Limitations to recording larger mammalian predators in savannah using camera traps and spoor

1. What they did

Spoor (tracks or pugmarks) has long been a low-cost way to find out which large mammals are present, and sometimes which individual. Camera traps are now widely used instead, but little is known about how well they perform against spoor in real field conditions. The researchers worked in a South African game reserve, as part of a wider study of leopards, and asked three questions:

• Camera traps versus spoor. Over July and August 2013, they placed one camera at a T-junction on dirt roads at four sites. Each morning, at least two people who had had a week’s spoor recognition training walked the roads and recorded spoor of leopard, brown hyena, African civet, Meller’s mongoose, genets and jackals, within the camera’s range and 25 m either side. Each spoor was photographed with a ruler, identified with a field guide and rechecked from the photo by an experienced tracker. The roads were then swept clean with a feather duster so only fresh spoor would be found the next day. Each species was counted once per night per site, by camera and by spoor.
• Do different people measure spoor the same way? They made a plaster cast of a clear leopard print and used it to press 20 prints into fine sand. Four people measured each print in four ways (full spoor length and width, hind pad length and width) with a ruler: three novices with a week’s training and one experienced field researcher.
• Can simple measurements help follow one leopard? An experienced tracker found and followed the trails of three leopards for 1 to 2 km each. Every 5 m, one person measured a hind footprint, noting whether the sand was under or over 2 mm deep.

2. What they found

Camera traps versus spoor

• More spoor was found inside the camera area than outside, so the cameras were well placed and animals did not seem to avoid them.
• Over 35 nights there were 153 records: 48 by camera and 105 by spoor. Spoor recorded species more than twice as often as the cameras.
• Leaving leopard out, cameras significantly under-recorded the carnivores compared with spoor. Under-recording was 64% for civet, 46% for genets, 45% for Meller’s mongoose, 30% for jackals and 22% for brown hyena.
• Leopard was the exception: leopards were photographed three times (individuals MS24, MS24 and FS7) but no complete leopard spoor was found inside the camera area. On two occasions leopard spoor was found just outside the camera zone while the leopard was photographed. The authors put this down to small changes in the sand, and partial prints were not counted.
• Shorter species tended to be missed more by cameras, though this trend was just short of statistical significance.

Different people measuring the same print

• Of the 20 cast prints, 19 could be used. The four observers’ measurements differed significantly for all four measurements.
• The experienced researcher was much more consistent than the three novices. For example, the novices’ full spoor lengths ranged over 18 to 23 mm; the experienced researcher’s over 8 mm.

Measuring known leopards in the field

• Full spoor length, full spoor width and hind pad length differed significantly between the three leopards; hind pad width did not.
• Sand depth changed spoor size: prints were larger in sand over 2 mm deep, perhaps because the toes spread further from the main pad as the foot sinks in.
• Full spoor length was the most distinct measurement between the leopards, followed by hind pad length.

3. What they concluded

• Camera traps under-recorded most of the carnivores that passed them, especially smaller species. Spoor can be used to calibrate camera traps, and camera trap studies should be checked against other methods.
• Measurements of spoor size taken by amateur volunteers vary too much to give more than presence or absence of leopards.
• Simple measurements are unlikely to identify unknown leopards. But an experienced tracker can use them to keep following a known leopard when the trail is broken or crossed by another animal, linking camera records with the leopard’s wider movements and habitat use.
• The authors advise taking direct field measurements in sandy substrate under 2 mm deep, as this reduces variation.
• Camera traps are a very helpful tool, but the benefits of good field craft should not be neglected.

4. How tracking was or was not used

Tracking was central to the study. Trained volunteers searched dirt roads every morning for carnivore spoor, when low sun casts shadows in the tracks, and recorded only complete, clear prints. An experienced tracker checked each identification from the photograph. An experienced tracker also found and followed the trails of three leopards over hard ground covered with fine sand, treating a break of more than 5 m as the end of a trail. Hind footprints were measured because leopards register (the hind foot lands on the front print).

5. Tracker or observer certification

Reported but not validated. The survey team had a week’s spoor recognition training; the people who checked identifications and followed leopard trails are called “experienced”, with no certification or test of their tracking skill. The inter-observer test measured how consistently people measured a print, not how well they identified or followed spoor. The experienced field researcher in that test (observer 3) is not named.

6. Other technologies, skills or innovations

• Little Acorn 5210A camera traps: one-second trigger, three-photo bursts, 30-second interval, lens at about 45 cm and angled 45° to the road.
• Photos of each spoor (Nikon D3100) with a millimetre ruler zeroed at the back of the main pad.
• Plaster casting of a leopard print: hairspray to fix the sand, a plastic ring, baby powder, and plaster poured over the back of a spoon.
• Sweeping the roads clean daily with a feather duster.

7. Implications for conservation (from the paper)

• Camera traps may miss more animals than thought, which matters for estimates of distribution and abundance. How likely a camera is to record an animal depends on the camera model, the habitat and the species.
• Spoor and the skills of tracking in the field still have a place in monitoring large savannah predators.
• Following individual leopards by their spoor can add information on movement and behaviour, useful for developing conservation strategies.