it is impossible to determine a precise time of death and the view given by a forensic pathologist will be a likely estimate or a time range.
As a general rule, the sooner after death a body can be examined, the more accurate the time of death determined is likely to be. In practice, there can be three different times of death:
- Physiologic death: the point when the victim’s vital functions completely ceased.
- Legal time of death: the time stated on any official death certificate
- Estimated time of death: the time that a medical examiner estimates as point of death based on examination of the body.
These times may end up being very different, especially where a body is not discovered quickly.
A final word…
There are a number of clues that a forensic witness can take from examining a corpse which will help them to determine an approximate time of death. The most important of these is probably temperature, but stomach contents, status of injuries, level of rigor mortis and other physical signs can also play a role. Forensic entomology is a newer field of forensic science which can also make an important contribution.
The professional views of an expert witness will be just one component considered by courts, police and other juries considering time of death. Electronic and digital evidence, CCTV and eye-witness reports may be equally important in establishing the true time and circumstances of a death.
Whether you’re a Crime Scene Investigation TV series fan, interested in studying forensic science or just curious about the subject of how forensic witness can determine the time of death, we hope this article has answered some of your questions.