| Introduction |
| As in human forensic cases, determination of the time of death may
be necessary to help place a defendant at the scene. In addition, wildlife forensic
cases may involve the harvest of an animal during a non-sanctioned time of day, such as
illegal night hunting. We will discuss some techniques used by investigators to
estimate post-mortem interval; however, most of these methods are applied in the field and
have little use in the laboratory. |
| Many different techniques may be used, and it is important to
remember that there is no single accurate determinant of time of death. Of the
following methods, body temperature loss and response to muscle stimulus are regarded as
the most reliable. |
| Body Temperature |
Body temperature may be a useful indicator of the post-mortem
interval during the first 12 to 24 hours following death. In mammalian species,
intranasal temperature (measure at caudal extent of the nasal cavity) and temperature at
the center of muscle mass in the hindleg usually are taken.
In smaller
mammals, rectal temperature is used (3 inch insertion).
For avian species,
measurements usually are taken by intrathoracic or cloacal insertion (3 inch insertion for
both). Loss of body temperature is dependent on the following variables: |
1. Initial body temperature
The initial body temperature is fairly constant for a given
species and the information is published in various reference texts. However, body
temperature may be elevated by pyrexia or hyperthermia as occurs with disease and physical
exertion, respectively.
|
2. Ambient temperature and humidity
Ambient temperature and humidity greatly affect the cooling of a
carcass. These parameters are relatively easy to measure. It is necessary to
account for weather fluctuations over the relevant time period. Water temperature
also may be a consideration, such as in the case of waterfowl.
|
3. Body surface area
Body surface area is relatively constant for a given species.
|
4. Body mass
Investigators may estimate body mass by measuring the girth of the
thorax at the level of the heart (heart girth).
|
5. Carcass handling
The manner in which a carcass is handled during the post-mortem
interval also will influence body temperature. If a carcass is field dressed, the
body mass is greatly decreased by removal of the internal organs and the surface area is
increased. These changes will increase the rate of body heat loss. Skinning a
carcass will produce a similar effect.
If the carcass is transported in a vehicle,
air flow will cool it much more quickly than exposure to still air. Typically,
intranasal temperature drops faster than intramuscular temperature during transport.
Finally, carcasses may be insulated, either by being piled with other carcasses or being
kept at a higher ambient temperature.
|
| Taking several temperatures over several hours may improve
accuracy of the time of death estimate. Also, a computer program is available for
helping investigators plot white-tailed deer body temperatures. |
| Rigor Mortis |
Rigor mortis is the progressive stiffness of muscle groups that
occurs following death due to the depletion of ATP and phosphocreatine. In general,
full body rigor takes about 6 - 12 hours to develop and is lost by 24-48 hours.
Relaxation of rigor occurs by autolysis. The extent of rigor is measured by flexion
of joints.
The sequence of detectable rigor has been experimentally determined for
several species. In cervids, rigor begins with the jaw followed in order by the
knee, elbow, tarsus, neck, and ends with the carpus. Investigators typically test
both sides and use the side that is most advanced for their estimate. |
It is important to understand the limitations of these
methods. First, the assessment itself is relatively subjective. Also, many
variables may affect the rate for rigor. The process occurs more slowly at cooler
temperatures and is more rapid in animals that died following physical exertion.
Rough handling of a carcass may delay or prevent the onset of rigor mortis. It is
recommended to avoid the muscles adjacent to wounds and legs that are stretched when a
carcass is hung. Also, freezing must be differentiated from rigor in some
situations. |
| Electrical Stimulus |
| Electrical stimulation provides a less subjective method for
measuring the availability of ATP; however, this techniques is affected by the same
processes as rigor mortis. In addition, severe brain injury may affect
response. The apparatus for performing the test is a simple circuit powered by the
ignition system of a vehicle. The response of various muscle groups are ranked as
very good, good, fair, and poor. Responses usually remain good for 4 hours or less
following death. |
 |
Muscle groups stimulated in the deer include the eye, muzzle,
ear, proximal forelimb, epaxial muscles, tail, exposed inner thigh, tongue, exposed flank.
(Diagram credit: Wildlife Forensic Manual 1992) |
|
| This technique may be applied to waterfowl as well. If a
reaction is detectable in the wing, it usually indicates that a duck has been dead for
less than 1 hour. The same finding suggests a goose has been dead less than 2 hours. |
 |
Muscles tested in water fowl include extraocular, bill, epaxial,
tail, wing.
(Diagram credit: Wildlife Forensic Manual 1992) |
|
| Changes in the Eye |
| Ophthalmic examination also may provide an indication of the
post-mortem interval. In cervids, parameters that are examined include pupil
diameter (vertical), tapetal luminescence, and intraocular fluid character. Here is
the general sequence of changes that have been reported for the cervid eye: |
| First 30 minutes following death |
| |
- Intraocular fluid and lens is transparent
- Tapetum brilliant luminous green
- Pupil size dependent on light response at the time of death, after
a brief interval following death, the pupil is fully dilated
|
| 30 minutes to 6 hours |
 |
- Slight loss of color and luminosity of the tapetum
- Subtle wrinkling of cornea
- Lens and fluid transparent
(Photo credit: Wildlife Forensic
Manual 1992) |
| 6 to 10 hours |
 |
- Lens and fluid opacity develops
- Luminosity fades
- Pupil narrows to one third or less of original diameter (use
vertical diameter)
(Photo credit: Wildlife Forensic
Manual 1992) |
| 11 to 18 hours |
 |
- Opacity increases
- Luminosity fades away
- Pupil narrows to one-third or less of original diameter
(Photo credit: Wildlife Forensic
Manual 1992) |
| After 30 hours |
| |
- Color and pupil diameter remain the same
- Brown iris becomes a hazy blue after ~ 48 hours
|
|
| Freezing increases opacity, but halts pupil constriction.
Thus, frozen carcasses may still be evaluated. |
| Forensic Entomology |
 |
|
| Forensic entomology measures post-mortem interval by assessing
the life stages of necrophagous insects. This field has not been applied extensively in
veterinary forensics. |
| Determination of Sex >> |
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Veterinary Forensic Pathology | Time of Death