The Use of Fluorescent Powdered Pigments as a Tracking Technique for Snakes

1. What they did

Radio transmitters and thread trailing (a spool of thread attached to the animal, which leaves a line as it moves) have both been used to follow snakes, but both have drawbacks: the weight and bulk of the device, surgery and anaesthetic for implanted transmitters, and thread spools that snag in burrows and dense plants. Fluorescent powder, first developed for small mammals and since used on frogs, toads, salamanders, turtles and lizards, had not been reported for snakes. The authors tested whether it works for garter snakes.

• Catching the snakes. In May and June 2009 they caught 19 garter snakes of three species by hand in ravine habitat in Edmonton, Alberta, Canada. They measured and weighed each one and marked them individually with a cautery iron. Most were held an hour or less before release (range 20 minutes to five days).
• Powdering. Each snake was gently tipped about in a plastic bag holding about 2.5 cm of orange or green fluorescent powder, with the head kept out of the bag so no powder reached the eyes or mouth. The colour was chosen so it would not be confused with other snakes released that day or with older trails.
• Release. Snakes were released within 5 m of where they were caught, between late morning and early afternoon, when it was warmer than 15 °C.
• Following the trails. About 36 hours later, from around 23:00, they followed each trail with an ultraviolet (UV) lamp powered by a portable generator on a 100 m extension cord. They flagged the release point and every change in direction greater than 10°, and followed until the powder ran out or the snake entered water or went underground.
• Measuring. The next day they measured the direction and distance between flags (the “actual distance”) and the straight-line distance from release point to the last powder, then rebuilt each path in ArcGIS, a Geographic Information System (GIS).
• Analysis. They tested whether bigger snakes left longer trails, and whether the first six turns after release were more winding than the next six and than later sections of the longest trails.

2. What they found

• Trails started as distinct lines of powder, then broke into patches on plants, and finally into small flecks.
• Trail lengths varied widely: 3.08 to 203.4 m of actual path and 1.28 to 120.4 m in a straight line. Some individuals were tracked for more than 200 m.
• Some trails ended early when snakes entered water (4 snakes), went into mammal burrows (2) or shed their skin (2). The authors do not know whether the powder could have triggered shedding.
• Heavier snakes left longer trails. Seven small snakes under 10 g, thought to be young of the year, left short trails (mean 11.9 m of actual path), never longer than 30.2 m. The 13 larger snakes averaged 56.1 m, and their trails ranged from 11.5 to 203.4 m.
• Snakes moved in a more winding way just after release. The ratio of actual to straight-line distance was about twice as high in the first six turns as in the next six, so the paths then became straighter. Along the four longest trails, the later sections did not differ significantly from each other: once the snakes had settled, their paths stayed fairly straight.

3. What they concluded

• Fluorescent powder is a useful, non-invasive, low-cost way to study how garter snakes move and orient themselves in relation to habitat features, especially for short-term, small-scale habitat use. It worked well for snakes over 10 g.
• The first six or so turns (roughly the first 7 m) after release may reflect handling, disorientation or attempts to rub off the powder, and should be treated with care in future studies.
• Unlike radio telemetry, the powder records the exact path and the habitat features the snake actually used, without the observer needing to stay in sight of the snake. Unlike thread, it does not snag, is hard to disturb, and weighs very little (under 0.5 g).
• Smaller snakes leave shorter trails, probably because they carry less powder. Garter snakes have keeled (rough) scales that may hold powder well, so the method may work less well on smooth-scaled snakes (the paper gives Pythonidae as an example).
• Shallow water usually washed the powder off and ended 20% of the trails, and rain can make trails much harder to see. Because 10% of the trails ended in tunnels or burrows, the method may help find hibernacula.
• The authors recommend fluorescent powder for tracking snakes as part of ecological studies.

4. How tracking was or was not used

The whole study is a tracking method. Instead of following natural tracks and signs, the researchers dusted the snakes with fluorescent powder and then followed each trail at night under ultraviolet (UV) light, flagging every turn of more than 10°, until the trail was lost in water, underground or as the powder ran out. Trails ran from clear lines to patches and small flecks of powder on vegetation. Some trails led into mammal burrows.

5. Tracker or observer certification

Not reported. The authors followed and recorded the trails, with help from Nick McCrea and Logan Wiwchar, who are thanked “for help tracking snakes”. No tracking experience or training is described; the authors note that the method can be done successfully with little training.

6. Other technologies, skills or innovations

• Fluorescent powder (Radiant Color Series T1, DayGlo Color Corporation, orange or green, about US $10 per pound), with colours chosen so trails from different snakes would not be confused.
• A fluorescent industrial lamp with two 90 cm black light bulbs, run from a gas-powered generator on a 100 m extension cord.
• Wire flags at every turn, compass and tape measurements, and path reconstruction in ArcGIS.
• Individual marking of snakes with a disposable medical cautery iron.

7. Implications for conservation (from the paper)

• Powder tracking avoids the surgery, anaesthetic risk and weight of radio transmitters and the snagging risk of thread spools, which matters especially for small snakes.
• Trails that lead into burrows may help locate hibernacula.
• Researchers should be aware that the powder could make snakes easier for predators to see, although harmful effects are thought to be few.