Semen, Saliva and Other Body Fluids as Evidence

Semen is a composite fluid consisting of spermatozoa suspended in seminal plasma. The plasma is contributed by the seminal vesicles (approximately 65% of volume), the prostate gland (approximately 25%), and the bulbourethral glands and epididymides (the remainder). Each component contributes distinct biochemical markers. The seminal vesicles secrete fructose, the primary energy substrate for sperm motility. The prostate secretes PSA, acid phosphatase, and zinc. These markers are the targets of presumptive and confirmatory tests.

A mature spermatozoon has three parts. The head contains the haploid nucleus with 23 chromosomes, covered anteriorly by the acrosome, a membrane-bound organelle containing hydrolytic enzymes needed for fertilisation. The midpiece connects the head to the flagellum and is packed with mitochondria that power the beating motion of the tail. The tail is a flagellum approximately 50 micrometres long. The head is roughly 4.5 by 3 micrometres, making spermatozoa visible and morphologically distinctive under light microscopy.

In casework, semen is first detected by ultraviolet or alternate light source examination, which causes seminal stains to fluoresce. A presumptive test for acid phosphatase follows: a colour reaction produced on a filter paper pressed against the stain. Confirmation uses the p30 (PSA) immunological assay. Microscopic examination searches for spermatozoa using haematoxylin-based stains such as Christmas tree stain, which colours sperm heads red and tails green. For DNA profiling, differential extraction separates the sperm fraction from any co-deposited non-sperm cells, producing separate contributor profiles. Detailed serological confirmation methods are covered in the Forensic Serology subject.

Saliva is produced by three paired major salivary glands: the parotid, submandibular, and sublingual. It also contains minor gland secretions from the oral mucosa. Whole saliva is approximately 99% water. The remaining 1% includes mucins, immunoglobulins, electrolytes, and enzymes, the most forensically important of which is salivary amylase (alpha-amylase, EC 3.2.1.1). Amylase is present in saliva at concentrations approximately 40-fold higher than in serum and approximately 1000-fold higher than in vaginal secretions or sweat, making it a useful presumptive marker.

Saliva also contains buccal epithelial cells shed from the oral mucosa at a rate of approximately 10,000 cells per millilitre. These cells carry nuclear DNA and are the primary DNA source in saliva evidence. Because the cells are already shed and relatively unprotected, DNA from dried saliva stains degrades faster than DNA from semen stains (where the sperm head provides a protective structure). A saliva stain on a porous surface such as paper or cloth can yield a profile after months of storage in dry, cool conditions, but the same stain exposed to heat and humidity may be unrecoverable within days.

Crime scene locations where saliva evidence is commonly encountered include bite marks on skin or food, cigarette ends, drinking vessels, envelopes and postage stamps, ski masks, balaclavas, and any surface that has been licked. Bite mark evidence requires careful documentation of the mark geometry before swabbing, as swabbing destroys the surface. The double-swab technique (wet swab followed by dry swab over the same area) maximises cellular recovery from skin surfaces.

Vaginal secretions are a complex mixture originating from multiple sources: transudation from the vaginal wall, secretions from Bartholin and Skene glands, cervical mucus, and desquamated vaginal epithelial cells. No single high-specificity biochemical marker equivalent to amylase or PSA has been established for routine casework. Research has focused on human beta-defensin 1, CD45 (a leucocyte marker), and, more recently, microRNA expression profiles characteristic of vaginal epithelial cells. None of these is yet standardised in routine practice. In sexual assault casework, vaginal secretions are inferred by context (swabs from a vaginal area of a complainant's garment or from a penile swab) rather than confirmed by a single positive test.

Sweat is secreted by eccrine glands distributed across the body surface and by apocrine glands concentrated in the axilla and groin. The fluid itself is primarily water with electrolytes and trace organic compounds. Sweat contains almost no nucleated cells, but it carries shed corneocytes from the stratum corneum. These anucleate cells do not contribute DNA, but nucleated keratinocytes from deeper layers are also deposited at low levels through physical contact. The result is touch DNA, covered in more detail in the Touch DNA and Trace Biological Material topic.

Urine is filtered plasma with added secretions from renal tubular cells. Its primary forensic value is as evidence of presence at a location rather than as a source of identity. Urothelial cells shed into the urine carry nuclear DNA, and it is possible to obtain a STR profile from urine-stained substrates, though the cellular yield is low and variable. Urine is detected in casework by creatinine testing or by chemical spot tests for urinary components, but these are not highly specific.

Correct collection begins with scene documentation: photograph the stain in situ with and without a scale before any physical interaction. The stain's position, size, shape, and relationship to the scene are recorded because these may be relevant to reconstructing events. Only after documentation should collection proceed.

Wet stains require air drying before packaging. A stain collected wet and sealed in a plastic bag will support bacterial and fungal growth within hours, degrading cellular material and fragmenting DNA. The preferred sequence is: allow drying in a secure location (or use a portable drying box with filtered airflow), then package in paper. Plastic packaging is only acceptable for dry stains and only when paper is unavailable, as moisture trapped in plastic accelerates degradation. Items large enough to be packaged whole, such as clothing, should be packaged whole where possible rather than cutting out the stain, because adjacent areas may carry additional evidence.

Small liquid samples, such as vaginal swabs collected at clinical examination, are placed in swab transport tubes and refrigerated. They should not be frozen and thawed repeatedly because freeze-thaw cycling lyses cells and fragments DNA. Samples intended for long-term storage are best stored frozen at minus 20 degrees Celsius or below after drying.

Biological evidence degrades through four main pathways: enzymatic autolysis (endogenous enzymes within cells break down nucleic acids and proteins after cell death), microbial activity (bacteria and fungi produce nucleases, proteases, and lipases that attack biological material), photolysis (ultraviolet light cleaves nucleotide bases and cross-links DNA strands), and physical abrasion (repeated handling, washing, or exposure to water dilutes and fragments cellular material).

Temperature and humidity are the dominant environmental variables. At room temperature in dry conditions, DNA in bloodstains, semen stains, and saliva stains can remain amplifiable for years. At 37 degrees Celsius in humid conditions, the same stains may lose amplifiable DNA within 72 hours. Field conditions in tropical and subtropical climates, including much of South Asia and sub-Saharan Africa, present a genuine challenge: evidence must reach controlled storage within hours rather than days if DNA recovery is a priority.

Washing degrades body fluid evidence dramatically. A single machine wash at 40 degrees Celsius reduces cellular content in semen stains by approximately 90%, though spermatozoa embedded in fabric fibres can survive multiple washes because the fibre protects them from mechanical disruption. Saliva stains on clothing frequently survive one gentle wash at low temperature, detectable by amylase testing, but DNA recovery is reduced. This is why items of clothing collected from complainants should be air-dried, not washed, before submission.

The concept of biological degradation ties directly to the broader principles of DNA replication and mutation covered in the DNA Replication and Mutation topic. Understanding what DNA looks like when intact helps the analyst recognise the signature of degraded material: short fragment lengths concentrated at the low end of the electropherogram, high drop-out rates, and allelic imbalance in heterozygous positions.

Body fluid evidence does not stand alone. It is the starting point for several analytical pipelines, each with its own subject area. Serological fluid identification methods belong to Forensic Serology, which covers immunological and biochemical tests in depth. Once a fluid is identified, DNA profiling connects to the chromosomes, genome organisation, and STR marker systems. Hair evidence connects to Hair Anatomy and Growth Cycle within this subject. Bone and tooth evidence connects to Forensic Anthropology, which covers hard tissue DNA extraction from degraded material.

Insect evidence collected alongside body fluids, such as blowfly larvae feeding on a decomposing body, can be submitted for species identification and post-mortem interval estimation. That pipeline belongs to forensic entomology. Plant material or pollen deposited alongside biological stains can be submitted for botanical analysis. These cross-disciplinary connections make the scene examiner's role critical: collecting only the obvious liquid stain and ignoring adjacent biological material misses evidence that could be independently valuable.

Data protection law intersects with body fluid evidence wherever biological profiles are stored on databases. In the United Kingdom, the National DNA Database operates under the Protection of Freedoms Act 2012 and the Forensic Science Regulator's codes. In the European Union, the Prüm Convention framework governs cross-border DNA profile exchange. In India, the DNA Technology (Use and Application) Regulation Bill, pending as of 2026, proposes a national DNA databank with specified retention categories. In the United States, CODIS is governed by the DNA Identification Act and regulations at 34 U.S.C. 12592. Anyone working with body fluid DNA profiles must understand the legal framework governing storage, matching, and deletion in their jurisdiction.