Distinguishing tracks of mink Mustela vison and polecat M. putorius

1. What they did

American mink are an introduced pest in the United Kingdom (UK) and mainland Europe. They are monitored and trapped using floating rafts fitted with clay tracking plates: when mink tracks appear on a raft, a trap is set there. Mink tracks are easily told from stoat and otter tracks by size, but polecats are a similar size, and their tracks are hard to tell from mink tracks. Polecats are also found on mink rafts, and in the UK they are protected, so trapping them on purpose is prohibited. The authors set out to find a reliable, measurement-based way to tell the two species’ tracks apart.

• Collecting tracks (2005). Wild mink and polecats trapped in the Upper Thames valley, Oxfordshire, were released into a wooden tunnel (0.2 × 0.2 × 2.4 m) over a layer of moist clay, built to imitate a tracking raft but long enough to record several tracks from each animal. A temporary obstruction at the far end encouraged the animals to walk rather than run. All animals were adults.
• Photographing and measuring. Each footprint was photographed with a digital camera beside a ruler. Overlapping tracks, tracks with fewer than three clear toe prints and tracks with poorly defined pads were left out (under 10%). Front and hind, left and right feet were all included and not told apart, so that the method could be used in the field without having to identify which foot made the track. Tracks were measured on screen in ArcMap, a Geographic Information System (GIS) program, to 0.1 mm: 15 measurements of the track and the spacing between its pads, plus eight ratios between them, because the authors expected differences in shape rather than size. They also recorded whether claw marks were present and their shape.
• Checking the measurements. Six measurements were repeated five times on five mink tracks to see how much the same measurement varied.
• Building and testing a formula. Tracks from five mink and three polecats were set aside as a test set. The remaining tracks were used to choose the most useful measurements and build a discriminant function (a formula that sorts a track into one species or the other), which was then tried on the test set.

2. What they found

• They collected 56 tracks from 12 mink and 63 tracks from nine polecats. Mean weights: male mink 1,460 g, female mink 840 g, male polecats 1,510 g, female polecats 1,030 g.
• Measurements were repeatable: measurement error was between 1.11% and 5.79%.
• Size alone does not work. Male mink tracks were wider than those of polecats and of female mink, but the ranges overlapped. Males of both species had longer tracks than females, while same-sex mink and polecats had similar track lengths.
• Claw marks: claw marks showed in 74% of mink tracks and 98% of polecat tracks. “Crooked” claw marks, described in earlier work as typical of polecats, were more common in polecat tracks (37%) than mink tracks (3%), but 63% of polecat claw marks looked straight, so claw shape was not a reliable guide.
• Rules of thumb: where long claws show, a distance of more than 6 mm from the top of the middle toe pad to the tip of its claw mark means a polecat; below 6 mm it could be either. A gap of more than 4 mm between the two middle toe pads (d1) suggests mink; below 4 mm it could be either.
• The formula: six measurements together classified 98.6% of tracks correctly, and 100% of the test tracks. The simplest model used only three: overall width across the toes (W1), the gap between the two middle toe pads (d1), and the mean gap between each outer toe and its neighbouring inner toe (d7 and d8). It classified 97.3% correctly, and 93.9% of the test tracks.
• The rule: if 1.70 − 0.75(W1) + 2.43(d1) + 1.85(mean of d7 and d8) is above 0, the track is classed as mink; below 0, polecat.
• What it looks like: mink toe pads are more widely spaced, especially the two middle ones, and the outer toes sit so that mink tracks look much wider for their length than polecat tracks.

3. What they concluded

• Three simple measurements can separate mink and polecat tracks on clay tracking plates with high success.
• Clear, sharp tracks are essential. Where polecats are known or suspected, the authors recommend renewing and smoothing the clay often.
• Measurements can be taken in the field with calipers, or from photographs with a ruler, measured later with calipers on prints or with software such as ArcGIS or Photoshop.
• Limits: the formula was built from adult animals walking on clay. The authors do not know how well it works on natural ground or with running animals, and it cannot be used for juveniles (a potential problem only in summer). Further work is needed to include juveniles.
• Being able to recognise polecat tracks suggests terrestrial tracking plates could be developed to survey and monitor polecats, whose current survey methods are limited or very labour intensive. This warrants further attention.

4. How tracking was or was not used

The study is about tracks and signs identification: telling mink and polecat footprints apart on clay. The authors measured toe pad spacing, track width and length, and claw marks, tested an earlier qualitative description of claw shape (Sidorovich 1999), and turned the most useful measurements into a formula and two field rules of thumb. No trailing was involved; the species of each track was known because the animals had been trapped.

5. Tracker or observer certification

Not reported. The measurements were taken from photographs in software, and their repeatability was checked statistically (measurement error 1.11% to 5.79%), but the paper does not say who measured the tracks or what tracking experience they had.

6. Other technologies, skills or innovations

• A long clay-lined tunnel that imitates a mink tracking raft, with a temporary obstruction so animals walk, giving several tracks from each known animal.
• Digital photographs with a ruler, measured to 0.1 mm in ArcMap (ArcGIS 9).
• A separate test dataset from different animals, to check the formula fairly.

7. Implications for conservation (from the paper)

• Mink control on rafts depends on correctly identifying mink tracks. Recognising polecat tracks helps avoid trapping polecats, which are protected in the UK, and saves time and effort when starting a mink control programme.
• Tracking plates might become a survey and monitoring tool for polecats.