How DNA degradation affects wildlife crime investigations
A research-led explanation of why degraded wildlife evidence is not only about recovering DNA, but about interpreting what the remaining DNA can still prove.
Contents
Wildlife forensic evidence is often degraded before it reaches a laboratory. A blood trace may have been exposed to weather for days or weeks. A tissue sample may have dried out. A meat product may have been salted, seasoned, mixed, dried, or heated before anyone asks what species it contains.
By the time the sample is tested, the important question is not only whether DNA can be recovered. It is what the recovered DNA can still support. A result that is useful for species identification may not be strong enough for individual-level comparison. A sample that gives a measurable DNA concentration may still fail to amplify the target marker. A processed product may contain several species, but the strongest DNA signal may come from only one of them.
That is the part of wildlife forensic genetics my ongoing MSc research is concerned with: the point where biological evidence is no longer ideal, and interpretation has to be careful.
The evidence is often already imperfect
In a wildlife crime investigation, biological evidence may not be collected immediately. It may sit on metal, wood, soil, grass, fabric, a vehicle surface, or another substrate while heat, moisture, sunlight, rainfall, microbes, and time act on it. Those conditions can break DNA into smaller fragments, reduce the amount of amplifiable target DNA, and introduce substances that interfere with later testing.
Processed wildlife products create another layer of difficulty. Biltong and dry sausage are not the same type of evidence as a fresh blood trace. Salt, vinegar, spices, drying, mixing, and storage can all affect what is recovered. A single-species whole-muscle product and a mixed-species product can behave very differently, even when both contain animal tissue.
The goal is not only to get DNA. The goal is to understand what forensic conclusion can responsibly be drawn from the DNA that remains.
Species identification and individual comparison are different questions
A common mistake is to treat any DNA result as if it answers the same kind of identity question. In wildlife forensics, species identification and individual-level comparison are related, but they are not interchangeable.
Species identification asks which animal species the biological material most likely came from. In my research, mitochondrial COI barcoding is used for that species-level question. Mitochondrial DNA occurs in many copies per cell, which can make it more robust when a sample is old, trace-level, exposed, or processed.
Individual-level comparison asks whether a trace can be associated with a particular animal. That requires nuclear markers such as microsatellites. These markers can provide more individual-level information, but they are usually more vulnerable when DNA is degraded or present in low amounts. As samples age, locus recovery and allele recovery can decline, which weakens the strength of the comparison.
This is why a species result can remain useful even after the nuclear profile becomes partial or uninformative. The two methods answer different forensic questions, and the interpretation has to respect that difference.
What my MSc work examines
My ongoing MSc research uses blesbok as a model species to test how wildlife DNA behaves under less-than-ideal conditions. One part examines blood deposited on metal, wood, rubberized car mat material, cotton, soil, and grass. The samples were exposed indoors, in a semi-covered outdoor setting, and under fully exposed outdoor conditions.
That design allows the work to ask how substrate type, exposure, and time affect species identification and individual-level comparison. Metal, wood, and car mat samples produced blesbok identification early in the study across conditions, and indoor protection helped target DNA remain useful for longer. Cotton did not produce a species-specific blesbok result, while soil and grass produced non-target species.
Another part of the work looks at processed products. Blesbok biltong, a single-species whole-muscle product, remained identifiable after seven months using DNA barcoding. Mixed dry sausage was more difficult. Sequences suitable for species identification were obtained, but standard DNA barcoding detected cattle, Bos taurus, rather than blesbok.
That result does not prove blesbok was absent from the whole product. Mixed products are uneven. One portion may not represent the entire sausage, and the dominant species may amplify more strongly than a minor contributor. Mixture heterogeneity, dominant-template amplification, and preferential amplification all matter.
Where qPCR fits in the work
The qPCR part of my MSc work should be understood as targeted assay development. It was explored to detect blesbok DNA specifically within processed or mixed samples, especially where standard barcoding may miss a minor contributor.
That work sat apart from the extract assessment. DNA concentration and purity were assessed separately using a NanoDrop spectrophotometer. COI barcoding was used for species identification. Microsatellite markers were used for individual-level comparison. qPCR and multiplex qPCR were explored for species-specific detection and targeted amplification.
The primer screening step was successful, but the probe component had limitations. The probes were too short and lacked sufficient binding stability, which prevented successful probe-based detection and full multiplex validation. That matters because a targeted assay cannot be treated as a forensic solution until primer specificity, probe behaviour, sensitivity, controls, and validation are strong enough to support the intended conclusion.
Why interpretation matters
DNA degradation does not automatically make a sample useless, but it changes what can be said from the result. A degraded blood trace may still support species identification. It may not support individualization. A processed product may yield DNA, but the result may reflect the dominant species rather than every species present in the mixture.
This is why wildlife forensic genetics cannot stop at a positive or negative result. The method, marker, sample condition, substrate, exposure history, processing, mixture composition, and reference data all shape the conclusion.
A careful result is not weaker because it names its limits. It is stronger because it says exactly what the science can support and what it cannot.