Gender Discrimination of Tigers by Using Their Pugmarks

1. What they did

Indian foresters and researchers have long claimed that you can tell a male tiger from a female by the shape of the hind pugmark. The most widely used field rule says a male’s hind pugmark fits in a square frame and a female’s in a rectangular one, with slender, longer toes. Other rules used toe angles, pad widths, or the difference between length and width. The authors point out that most of these rules were built from census pugmarks of tigers whose sex and identity were not known, traced by several different people, and had never been checked against known tigers.

To test this, they:

• Collected pugmark sets from 13 tigers of known sex (3 males and 10 females) in four tiger reserves and parks and one zoo in India, between November 2000 and April 2001. Wild tigers’ sex was confirmed by sightings. A set was a series of tracings of left and right hind pugmarks from one tiger’s fresh trail, taken only where the tiger had walked in a normal gait.
• Kept the tracing consistent. The first author traced every pugmark onto acetate sheets, on flat ground with fine, loose soil. Captive tigers walked over impression pads of fine soil.
• Measured each pugmark. The tracings were scanned and 9 measurements taken from each, chosen from 93 because they best told tigers apart (Figure 2).
• Tested several statistical methods to see how well the measurements separated males from females, first with all 9 measurements and then with only the pugmark’s length and width (the measurements behind the square-versus-rectangle rule). They checked the models in three ways: leaving pugmarks out one at a time, building a model on half the pugmarks and testing it on the other half, and testing a model built from 10 tigers on 3 tigers it had never seen (1 male and 2 females).

2. What they found

All 9 measurements

• Two summary components from the measurements explained 75.8% of the variation, and plotting them showed two distinct clusters, one male and one female (Figure 3).
• Three measurements did most of the work: the distance between toes 2 and 3, the distance between two notches at the back of the main pad, and the distance from the top of the main pad to the base line of the toes.
• These classified 99.1% of pugmarks correctly, and 98.1% when checked by leaving pugmarks out one at a time. Tested on half the data, the model sexed every tiger correctly (100% of pugmark sets) and 99.1% of individual pugmarks.

Length and width only

• Using only the pugmark’s length and width, the model classified 100% of pugmarks correctly, including when checked by leaving pugmarks out and when tested on half the data.
• On the 3 new tigers, both this model and a logistic regression model sexed all three correctly. At the level of single pugmarks, 97.9% of 97 female pugmarks and 94.1% of 34 male pugmarks were classified correctly, 97.0% overall. The authors note that one male pugmark was classified as female.
• In the logistic regression, both length and width were significant, with width the stronger of the two (Table 2).

3. What they concluded

• The measurement models were highly accurate in telling male from female tigers, and pugmark length and width were robust predictors. The most accurate and simplest model used length and width only.
• The authors acknowledge that the sample was small. Good sets with enough clear pugmarks from individually known tigers were hard to get, because tigers are elusive, mostly nocturnal, wide-ranging and sparsely spread. Male pugmarks were harder to obtain than female ones, because the sex ratio is biased towards females and males have larger home ranges.
• Collecting and analysing pugmarks in this way could be a cost-effective, reliable and accurate tool for monitoring tiger sex ratios and the movement patterns of each sex, where pugmarks are easy to find.

4. How tracking was or was not used

The whole study rests on tracks. Pugmarks were traced from the fresh trails of tigers of known sex, choosing only trails with at least 5 clear impressions of both left and right hind feet. The gait was checked for a normal walk by looking at the consistency of stride length, how the hind feet landed on the front prints, and whether the front prints were under- or over-shot. The hind pugmarks were traced, photographed and measured, and the measurements were used to identify the tigers’ sex. The authors also review earlier field rules for sexing tigers from pugmarks.

5. Tracker or observer certification

Not reported. The first author traced all the pugmarks, which the authors say removed variation between tracers. No tracker qualification or test of tracing skill is described. His biography notes that his master’s project studied pugmarks for identifying individual tigers and evaluated India’s pugmark census technique.

6. Other technologies, skills or innovations

• The standard pugmark-tracing technique, with tracings on acetate sheets in indelible ink.
• Pugmark Impression Pads for captive tigers.
• Scanned tracings measured with Arc Info and Sigma Scan Pro software.
• Simple equations that give a male or female score from a pugmark’s length and width.

7. Implications for conservation (from the paper)

• Tigers are shy, mostly nocturnal, wide-ranging and hard to see, so researchers often have to rely on their signs.
• This non-invasive method could provide sex ratio and other demographic data. Together with the authors’ method for identifying individual tigers from pugmarks, it could give information on population structure and density and support continuous monitoring.
• Because many big cats have similar pugmarks, the approach might help develop monitoring protocols for other species.