Diets of Free-Ranging Mexican Gray Wolves in Arizona and New Mexico

1. What they did

Mexican gray wolves were wiped out of the wild in the southwestern United States by the late 1960s, before anyone studied what they ate. In April 1998 the United States Fish and Wildlife Service began releasing captive-reared Mexican wolves into the Blue Range Wolf Recovery Area in Arizona and New Mexico. The authors set out to describe the diet of these free-ranging wolves, to check for differences among years, seasons and packs, and to compare it with the diets of other North American gray wolves.

• Collecting scats (April 1998 to October 2001). They searched where released, translocated and wild-born wolves were known to spend time, following the radiocollared wolves as they moved. Scats were picked up along forest roads, trails, ridgelines and riparian areas, and at opened release pens, campsites, den sites, and kill and carcass sites. The Mexican Wolf Interagency Field Team also collected scats when they came across them. In total they collected 1,682 carnivore scats.
• Deciding which scats were wolf scats. Scats were first identified to species with traditional field methods: diameter, location, and tracks near the scat. The dried scats were then measured with calipers. Based on earlier DNA work by the same team (Reed et al. 2004), they used 28 mm or more as the cut-off for Mexican wolf scats. This left 251 wolf scats.
• Analysing the contents. Scats were broken apart by hand, and hair, bone, teeth, claws and hooves were identified by eye against a reference collection.
• Calculating the diet. They worked out how often each food item appeared (percent frequency of occurrence, PFO) and estimated the mass of prey eaten (biomass). They tested for differences with G-tests and compared the results with seven earlier gray wolf diet studies.

2. What they found

What the wolves ate

• Large food items made up 92.8% of occurrences. Elk dominated: adult elk 36.6% and elk calves 36.2% (72.8% together).
• Other native ungulates made up 15.8% (deer adults 2.6%, deer fawns 2.3%, unknown native ungulates 10.9%).
• Domestic cattle made up 4.2%.
• Smaller mammals made up 5.3%, birds 0.4%, insects 0.8% and vegetation 0.8%.
• Most scats held only one food item (1.06 items per scat on average).

Differences

• There were no differences in diet among years, between seasons (spring and summer versus autumn and winter) or among four packs, although two packs included medium and small food items.
• Mexican wolves ate a higher share of large food items (92.8%) than reported in the seven other North American gray wolf studies (58.5% to 82.9%), and the difference was significant.

Biomass

• The scats represented an estimated 414 kg of prey. Elk made up 76.7% of this: adults 57.2% and calves 19.5%. Cattle made up 8.0%.
• Frequency of occurrence underestimated the importance of adult elk and overestimated elk calves compared with biomass. Even so, the rankings of prey were the same under both methods.

Livestock

From May 1998 to October 2001 a compensation fund paid US$16,612 for damage linked to Mexican wolves: 4 adult cattle, 11 calves and 1 herding dog killed, and 2 horses, 1 calf and 1 guard dog injured.

Possible biases

The authors name two: scats from pack mates collected at the same time may not be independent, and the strict 28 mm cut-off may have left out smaller wolf scats with smaller prey, inflating the share of elk. They doubt the first mattered much, because few scats came from kill sites.

3. What they concluded

• Free-ranging Mexican wolves ate mostly elk, the largest of the five large native ungulates in the area, rather than the smaller deer. They ate more large prey than their northern relatives.
• The authors suggest the high share of elk came from the early, low-density stage of wolf recovery: elk were probably the most numerous ungulate, and after about 40 years without wolves they were likely naive to wolf predation and easier to catch.
• Together with kill reports and compensation records, their results show Mexican wolves ate more large native ungulates than livestock.
• Unless deer numbers greatly increase, they predict elk will continue to dominate the Mexican wolf’s diet.
• Comparing two methods (frequency and biomass) gives a better picture of a carnivore’s diet than either alone.
• The results give baseline diet information for a newly re-established wolf population. A study of prey selection across all the large carnivores sharing this prey (mountain lion, black bear, coyote and wolf) could show why Mexican wolves ate mainly elk.

4. How tracking was or was not used

Tracks and signs identification was central to the study. Scats were the whole data source, and the researchers first identified each scat to carnivore species in the field by its diameter, where it lay, and tracks near the scat, following methods in the literature. They then applied a measured diameter cut-off (28 mm or more), based on earlier DNA results, to separate Mexican wolf scats from coyote and fox scats. Searches followed the movements of radiocollared wolves, and scats were also collected at den sites and kill and carcass sites. No trailing was reported.

5. Tracker or observer certification

Not reported. The paper does not say who identified the scats in the field or what experience they had. Scats were collected by the research team and by the Mexican Wolf Interagency Field Team (staff of several federal and state agencies). The final choice of wolf scats rested on a diameter cut-off checked against DNA in an earlier study.

6. Other technologies, skills or innovations

• Radiocollars on 31 of about 37 free-ranging wolves, used to guide where scats were searched for.
• Dial calipers to measure the maximum diameter of each dried scat.
• A scat diameter cut-off for Mexican wolves (28 mm or more) set from earlier DNA analysis.
• A reference collection for identifying hair, bone, teeth, claws and hooves in scats.
• Biomass equations (Weaver 1993) to convert scat counts into estimated mass of prey eaten.

7. Implications for conservation (from the paper)

• The diet information is useful for managing Mexican wolves and their prey and for investigating livestock losses where wolves are recovering.
• Mexican wolves ate more large native ungulates than domestic livestock.
• Historic records of the Mexican wolf’s diet focused on wolves that killed cattle, and these reports were likely exaggerated and biased.
• Little research has been done on the other large predators that now share the same prey, and a joint study is suggested.