# Foraging and Feeding Ecology of the Gray Wolf (Canis lupus): Lessons from Yellowstone National Park, Wyoming, USA

> What does a wolf pack eat, and in what order? Ten years after wolves returned to Yellowstone, researchers found they mostly kill elk, choosing calves, old cows and weakened bulls, and eat the organs first and the bone and hide last. One lone wolf went 10 weeks without making a kill, getting by on carcasses up to 4 months old.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2006-stahler-smith-foraging-feeding-ecology-gray/

## Citation and access

- **Authors:** Daniel R. Stahler, Douglas W. Smith and Debra S. Guernsey
- **Year published:** 2006
- **Journal or source:** The Journal of Nutrition
- **Full citation:** Stahler, D. R., Smith, D. W. and Guernsey, D. S. 2006. Foraging and feeding ecology of the gray wolf (Canis lupus): lessons from Yellowstone National Park, Wyoming, USA. The Journal of Nutrition 136(7): 1923S–1926S.
- **DOI:** 10.1093/jn/136.7.1923S
- **Access status:** Hosted (freely available elsewhere)
- **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/2006_Stahler_and_Smith_etal_foraging-and-feeding-ecology-of-the-gray-wolf.pdf

## Tags

- **Species:** Mammals > Grey wolf
- **Region:** North America > United States
- **Tracking:** Kill sites; Scat; Tracking
- **Behaviour and ecology:** Behaviour; Feeding; Food caching; Hunting; Predation; Scavenging
- **Methods and technology:** Descriptive statistics; Direct observation; Global Positioning System (GPS) collars; GPS cluster analysis; Radio telemetry
- **Data type:** New field data; Review
- **Observer qualification:** Not reported
- **Discipline:** Behaviour and ecology
- **Year published:** 2006

## Detailed keywords

- **Species:** Studied: gray wolf (Canis lupus). Prey and food: elk (given in the paper as Cervus elephus, usually spelled Cervus elaphus), the main prey; also moose (Alces alces), deer, pronghorn antelope (Antilocapra americana) and bison (Bison bison) among winter kills; mule deer (Odocoileus hemionus), rodents, birds, invertebrates and plant matter, mainly grass (Graminae), in summer scats. Scavengers at wolf kills: common ravens (Corvus corax), black-billed magpies (Pica pica), golden eagles (Aquila chrysaetos), bald eagles (Haliaeetus leucocephalus), coyotes (Canis latrans) and grizzly bears (Ursus arctos). Also in the park but not studied: cougars (Puma concolor), black bears (Ursus americanus), white-tailed deer (Odocoileus virginianus), bighorn sheep (Ovis canadensis) and mountain goats (Oreamnos americanus).
- **Location and habitat:** Yellowstone National Park (8991 km²), Wyoming, USA, at the centre of the Greater Yellowstone Ecosystem (58,026 km², mostly public land in Idaho, Montana and Wyoming). Most kill data are from the park’s northern range. Elevation 1500 to 3800 m; forests among a mosaic of grassland, alpine, riparian, lake and thermal valleys. Wolves were reintroduced from Alberta and British Columbia, Canada.
- **Topics:** wolf reintroduction; prey selection by species, age and sex; kill rates; seasonal variation in predation; summer diet from scat analysis; plant eating; hunting behaviour and attack points; order of carcass consumption; carcass use and nutrition; daily food requirements and consumption; feast-or-famine foraging; scavenging old carcasses; food caching; pack size and structure; feeding hierarchy at carcasses; loss of food to scavengers.
- **Statistics or models:** Descriptive statistics only: counts, percentages, means and kill rates (elk per wolf per 30 days; ungulates per pack per day). Food consumption estimated from kill rates and live weights of prey. Inconsistencies and gaps: the kill-rate figure (Figure 3) covers 1995 to 2005 while the text and abstract cite 1995 to 2004; cows are defined as 1 to 9 years and old cows as over 10 years, which leaves 10-year-olds unplaced; the 2004 percentages add to 101% (rounding); the mean age of wolf-killed adult cow elk is given as 13.4 years with a range of 1 to 26, so the “adult” sample includes yearlings. Summer kill rates are preliminary, unpublished data.
- **Data type:** New field data from the Yellowstone Wolf Project: winter predation studies (30 days in mid-November to mid-December and in March, November 1995 to March 2004, with 2005 shown in Figure 3), year-round opportunistic observations, summer scats from 2003, and one lone wolf with a GPS collar in summer 2005. Also a short review of the literature on wolf feeding. Presented at a pet nutrition symposium (WALTHAM International Nutritional Sciences Symposium, Washington, DC, September 2005).

## Synopsis

### 1. What they did

Wolves were killed off in Yellowstone National Park by the 1920s and were brought back in 1995 and 1996. This short paper, given at a pet nutrition symposium, summarises the first decade of what the Yellowstone Wolf Project has learned about how these wolves hunt and feed.

- Following packs. At least one wolf in each pack wore a radio collar, so packs could be followed from the air and on the ground all year.

- Winter predation studies. Each early winter (mid-November to mid-December) and late winter (March), from November 1995 to March 2004, the team monitored radio-collared packs for 30 days to document kills, with the age and sex of each elk killed.

- Year-round observations. Predation and feeding behaviour were recorded whenever possible outside the winter studies.

- Summer diet. Scats were collected at summer den and rendezvous sites and analysed for the share of scats holding each food type.

- A lone wolf. In summer 2005, one lone wolf wore a Global Positioning System (GPS) collar recording a location every 30 minutes for 10 weeks, and the clusters of locations were investigated.

### 2. What they found

Prey and prey selection

- The wolf population grew from 31 reintroduced animals to 171 in 16 packs by the end of 2004.

- Of 2347 ungulate kills documented from 1995 to 2003, 2060 (88%) were elk, even though eight ungulate species live in the park.

- In early winter 1995 to 2003 (277 elk), wolves on the northern range killed 51% calves, 10% prime-age cows, 20% old cows and 19% bulls. In early winter 2004 (40 elk) this shifted to 18% calves, 25% prime-age cows, 15% old cows and 43% bulls.

- Wolves chose old elk: the mean age of wolf-killed cows from 1995 to 2004 was 13.4 years.

- Calves were strongly selected and cows selected against; bulls were killed in proportion to their numbers (1995 to 2000).

Kill rates

- From 1995 to 2000, each wolf killed on average 1.6 elk per 30 days in early winter and 2.2 in late winter, when elk are weaker.

- Since about 2000, late winter kill rates no longer rose, and wolves killed fewer elk overall (1.1 elk per wolf per 30 days from 2000 to 2004).

- Early evidence suggests kill rates drop by up to 25% in summer.

Summer diet

- Summer diets were more varied, including rodents, birds, invertebrates and ungulates not killed in winter. In 2003, mule deer were in 133 of 530 scats (25%).

- Plant matter, mostly grass, was in 392 of 530 scats (74%), matching observations of wolves eating grass. The authors consider this deliberate, perhaps for vitamins or to help clear intestinal parasites.

Hunting and feeding

- Packs usually hunted elk together, with two or three wolves doing the actual killing. Elk that ran were more likely to be killed than those that stood their ground.

- Calves and cows were often attacked at the neck; bulls more often from behind.

- Wolves first opened the body cavity and ate the organs (heart, lungs, liver, intestines, spleen, kidneys), then the large leg muscles, eating up to 10 kg of meat at a time and resting near the carcass for hours. Later they picked meat off ribs, leg bones and hide, and crushed bone, tendon, cartilage and hide.

- They did not eat the rumen contents, which, with large bones and some hide, were often all that was left. With many wolves and scavengers, carcasses rarely lasted more than 48 hours.

- A pack typically killed and ate an elk every 2 to 3 days. Estimated food intake was 5.7 kg per wolf per day in early winter and 10.4 kg in late winter, though actual intake was lower because of losses to scavengers and inedible parts. A minimum need of 3.25 kg a day has been estimated for a 35 kg wolf; Yellowstone wolves averaged 45 kg.

- The lone GPS-collared wolf made no kills in 10 weeks; they survived by scavenging more than 10 carcasses, 2 weeks to 4 months old, mostly bone and hide.

Packs

- Mean pack size over the first 10 years was 11 (range 2 to 37). Packs were either simple (a breeding pair and pups) or complex (several generations, or unrelated wolves).

- Unlike other studies that describe a strict feeding hierarchy, the authors saw little social tension at carcasses, likely because prey was still plentiful.

### 3. What they concluded

- Wolves in Yellowstone do not kill at random: they select prey by species, age and sex, looking for animals that are vulnerable enough to kill safely. They favour elk because elk are the most numerous ungulate and offer the best balance of encounter rate, injury risk and hunting success.

- Changing kill rates and prey selection since 2000 are not yet well understood, but are likely due to prey availability and vulnerability, competition between packs, and winter severity and drought.

- Wolves are adapted to feast or famine. They get through times without fresh kills by caching food, resting more and scavenging old carcasses, and they use almost every part of a carcass. Organs are rich in B vitamins, vitamin A, minerals and fatty acids.

- The authors predict that predation and feeding will change as prey density, prey vulnerability, wolf density and weather change.

### 4. How tracking was or was not used

The paper does not describe any tracking in detail. Packs were followed by radio telemetry from the air and on the ground, kills were documented during the winter studies (methods in an earlier paper), scats were collected at den and rendezvous sites for diet analysis, and the lone wolf’s GPS location clusters were checked on the ground to find which carcasses they had fed on and whether they had made any kills.

### 5. Tracker or observer certification

Not reported. The paper does not say who found and examined kills, collected scats or checked the GPS clusters. Scat contents were analysed by a laboratory (Big Sky Beetle Works, LLC).

### 6. Other technologies, skills or innovations

- Radio collars, with aerial and ground monitoring of every pack.

- A downloadable GPS collar taking a location every 30 minutes, with investigation of location clusters to reveal scavenging.

- Laboratory scat analysis giving the share of scats with each food type (this shows how often a food was eaten, not how much).

### 7. Implications for conservation (from the paper)

- Large carnivores play important top-down roles in shaping ecosystems, and wolves were the last missing carnivore restored to Yellowstone.

- Long-term study of the restored wolves’ foraging will help understanding of how ecosystems work and will matter to managers dealing with wolf and ungulate issues in the Greater Yellowstone Ecosystem.

### 8. Implications for tracking (Original Wisdom's observations)

_These are Original Wisdom's observations, not the authors'._

- The lone wolf shows how GPS clusters and ground checks work together: the collar showed where the wolf spent time, and what was found at those places showed that they were living on old carcasses rather than fresh kills.

- The described feeding sequence (organs first, then leg muscles, then ribs, bones and hide, with the rumen left behind) and the attack points (neck for calves and cows, from behind for bulls) may help trackers read wolf kills and feeding signs.

## Cite this summary

Lawrence, K. 2026. Summary of: Foraging and Feeding Ecology of the Gray Wolf (Canis lupus): Lessons from Yellowstone National Park, Wyoming, USA (Stahler and Smith et al., 2006). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2006-stahler-smith-foraging-feeding-ecology-gray/