Tahsin Tabassum
Sophomore
School of
Life Sciences
Independent
University, Bangladesh
June 23rd, 2017
The time of
death, as the phrase suggests, refers to the exact moment that someone has
died. It is a very important variable in forensic science, and can help solve
murders and identify murderers by matching and comparing with alibis and times
when suspects were present. It is also an interesting microbiological
phenomenon, as we shall see. Estimating the time of death is a tricky task and
it becomes harder to calculate if the body has been dead more than 48 hours. Scientists
can estimate the time of death based on many factors. Let’s take a look at some
of the important ones.
Body temperature
The average normal
human body temperature is around 37oC, which remains more or
less constant during one’s lifetime. This is because we are mammals, and
therefore endothermic, that is, capable of generating internal heat from
metabolic reactions such as respiration. From the time of death, the metabolic
reactions slow down and eventually stop. This causes the body temperature to
decrease until it becomes equal to the surrounding temperature. This process is
known as algor mortis. Forensic scientists can estimate the rate at which a
human body cools based on physique, amount of clothing and exposure, and the
total surrounding environment, which includes temperature, humidity, and wind
speed. The rate of cooling also varies depending on which part of the body is
measured. After taking all these variables into account, it is possible to work
out the approximate time of death from the temperature of the dead body when
discovered. This is a very useful method to determine the time of death but it is
only helpful if the body is found within a few hours of death.
Rigor mortis
During
aerobic respiration, energy is produced through the utilization (more specifically,
reduction) of oxygen. Not all of the energy is used immediately, and much of it
is stored as a molecule called ATP or adenosine triphosphate, which can be
broken down later to release that energy for use by the cell. In muscle cells,
ATP is used to maintain muscles in a relaxed state. Oxygen is provided to cells
via blood. When an animal dies, the heart stops pumping blood around the body,
but muscle cells are able to anaerobically generate ATP for some time, allowing
muscles to stay relaxed. Eventually the muscles run out of ATP, which causes
them to contract permanently, and stiffen. The process of contraction and stiffening
of muscle is called rigor mortis. Contraction of muscles starts from the head
and neck region 2-3 hours after death, and spreads throughout the body 6-8 hours
after death, but these ranges can vary by climate. The contraction of the
facial muscles creates an impression of smiling.
Approximately 24 hours after death, the body releases
degradative enzymes present in the cell’s organelles called lysosomes that
break down muscle cells by a process known as autolysis, after which the
muscles no longer contract. Rigor mortis is therefore not permanent and usually
passes between 36-48 hours after death. Forensic scientists can estimate a
range for the time of death based on the stage of rigor mortis the body is
found in.
Ecological Succession
The types of
organisms found in or on a dead body change over time, going through a number
of stages. This is known as ecological succession. The stages of ecological succession
for a body that has not been buried is known. Forensic scientists can
determine the time of death based on the stage of succession the body is in.
Microbes are
everywhere, and can collectively all organic materials. Immediately after the death,
conditions are very favorable for bacteria, including ones that inhabit our
guts. The bacteria begin to decompose the body tissues, and the conditions
become favorable for flies and their larvae. As they feed on the tissues and
organs, gases such as methane are produced which causes the body to become
bloated. The skin begins to blister and fall off. Soon the beetles move in,
initially to feed on the fly larvae and later to chew on body parts. As the
dead body dries out in a few weeks, the conditions are no longer suitable for
the flies, but the beetles continue to feed on the dead carcass. Other animals
that are involved include moths, wasps, and mites. When there are no tissues to
feed on, and only the skeleton remains, the series of invasions ends, but this
would take a very long time.
The stages
of decay remain more or less constant, but the speed at which it might occur
varies and depends on number of factors such as temperature. For instance, if the
body is kept in a freezer, it would not show any secondary stage of decay (no
flies or larvae present) and the decaying rate would be much slower compared to
a body in a desert that would have faster a rate of succession.
We have
considered the ecological succession of organisms for a body that is discovered
above ground in the interest of covering forensic investigations. Decomposition
in nature is an important means of recycling nutrients, and the succession and
metabolism of microbes in a buried setting is also fascinating in its own
right.
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