Edexcel A Level (IAL) Biology -6.20 Types of Data Provided by Forensic Analysis- Study Notes- New Syllabus

Determining Time of Death in Mammals

🌱 Introduction

Estimating the time since death (post-mortem interval, PMI) is crucial in forensic science. Indicators include decomposition, succession of organisms, insects, body temperature, and muscle contraction.

1. Extent of Decomposition

• Decomposition occurs in stages as tissues break down:
  • Fresh: No obvious signs; body appears normal.
  • Bloating: Gas accumulates from bacterial activity.
  • Active decay: Tissues break down; strong odor.
  • Advanced decay: Most soft tissue gone; bones may appear.
  • Skeletonisation: Only bones remain.
• Stage gives rough PMI estimate; environment affects rate (temperature, humidity, exposure).

2. Stage of Succession

• Microorganisms and invertebrates colonize in predictable order:
  • Early colonizers: Flies, maggots.
  • Later colonizers: Beetles, other decomposers.
• Species present helps estimate time since death.

3. Forensic Entomology

• Study of insects on the body.
• Blowflies are often first; larval stage indicates time since egg laying.
• Combining insect life cycle and environmental temperature gives PMI estimate.

4. Body Temperature (Algor Mortis)

• Body cools to match environment after death.
• Approximate: ~1–1.5°C lost per hour under normal conditions.
• Accuracy improves when combined with other indicators.
• Factors: clothing, ambient temperature, body size, wind, humidity.

5. Degree of Muscle Contraction

• Rigor mortis: stiffening of muscles post-death.
• Appears 2–6 h, peaks ~12 h, disappears 36–48 h.
• Useful for PMI in first 1–2 days.

📋 Summary Table