PMI estimation and influencing factors Flashcards
what is late stage decomposition
the other four stages after the fresh stage - bloat, active decay, advanced decay and skeletonisation
name 5 things that can occur during the bloat stage of decomposition
- Build-up of gas and pressure in intestines and abdomen
- Greenish discolouration, caused by sulfhaemoglobin
- Marbling
- Skin sloughing - skin coming away from the body
- Blisters in skin
what 5 things occur during active decay of decomposition
- Maggots hatch and destroy integrity of the body
- Bacterial action continues
- Gases are released – rupturing
- Temperature rises due to maggot mass – accelerates bacterial and enzymatic action
- Bacteria and insects are main drivers of decomposition
what 5 things occur during the advanced decay stage of decomposition
- Gases still released
- Decomposition fluids leach into surroundings
- Drying out of remains
- Maggots retreat, beetles more active
- Insects and mechanical forces are main drivers of decomposition - Bacterial action reduced
what 5 things occur during the skeletonisation stage of decomposition
- 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 etc.
name the 12 main extrinsic factors that influence the rate of decomposition
- Temperature
- Access by insects
- Moisture/ humidity/ rainfall
- Sun/shade
- Burial environment
- Burial depth
- Coffins
- Submersion in water
- hanging above ground
- Scavengers
- Dismemberment/ fragmentation
- Desiccation - ‘drying’
what is the difference in decomposition is hot/warm weather and cold weather
hot/ warm - maggots and flies thrive and a body can be skeletonised in 2-4 weeks
cold weather - maggot action is slowed outside the body but fine in body cavities and mould can develop
explain the access by insects extrinsic factor
insects are attracted by the odour/VOC’s produced by a dead body - if this odour is hidden in some way decomposition may be slowed
explain how moisture affects decomposition
water and rain can help to liquefy the body and increase the speed of decomposition, but can also rehydrate mummified remains and wash away maggots to slow down decomposition
explain how sun/ shade affects the rate of decomposition
exposure to sunlight can warm remains and promote bacterial decay, however insects avoid laying eggs in areas exposed to direct sunlight so a bit of both is preferred for optimum decay
what kind of burial environment will promote decomposition
well aerated, coarse textured, free draining soils, or acidic sands/ gravels
how can burial depth affect the rate of decomposition
decomposition is up to 4x slower at greater depths as the temp is lower, less insect access and limited release of gases
how can coffins affect the rate of decomposition
they act as a physical barrier to insects, therefore slowing the process down
how can submersion in water affect the rate of decomposition
insects can only access the exposed body parts, bacterial activity declines as they require oxygen to respire, and overall decomposition rate slows in half
how can hanging above ground affect the rate of decomposition
it will slow the rate of decomposition because insects favour the shaded area underneath a body, but a hanging one won’t have any, as well as they might fall off the body
how can scavengers affect the rate of decomposition
different populations based on location will cause different amount of decomposition e.g. coyotes will do more damage than badgers
what do scavengers do regarding decomposition
mechanical decomposition - break up the body, pulling apart ligaments, muscle, damaging bone to get to the bone marrow
how can we identify what scavengers were present at a scene
size, shape and positioning of scavenging marks e.g. canines eat at the ends of bone with puncture looking marks
how can dismemberment/ fragmentation affect the rate of decomposition
it increases the surface area to volume ratio of exposed flesh to insects and scavengers therefore increasing the rate of decomposition
name the 6 intrinsic factors that affect the rate of decomposition
- Body size
- Clothing/ covering
- Trauma
- Burning
- Disease
- Drugs
explain how different body sizes will affect the rate of decomposition
changes in the surface area to volume ratio can affect decomposition massively - Large SA:V ratio = mummification, whereas small SA:V ratio = faster decomposition - due to the fat preventing moisture loss, therefore increase in decomposition
how can clothing/ coverings affect the rate of decomposition
they can slow decomposition due to it; preventing easy access by insects, excluding oxygen and preventing access for scavengers
however it can also increase decomposition as the covering acts as insulation which can maintain higher temps for enzyme activity
how can trauma affect the rate of decomposition
wounds allow entry of air and insects into the body which speeds up decomposition, but severe blood loss can deprive gut bacteria of food and therefore slow bacterial colonisation of the body
how can burning affect the rate of decomposition
sterilises skin surface and dries underlying tissues making them unsuitable for bacterial growth, however destruction to skin exposes unaffected internal tissues which are more likely to attract insects or scavengers
how can disease affect the rate of decomposition (use examples)
septicaemia (blood infection) - increases bacteria in bloodstream = faster decomposition
anaemia (low red blood cell counts) - less food for bacteria = slower decomposition
fever (higher body temp just before death) - warmer climate for bacterial activity = faster decomposition
how can drugs affect the rate of decomposition
they can influence insect activity, and alter the physiological conditions at the time of death e.g. body temp
what factors accelerate decay
unrestricted oxygen
warm temperatures
humid atmosphere
presence of insects
wounds permitting access to internal body tissues
obesity
blood infection
etc
what factors decelerate decay
depleted oxygen
cold temperatures
dry atmosphere
absence of insects
intact body
small/ thin body
submersion in water
what is differential decomposition and when does it happen
where body parts decompose at different rates - occurs when the deposition environment is extreme e.g. in water
what is PMI estimation in the late post-mortem period depended on
accumulated temperature
what is the total body score (TBS) and how do we use it for PMI
a visual scoring method to record the transformative taphonomic changes of a body during decay
we score different sections (head, torso, limbs) a number and add them up to get a TBS - min is 3 (fresh stage), max is 35 (skeletonised)
what does ADD/H stand for in regards to decomposition and what does it mean
accumulated degree days/ hours - a tool used to estimate PMI whilst taking into account temperature
how is ADD calculated
○ Sum of daily temperature variations at the crime scene or recovery site.
○ Accumulation of thermal energy (degrees Celsius) over time (days)
○ Reflects cumulative heat exposure to the body.
○ Base temperature needs to be selected – it’s not always 0°C, as it should represent the temperature at which these processes stop
not including ADD, what are the other 5 methods for late PMI estimation
- Entomological
- Microbial
- Forensic botany/ geology/ pedology/ geoscience
- Volatile Organic compounds
- Bone diagenesis and biochemistry
what is the human necrobiome
the microbiome of the human body after death
