Differentiating Mountain Lion and Bobcat Scats

1. What they did

Scats are often the only physical evidence that a mountain lion is present, and many diet studies rely on scats, but looking at a scat is not always enough to say who left it. Several cat species may leave scats in scrapes, and mountain lion and bobcat scats and scrapes overlap in size. Earlier work suggested that the bile acids in mountain lion scats are distinctive, but some bobcat scats looked chemically similar. The authors tested whether a simple laboratory method, thin-layer chromatography (TLC) of the bile acids in scats, can tell the two species apart.

• Reference standards. Scats from 10 captive bobcats and 10 captive mountain lions were analysed to find the bands each species shows on a TLC plate.
• Bile acid make-up. Four scats from each species were analysed in more detail by gas chromatography (GLC), to measure which bile acids were present and in what proportions.
• Blind test. Belden mailed 144 numbered but otherwise unlabelled scats to Johnson. Most were collected in the field by Belden and others, but hidden among them were 14 scats from captive bobcats and 24 from captive mountain lions. Johnson first looked at every scat and tentatively identified it, mainly by size, then analysed 1 g of each by TLC.

A scat was called bobcat if it showed the bands both cats share plus one or both bobcat bands, mountain lion if it showed only the two shared bands, and chemically unidentifiable if it showed only one band or no bile acids.

2. What they found

The chemical signatures

• Mountain lion scats showed two bands on the TLC plate; bobcat scats showed those two plus up to two more. Not every band appeared in every sample. The two extra bands were less common in bobcat scats and never appeared in the 10 known mountain lion scats.
• The same four bile acids were found in both species. One bobcat scat also held hyodeoxycholic acid, and three held an unidentified bile acid.
• Cholic plus deoxycholic acid made up on average 75.2% of the bile acids in bobcat scats and 94.4% in mountain lion scats.

Known scats from captive animals

• Chemistry alone correctly identified 79% of the mountain lion scats (19 of 24) and 64% of the bobcat scats (9 of 14). Four mountain lion scats were called bobcat, and four bobcat scats were called mountain lion; one of each could not be identified.
• By eye, Johnson correctly identified 88% of the mountain lion scats (21 of 24). One small mountain lion scat was misidentified, and three amorphous (shapeless) scats could not be identified. Of the 21, 18 were also correct chemically.
• By eye, he correctly identified 64% of the bobcat scats (9 of 14). One was called mountain lion and four could not be identified. Of the nine, seven were also correct chemically.
• When the visual and chemical identifications agreed, there were no errors. Agreement gave a correct identification for 75% of the mountain lion scats and 50% of the bobcat scats.

Field-collected scats (106)

• 34 (32%) were amorphous and could not be identified by eye; 29 (27%) looked like mountain lion and 43 (41%) like bobcat.
• Chemistry confirmed 25 (86%) of the visual mountain lion scats and 29 (67%) of the visual bobcat scats.
• Of the 34 visually unidentifiable scats, chemistry identified four as mountain lion and one as bobcat. The other 29 had too little bile acid to identify.

3. What they concluded

• TLC appears to be only about 80% accurate for mountain lion scats and about 65% accurate for bobcat scats.
• Mountain lion scats contain so little bile acid that only cholic and deoxycholic acid show on the plate. Bile acids dissolve in water, and many field scats were weathered, so a bobcat scat that has lost bile acids is much more likely to be mistaken for a mountain lion scat than the other way round. Scats exposed to rain cannot be reliably identified this way.
• Fresh scats usually held the expected bile acids. Combining chemical and visual evidence gave more reliable identifications than visual identification alone, so using both should give the most accurate results.
• For now, TLC should be used as supporting evidence to confirm the visual identification of fresh mountain lion scats.
• Johnson identified the scats without knowing whether they came from a scrape or what tracks were at the site. A field biologist may have these extra clues, so field identification would probably be more accurate than identification in the laboratory. However, collectors may be inexperienced, extra signs are not always present, scats may lack the typical cat smell, and the look of a scat is affected by the animal’s health and size, insects, diet and weathering. TLC can therefore help make sure that all scats in a study come from the target species.
• Gas chromatography would probably be more accurate, because the two species differ clearly in the share of cholic plus deoxycholic acid. The authors did not test this, as it cost too much for this many scats, but suggest the extra cost may be justified for reasonably fresh scats where great accuracy is needed.

4. How tracking was or was not used

The study is about scat identification. Johnson identified every scat by eye, mainly from its size, without knowing the collection site, whether it was in a scrape, or what tracks were nearby, and these identifications were checked against known scats from captive animals. The field scats were collected by Belden and others; the paper does not say how they found or identified them in the field. The authors note that other tracks and signs at the site, such as scrapes, tracks and the typical cat smell, can help a field biologist identify a scat.

5. Tracker or observer certification

Not reported. Johnson, a co-author, made all the visual identifications; his experience or qualification is not described. His visual identifications were, however, tested in this study against known scats from captive animals (88% correct for mountain lion and 64% for bobcat). Nothing is said about the experience of the people who collected the field scats.

6. Other technologies, skills or innovations

• Thin-layer chromatography (TLC) of bile acids extracted from 1 g of dried scat, following Major et al. (1980), with each band’s position compared with standards on the plate.
• Gas chromatography (GLC) of 5 g samples to identify and measure each bile acid.
• A blind design: known scats from captive animals were mixed, randomly numbered, among field-collected scats.

7. Implications for conservation (from the paper)

• Scats are a source of physical evidence for mountain lion sighting reports in the eastern United States, where credibility depends on such evidence.
• Chemical analysis can help make sure the scats used in food habits studies come from the species being studied.