Differentiating Mexican gray wolf and coyote scats using DNA analysis

1. What they did

Mexican gray wolves were extirpated from the United States by the late 1960s, and from 1998 captive wolves were released into the Blue Range Wolf Recovery Area in Arizona and New Mexico. Mexican wolves are the smallest North American gray wolves, so their scats could be confused with those of the coyotes that share their range. Diet studies depend on knowing which animal left each scat.

As part of a study of Mexican wolf diets, the authors and an interagency field team collected 1,682 carnivore scats between 1998 and 2001. They looked for scats along forest roads, trails, ridgelines and streams, and at campsites, opened release pens, dens and kill or carcass sites in areas the wolves were known to use. Each scat was identified in the field by traditional methods:

• Diameter: a scat over 30 mm across was taken as wolf.
• Location and sign: where it was found, plus tracks and sightings of wolves.
• Odor: wolf recovery staff taught them that a sweet, musky smell meant wolf. No odor description existed for coyote scats.

They then tested 203 of these scats (mostly ones identified as wolf) with DNA analysis of mitochondrial DNA scraped from the scat surface, and compared the DNA answer with the field identification. Finally, they used the DNA-confirmed scats to test whether diameter, mass and length could sort wolf from coyote scats.

2. What they found

DNA results

• DNA could be read from only 49 of the 203 scats (24%): 28 were Mexican wolf and 21 were coyote. Two wolf scats could not be measured, leaving 47.

Size

• Wolf scats measured 16.3 to 35.8 mm across, coyote scats 17.4 to 27.8 mm, a 79% overlap.
• On average wolf scats were wider (26.0 mm against 22.8 mm), a significant difference.
• Every scat 28 mm or wider was a wolf scat.

How well the field methods did

• Of 45 scats that would have been called wolf by location and odor, 19 (42%) were coyote.
• Of 41 scats that would have been called coyote because they were under 30 mm, 20 (49%) were wolf.
• A published diameter guide (Halfpenny 1986) would have called 3 scats fox (none correct), 24 coyote (62% correct) and 20 wolf (75% correct).

Measurements combined

• Diameter alone correctly sorted 81% of coyote scats but only 50% of wolf scats.
• Diameter with mass did best: 86% of coyote scats and 65% of wolf scats. Even so, 35% of wolf scats were classed as coyote.
• Diameter with length (68% coyote, 59% wolf) and all three measures (79% coyote, 55% wolf) did worse.

3. What they concluded

• Scat diameter and length vary too much to identify wolf and coyote scats reliably, and diameter with mass was unsatisfactory for wolf scats.
• Odor is subjective and, on these results, not a reliable way to identify wolf scats, although some wolf biologists say they can.
• Scats at dens and rendezvous sites probably give accurate wolf diet data because coyotes rarely visit them, but only for late spring and summer. Scats at kill sites are more doubtful, because coyotes often scavenge wolf kills.
• DNA identification is rapid (3 to 4 days), repeatable, definitive and relatively cheap (about US$5 per sample in consumables), but the low extraction success (24%) is a concern. Old scats (up to 5 years) were part of the reason, and a scat’s appearance did not predict whether its DNA would work.
• With only 47 scats, the results should be interpreted with caution. Even so, earlier diet studies that used traditional identification may have misrepresented wolf and coyote diets where the two live together.
• Future work would need fresh scats from systematic transects, with proper preservation, extraction and species-specific primers.

4. How tracking was or was not used

The study tests scat identification, a core tracks and signs skill. Field identification combined scat diameter, where the scat was found, sign nearby (tracks and sightings of wolves) and odor. The authors aged scats as old, recent or fresh from their appearance, how exposed the site was and the weather. Searches followed where the wolves were reported to be moving (from the US Fish and Wildlife Service), not their trails. DNA analysis was then used as the check on these field identifications.

5. Tracker or observer certification

Not reported. The field identifications were made by the authors following set criteria; their own experience is not described. Wolf recovery field staff showed them how to recognise wolf scats by odor, and wolf biologists gave them scats they had identified as wolf. The study tests the identification criteria rather than the skill of individual observers. The authors cite Halfpenny (1986) as reporting error rates of 50 to 66% when experienced naturalists identified scats by eye.

6. Other technologies, skills or innovations

• Fecal DNA: mitochondrial DNA (control region) from cells on the scat surface, extracted with a guanidine thiocyanate (GuSCN) and silica method, then polymerase chain reaction (PCR) and restriction fragment or sequence analysis.
• A reference collection of DNA from wolf blood, coyote tissue, and scats of captive wolves, coyotes and domestic dogs.
• Scat diameter taken as the average of two calliper measurements at the widest point; dry mass and total length also measured.

7. Implications for conservation (from the paper)

• Accurate scat identification matters for measuring diet overlap between wolves and coyotes, managing wolf and prey populations, and investigating livestock losses where wolves are being reintroduced or recovering.
• Fecal DNA can identify species without seeing or disturbing the animals, and could also be used to identify individuals, sex, diet, home range, kinship, population size and hybridization, and to confirm wolf presence at livestock depredation incidents.