Individual Identification of Raccoons (Procyon lotor) Using Track Plate Foot Printing

1. What they did

Counting animals is central to managing and conserving them, but many survey methods are costly, slow, biased or risky for the animals. Genetic sampling can identify individuals but is often expensive and time consuming. Earlier studies showed that fishers can be identified as individuals from the fine ridge patterns (papillae) on their foot pads, recorded on track plates. The authors tested whether the same method works for raccoons, and whether it can be used to estimate how many raccoons live in an area.

• Collecting prints. They set out 27 track plate stations, about 300 m apart, in a 225 ha rural reserve in Barry County, Michigan, from 18 June to 17 July 2014. Each station was a triangular plastic enclosure, baited with peanut butter at the back. Raccoons walked over a strip of copy toner and then onto sticky shelf liner, which kept a print of their feet. Stations were checked every other day, and prints were photographed with a macro lens.
• Reading the prints at three scales. At the coarse scale, the size and shape of each print were used to remove other species and to tell which foot made it. At the intermediate scale, scars and creases on the pads were used to tell individuals apart. At the fine scale, the papillae patterns were compared on screen in ImageJ, by laying one image over another and flipping quickly between them. Two evaluators compared all prints independently, without knowing each other’s results.
• Measuring how unique each print is. They measured the distances between 10 pairs of neighbouring papillae on each print and calculated the probability of identity (PID): the chance that two different raccoons would share the same pattern.
• Estimating numbers. They counted the minimum number of raccoons known to be alive (MNKA) and treated the identified prints as captures and recaptures in the program Capture. Back left and back right feet were analysed separately.

2. What they found

Prints collected

• 159 sheets with raccoon prints over 810 trap days. 74 sheets were usable, with 35 back left, 32 back right, 50 front left and 51 front right prints.
• Heavy rain ruined 15% of sheets, wet feet 13% of prints, and debris or overlapping prints another 9%.
• Slugs ruined 45% of prints in the first 5 days. Sprinkling diatomaceous earth around the enclosures cut this to 16% for the rest of the study.
• Front feet were not useful: they lacked scars and creases, and their papillae were often too fine to tell individuals apart.

Where raccoons were active

Raccoon activity was uneven. Most sheets came from forest and areas of human use (an average of 5.5 sheets per enclosure), then fields (3.5), with the fewest in prairie and wetland (1).

Individuals and numbers

• Most raccoons were recorded only once.
• Intermediate scale (scars and creases): at least 15 raccoons from back left feet and 12 from back right feet.
• Fine scale (papillae): at least 17 raccoons from back left feet and 16 from back right feet. The fine scale found more individuals because not all raccoons had distinctive scars.
• Program Capture estimates: 38 raccoons (95% confidence interval 17 to 108) from back left and 34 (21 to 93) from back right feet at the intermediate scale; 36 (18 to 122) from back left and 30 (15 to 103) from back right feet at the fine scale.
• The chance that two raccoons share the same papillae pattern was very small: from 5.72 × 10⁻⁹ to 6.71 × 10⁻¹² for back right feet, and from 3.34 × 10⁻⁸ to 3.55 × 10⁻¹⁰ for back left feet. The Discussion gives the highest value as 5.72 × 10⁻⁸, about one in 17 million raccoons, which does not match the Results.
• The authors put raccoon density at 6.1 to 16.9 raccoons per km², within the range of 4.7 to 19.1 per km² reported for other rural areas.

3. What they concluded

• Track plate footprinting is a workable, non-invasive way to identify individual raccoons, and is likely to be useful in mark-recapture studies if stations are left out for longer.
• Their confidence intervals were too wide to give biologically useful population estimates, probably because the short season gave few recaptures. A longer season would narrow them.
• If a PID is not considered small enough, more interpapillae distances can be measured.
• Suggested improvements: spread diatomaceous earth in front of enclosures against slugs, extend the roof beyond the entrance, and fully seal all seams and the back.
• Combined with bait stations, the method could show which raccoons are taking oral rabies vaccines, and when the 60% vaccination level thought necessary is reached.
• Since it now works for fishers and raccoons, it is likely to work for other mesocarnivores (medium-sized carnivores), but only for animals with large enough pads, as the smaller front feet of raccoons showed.

4. How tracking was or was not used

The whole study rests on reading footprints. Tracks and signs identification was used at the coarse scale to remove other species and to tell which foot made each print, from its size, shape and the position of the first toe. Individual raccoons were then identified from their prints: first from scars and creases on the pads, then from the fine papillae patterns. No trailing was reported; prints were collected on track plates, not from natural ground.

5. Tracker or observer certification

Not reported. Two evaluators compared all prints, working blind, but the paper does not say who they were or how experienced they were. Their agreement is not reported either.

6. Other technologies, skills or innovations

• Baited track plate enclosures made from corrugated plastic (coroplast), with aluminium plates, copy toner as the ink and sticky shelf liner as the print surface.
• Diatomaceous earth around the enclosures to keep slugs out.
• Prints photographed with a digital camera and a 50 mm macro lens, then compared and measured in the free image software ImageJ.
• A Global Positioning System (GPS) unit to place the enclosures.

7. Implications for conservation (from the paper)

• A simple, cheap and non-invasive way to identify individuals can support population studies without the costs of genetic sampling or the risks of live trapping.
• Raccoons can be invasive and spread diseases such as rabies and raccoon roundworm. Better counts can support management, since infection rates often depend on density.
• The method could help rabies vaccination programmes check how many raccoons are taking up vaccine baits, freeing effort for other areas.