Identification of the Anteroposterior and Mediolateral Position of Lion Paws and Tracks Using 3D Geometric Morphometrics

1. What they did

Tracks are a cheap, non-invasive way to study elusive animals such as big cats, and several studies have tried to tell individual felids apart from their footprints. A known problem, raised in India’s tiger “pugmark census”, is that a track from one foot can be wrongly compared with a track from another foot of the same animal, so one animal is counted as two. The authors set out to build an accurate, consistent and objective way to tell which foot made a lion track: front or hind, right or left.

• Paws. In Hluhluwe-iMfolozi and Tembe Elephant Parks, South Africa, they photographed 132 paws from 39 lions of both sexes and all ages (adults, subadults and juveniles) while the lions were captured at night for routine management by Ezemvelo KZN Wildlife staff, including a wildlife vet.
• Tracks. They photographed 182 tracks from 12 trails (7 to 31 tracks each) in riverbeds and on dirt roads, after seeing the lions directly or after a lion passed one of six camera traps placed along game paths and near waterholes. All trails were less than 24 hours old and showed a walk in which the hind foot landed beyond the front track. Which foot made each track was worked out from its place in the sequence of tracks.
• 3D models. For each paw or track they took 9 to 15 overlapping photos from different angles, with a scale bar, and turned them into a 3D model with photogrammetry software.
• Shape analysis. They placed 12 landmarks on each model: four points on the lobes of the main pad and the two ends of each toe along its long axis. Five people with no experience placed the landmarks four times on 10 paws and 10 tracks to test how repeatable this was. They then compared shapes and tested how well a statistical classifier could assign each paw and track to the right foot.

2. What they found

• Repeatability: landmark placement was 88.8% repeatable on paws and 73.7% on tracks.
• Accuracy: the best classification reached 95.45% for paws and 91.21% for tracks. With only the first two shape components, accuracy was already 84.09% for paws and 88.46% for tracks.
• Paws versus tracks: the main shape difference was the spread of the toes.
• Right versus left: the outer (smallest) toe is shorter and sits closer to the main pad, while the toe two digits in from it (the middle digit, between the outer toe and the dewclaw) sits furthest from the pad and marks the highest point of the paw or track. In tracks, the inner bottom lobe of the main pad tends to sit higher than the outer one.
• Front versus hind: front paws and tracks are larger and rounder than hind ones.
• Missing tracks: 30 tracks in the trails were missing (destroyed by other tracks) or too poor to use; 80% of these were front tracks.

3. What they concluded

• With just 12 landmarks, lion paws and tracks have shapes complex enough to tell front from hind and right from left accurately and consistently.
• Tracks were classified a little less accurately than paws, probably because the paw and the ground interact in complex ways and distort the track. This seems stronger in front tracks, likely because lions, like most four-legged animals, carry most of their weight on their front legs.
• The study used good-quality tracks in riverbeds and on sandy roads; tracks on other substrates still need testing.
• Future work: automating landmark placement using the 3D information, adding more landmarks to identify age, sex and individuals, testing bias between people recording the tracks, and trying cheaper cameras and free software.
• The authors recommend this method in future studies that compare tracks between individual lions.

4. How tracking was or was not used

Tracks were the study material. The researchers found fresh trails (less than 24 hours old) of lions they had watched or that had passed camera traps, and recorded continuous sequences of tracks, defining a trail as a continuous sequence of tracks from one animal. They identified which foot made each track from its position in the sequence, using a walking gait in which the hind foot oversteps the front track, following Liebenberg’s field guide and Practical Tracking. They also judged track quality, discarding tracks where a landmark was missing or distorted.

5. Tracker or observer certification

Not reported. The paper does not say who found the trails or assigned each track to its foot. This matters because those foot assignments are the “known answers” the classifier was tested against. The five people who tested landmark placement had no prior experience of placing landmarks.

6. Other technologies, skills or innovations

• Close-range photogrammetry with ordinary digital single-lens reflex cameras (Nikon D7100 and D80) and a scale bar; 3D models built in Agisoft PhotoScan Professional.
• Landmark-based geometric morphometrics in R (Geomorph package).
• Camera traps (Cuddeback Attack), checked daily, to confirm which lions made the tracks.
• The authors thank the Carnegie Institution for Science for letting them use the tracking frequencies of collared lions.

7. Implications for conservation (from the paper)

• Telling individual lions apart from their tracks would be a practical, low-cost, non-invasive monitoring tool with major implications for conservation, management and research.
• The field kit is just a commercially available camera and a scale bar. The simple protocol could involve citizen scientists and community members, making photogrammetry suited to long-term monitoring in remote areas with few resources, provided someone else does the 3D processing and analysis (for example through an online platform).