PMI estimation in the late post mortem period and influencing factors Flashcards
decomposition:
decomposition is not always linear. dependent of intrinsic and extrinsic factors. stages can be extended, paused, overlap, or skipped entirely.
communication and leaching:
communication- physical/ mechanical breakdown of matter. insects, wind, sun, tides, scavengers.
leaching: soluble material removed by water
bloat stage:
uncontrolled activity of internal bacteria.
Build up of gas and pressure in intestines and abdomen: cadaverine, putrescine, ammonia, methane.
greenish discolouration, caused by sulfaemoglobin.
marbling, skin sloughing, blisters in skin.
bacterial putrefaction is main driver of decomposition.
active decay:
maggots hatch and destroy integrity of the body.
bacterial action continues.
gases are released- rupturing- butyric acid, trimethylamine.
smelliest stage
temperature rises due to maggot mass- accelerates bacteria and enzymatic action.
bacteria and insects are main drivers of decomposition.
advanced decay:
gases still released.
decomposition fluids lead into surroundings. drying out remains.
maggots retreat, beetles more active.
bacterial action reduced.
insects and mechanical forces are main drivers of decomposition.
skeletonisation:
decomposition slows down.
gradual exposure of the bones.
some tendons and ligaments still visible.
disarticulation.
scavengers can take away small bones.
main drivers of decomposition are more mechanical- wind, rain, soil, abrasion.
once body becomes skeletonised, PMI estimation is the most difficult.
extrinsic factors: temperature
ambient temperature has the greatest effect on rate of decay.
bodies decay best in warm damp environments.
Hoffs rule of 10- chemical reactions increase in speed up to 3x with every 10C increase in temp- up to max of 40-50C, when enzymes are denatured.
hot- maggots thrive in warm weather. a body can become skeletonised in 2-4 weeks.
cold- maggot action slowed on outside the body, but fine in body cavities (generate own heat) mould can develop.
not necessarily dependent on the weather- hot/cold microclimates can exist (freezer, radiator, sheltered).
extrinsic: access by insects
insects are very important agents of decomposition.
they are attracted by odour- if odour is covered, decomposition may be slow.
blowflies colonise during the fresh and bloat stages, dermestid beetles in the advanced decay stage.
maggots consume the tissues of the body- aid physical destruction.
indoor vs outdoor environments.
extrinsic: moisture/humidity/rainfall.
decomposition requires moisture/humidity to progress.
dry environments slow decomp.
water and rain can help to liquefy the body- speeds up decomp.
can rehydrate mummified remains- kick start bacterial decomp again.
however, water can physically wash away maggots and slow their destruction of the body.
extrinsic: sun/shade.
exposure to sunlight- can warm remains and promote bacterial decay.
insects avoid laying eggs in areas exposed to direct sunlight because eggs are vulnerable to desiccation and UV exposure.
insects lay eggs just under skin or under covering.
exposure to sun can cause bleaching, cracking. weathering of bone.
extrinsic: burial environment
composition- moisture content/drainage, pH, o2, VOCs release, temp, microbial profile.
particle size, depth, coverings, wrappings, coffins.
promote decomp- aerated soil, coarse texture, free draining soil, acidic sand, sites containing sacosaphagous microorganisms (graveyards, manure dumps, cess pit).
slow decomp- poorly drained soils, clay based soil, poor aeration, adipocere formation and preservation of soft tissue.
extrinsic: burial depth
decomp is up to 4x slower at greater depths compared to surface deposition.
temp is lower.
access by insects/scavengers is reduced.
coffins slow decomp.
acidic soils accelerate decomp.
limits release of gases/VOCs.
extrinsic: coffins
physical barrier to insects. except megaselia scalaris that burrow down to >0.5m depth and get through cracks in coffin.
soil detritivores- earthworms, beetles.
cast iron coffins- delays onset of putrefaction by decades (if correctly sealed), soft tissues and hair present after >100 years, adipocere present, clothing preserved in very good condition, preservation of internal organs.
extrinsic: submersion in water
temp is usually more constant, but cooler than air.
insects can only access exposed body parts, once bloat has occurred.
O2 is depleted- aerobic bacteria need O2, cant respire underwater.
decomp rate slows by half in water.
marine and freshwater scavengers continue to contribute to destruction of the body.
chemical composition important- pH salinity.
mechanical destruction due to tides, currents, scavengers.
transportation of bodies and bones.
post mortem submersion interval (PMSI).
extrinsic: hanging above ground
a body left hanging will decompose more slowly.
no dark place underneath the body which is favoured by insects.
maggots may fall off or be washed off by rain.
more difficult for some scavengers to access the remains.
body fluids drain away- gravity.
differential decomp.
extrinsic: scavengers
break up the body, pulling apart ligaments, muscle, damaging bone to get the bone marrow- mechanical decomposition.
different populations based on location- badgers, foxes, coyotes, racoons, rodents.
aquatic scavengers- crustaceans, fish, eels, molluscs.
domestic scavengers- cats eat nose and mouth tissues, dog eat genitals.
size, shape, and position of scavenging marks can also help with identification of scavengers.
extrinsic: dismemberment/fragmentation
started destructive process already.
increase surface area to volume ratio.
more exposed flesh for oviposition.
but some parts may not have gut bacterial populations.
limbs may mummify.
extrinsic: desiccation
browning of the skin/membranes due to drying/exposure to heat or air.
skin retreats, hair and nails do not grow.
desiccation of extremities can happen first- differential decomposition.
body composition and surface area to volume ratio.
drying of the eyes= tache noir.
intrinsic factors: body size
surface area to volume ratio.
large sa:v (small body)- mummification.
small sa:v (larger body)- faster decomp.
fat can act as waterproofing- prevent moisture loss.
very thin people decompose slower- more likely to desiccate/mummify.
infant remains who are yet to be weaned (still breast fed) do not have the same intestinal bacteria/gut microbes- decomp is slower- infant remains mummify.
intrinsic: clothing/covering
slow decomp.
prevents easy access by insects and scavengers and excludes o2.
act as insulation- can increase/maintain high temps for enzyme activity.
depends on the material- synthetic, natural, absorbency, weave.
intrinsic: trauma
wounds allow entry of air and insects into the body- speed up decomposition.
scent of blood attracts insects more quickly.
blowflies less attracted to artificial orifices than natural ones.
severe blood loss from trauma can deprive gut bacteria of food, and can slow bacterial colonisation of the body.
intrinsic: burning
sterilises skin surface and dries underlying tissues- unsuitable for bacterial growth.
destruction of skin allows access to unaffected internal tissues.
reduces likelihood of oviposition.
if temp is high enough, body will be carbonised, and organic matter lost. so in that respect, destruction is accelerated.
limits PMI estimation and identification.
intrinsic: disease
septicaemia (blood infection)- higher amounts of bacteria spread throughout bloodstream- faster decomp.
anaemia- low RBC count- less food for bacteria- slower decomp.
fever- higher body temp just before death- can accelerate decomp.
intrinsic: drugs
certain drugs have been shown to have an effect on decomp rate.
influences insect activity.
alters the physiological conditions at the time of death- raised body temp- impacts PMI estimation.
cocaine in blood stream and tissues- speed up action of insects.
morphine slows decomp- underestimation of PMI by 24 hours is possible.
intrinsic: differential decomp
body parts decomp at different rates.
highly influenced by the deposition environment.
mummification= dry environments, preservation, particularly at the limbs.
adipocere formation= wet environments- preservation.
exposure to scavengers/insects- accelerated changes.
can lead to confusing estimations.
PMI estimation:
PMI estimation is the last post mortem period.
total body score (TBS) and accumulated degree days (ADD): used quantitatively to estimate the PMI.
PMI estimation: total body score.
visual scoring method to record the transformative taphonomic changes. score different sections- head, torso, limbs.
assign each section a score based on features of decomp.
add up the score to get TBS and assign a stage of decomp. 3=fresh. 35=skeletonised/
subjective, need to be adapted for waver environments.
based on unclothed body.
ADD is required- temperature
PMI estimation: accumulated degree days/hours
takes into account the temp- body accuracy of PMI estimation.
sum of daily temp variations at the scene, accumulation of thermal energy over time, reflects cumulative heat exposure to the body.
base temp represents the temp at which these processes stop, anthropologists debate where to put the base temp (4/6/10), the choice of base temp is important as it can vary depending on the region and local environmental conditions.
PMI estimation challenges
TBS is subjective- interobserver reliability, current methods based on animals.
deposition environment- aquatic, terrestrial, buried.
worldwide differences- climate.
population differences- genetics, diet.
PMI estimation: other methods
entomological.
microbial- microbiome, necrobiome.
forensic botany.
forensic geology, pedology, geoscience.
volatile organic compounds (VOCs).
bone diagenesis and biochemistry.
entomology:
insects attend the body in less than an hour- blowflies.
facilitated by VOCs.
life cycles and development- instar.
factors that influence development- temp and access.
microbial:
the microbiome to the necrobiome.
the human necrobiome= microbiome of the human body after death.
post burial interval and post mortem submersion interval.
data from the cadaver and environment.
volatile organic compounds:
detection dogs to locate bodies and graves. characterisation and concentration for PMI estimation
forensic botany and geoscience:
palynology- flowering.
growth rate of plants, disturbed growth, dendrochronology.
bone diagenesis and bone biochemistry:
bone degeneration.
weathering- colour changes, cracks, delamination.
proteomics.
histological changes- collagen degradation, microbial tunnelling, bacterial infiltration, fluorescence.
