Identification of mustelid species: otter (Lutra lutra), American mink (Mustela vison) and polecat (Mustela putorius), by analysis of DNA from faecal samples

1. What they did

Otters, American mink and polecats can all live along rivers and lakes in Europe, and all may eat fish. Surveys of their abundance and diet often rely on finding and analysing their scats. Otter spraints usually have a sweet smell and a distinctive look, but when mink and otter scats contain the same food (mostly fish), telling them apart can be uncertain, and mink and polecat scats may be hard or impossible to tell apart. The authors set out to develop a DNA test that tells the scats of these three species apart.

• Designing the test. They compared a short section of DNA (part of the mitochondrial cytochrome b gene) from the three mustelids and from common prey (fish, mice, frogs and birds). They designed primers (short starting pieces of DNA) that copy this section from the mustelids but not from the prey remains in the scat. Two restriction enzymes then cut the copied DNA at points that differ between the species, giving each species its own pattern of fragment sizes.
• Testing on known animals. They checked the test on muscle tissue from an otter killed by traffic and from a mink and a polecat shot by hunters.
• Testing on scats. They analysed 9 scats from various places in Northern Jutland, Denmark, which had been classed by smell and appearance as mink or polecat (not otter), and 14 scats collected along about 10 km of the River Trend. A few scats thought to be from red fox, and some of unknown but possibly mustelid origin, were also tested. DNA was extracted twice from 8 samples to check that the results could be repeated.

2. What they found

• The test worked on tissue from all three species, giving the expected fragment patterns.
• DNA was copied successfully from all 23 mustelid scats. No DNA was copied from the red fox scats or from the scats of unknown origin, and there was no sign of DNA from prey being copied.
• Repeat extractions from the same scat always gave the same result.
• Of the 9 scats classed by smell and appearance as mink or polecat, DNA showed 3 were mink, 5 were polecat and 1 was otter. On reinspection, the otter sample did have the smell and texture typical of otter spraint.
• Of the 14 River Trend scats, 13 were otter and 1 was mink, confirming that mink were present in the area.
• The amount of DNA copied varied between scats and was sometimes clearly lower than from tissue.

3. What they concluded

• DNA analysis of scats is useful for telling these mustelid species apart. It worked well, the results were repeatable, and every pattern matched one of the three species.
• Problems might arise only in extreme cases, for example if an otter had eaten a mink, when DNA from both species could be copied.
• Correct identification of scats is essential when scats are used to monitor distribution and abundance. This matters even more for an endangered species such as the otter: missing an otter sign could mean protection measures are not taken.
• DNA testing avoids subjective judgement of smell and appearance. The misidentified otter spraint, the separation of mink from polecat, and the confirmation of mink on the River Trend show its value where accurate identification is needed.
• In future, DNA from scats could also identify individual otters, supporting studies of home range, migratory behaviour, family relationships and population size, alongside radio-tracking and traditional surveys based on spraints and footprints.

4. How tracking was or was not used

Scats were found and collected in the field, and 9 of them were first identified by smell and physical appearance as mink or polecat scats. The DNA test was then used to check these identifications. The paper does not describe how the scats were searched for. The River Trend scats were collected in an area where otters were known from observations and earlier surveys.

5. Tracker or observer certification

Not reported. The paper does not say who collected or first classified the scats; the acknowledgements thank A. B. Madsen, C. Pertoldi and V. Reitz for supplying tissue and faecal samples. This matters because one of the 9 field identifications (an otter spraint classed as mink or polecat) was wrong.

6. Other technologies, skills or innovations

• Faecal DNA extracted with the method of Gerloff et al. (1995), based on guanidine thiocyanate and diatomaceous particles; other methods tried did not give usable DNA.
• Polymerase chain reaction (PCR) with new mustelid-specific primers designed not to copy DNA from common prey.
• Restriction enzymes Taq I and Nla IV, with fragments read on an automated DNA sequencer, so one test separates all three species.

7. Implications for conservation (from the paper)

• Scats are widely used to monitor the endangered otter in conservation programmes, and wrong identifications could lead to protection measures being taken or missed.
• Telling mink scats apart reliably helps assess the abundance and environmental impact of the American mink, which has spread in Europe after escaping from fur farms.
• Scats offer an easy, non-invasive way to sample many otters, where tissue samples have so far come only from animals killed by traffic or drowned in fishing nets. Molecular scatology could add substantially to the scientific basis for otter conservation.