International A Level Biology / Decomposition and forensics
Time of death: body temperature and rigor mortis revision notes
Edexcel International A Level Biology (YBI11), chapter 19: Decomposition and forensics, specification point 6.20. Free lesson.
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Body temperature and rigor mortis provide two ways to estimate time since death. Both methods have limits because conditions affect cooling and the development of stiffness.
How a body cools after death
Respiration releases heat and keeps your core temperature at about 37 °C. After death, respiration stops, so the body loses heat to its surroundings. This cooling is called algor mortis. A cooling curve shows body temperature, usually measured rectally, against time after death. Skin cools faster than the core. The curve depends on the conditions given, so use the curve supplied in the question. The curve assumes the body started at about 37 °C. Fever or hypothermia can make that assumption unreliable. The body cannot cool below the surrounding temperature, so the curve levels off…
How to use a cooling curve
Read the graph to find time after death, then work backwards from when the body was found to estimate the time of death.
What changes the rate of cooling
Cooling depends on how quickly the body loses heat. For each factor, explain its effect on heat loss and then state whether cooling is faster or slower. Colder surroundings create a bigger temperature difference, so heat is lost faster and the body cools faster. Clothing or covers trap insulating air, reducing heat loss and slowing cooling. A large body has a smaller surface area to volume ratio, so it loses less heat for its size. Fat also insulates. These factors slow cooling. Wind carries heat away by convection. Water conducts heat away better than air. Both can make cooling faster.
Why muscles stiffen after death
When the heart stops, blood stops circulating and muscle cells no longer receive oxygen. They respire anaerobically for a while, making a little ATP (the molecule that supplies energy for muscle activity), while lactic acid builds up. Lactic acid lowers pH, which inhibits the enzymes involved in respiration. Respiration stops, and the remaining ATP is used up. Myosin heads bind to actin, forming cross-bridges. ATP is needed for the heads to let go. When there is no ATP, the cross-bridges stay locked. Actin and myosin remain bound, so the muscles stiffen. This is rigor mortis.…
When rigor mortis develops and passes
Rigor mortis is another clock, but its timing is approximate and changes with conditions. Small muscles stiffen first, then stiffness spreads through the body. Enzymes from breaking-down cells later digest muscle proteins, so actin and myosin can no longer hold together and the stiffness passes off. It begins at about 3 hours, first in small muscles such as the eyelids, face and jaw, then spreads to the neck, trunk and limbs. By about 12 hours the whole body is rigid. It stays fully rigid for a while, then passes off by about 36 hours. Warmth makes rigor start and pass sooner; cold delays it.…