Wolf Effects on Elk Inhabiting a High Risk Landscape: The Madison Headwaters Study
by Robert A. Garrott, P.J. White, Claire Gower, Matthew S. Becker, Shana Dunkley, Ken L. Hamlin, & Fred G.R. Watson
The effects of wolves on elk in the Greater Yellowstone Ecosystem have been contested among laypersons, politicians, and scientists—with some claiming devastation, others suggesting healing restoration, and most seeing something in between. In 1991, Montana State University initiated a study of about 400 to 600 elk inhabiting the Madison headwaters area in the west-central portion of Yellowstone National Park. The elk herd was nonmigratory and remained within the park year-round; therefore, the animals were not subject to harvest by human hunters. This high-elevation area has complex terrain, accumulates deep snow, and supports a mosaic of habitats including large tracts of burned and unburned forests interspersed with geothermal areas, meadows, rivers, and small lakes. The area is also an important winter range for bison that seasonally migrate west from their summer range in Hayden Valley. Prior to wolf restoration, coyotes were the only abundant mammalian predator, with some grizzly bears during spring and a few mountain lions. The study was initiated seven years before reintroduced wolves recolonized this portion of the park and continued thereafter, providing a rare opportunity to compare the responses of individual elk and the population as a whole to the restoration of a top predator that had been absent for approximately 70 years.
The protocol for the study was based on maintaining a representative sample of radio-collared female elk with biologists conducting extensive field work from November to May each year to monitor their behavior, nutrition, movements, pregnancy, survival, and population trends in response to forage, snow, predators, and other conditions. From 1991 to 2009, scientists amassed more than 12,000 person-days of field work and evaluated 15,000 observation periods of elk groups; 6,500 snow urine samples for assessing elk nutrition; 2,000 serum and fecal samples for assessing elk pregnancy; 1,000 plant samples for assessing biomass and nutrition; 17,000 measurements of vegetation; 4,175 kilometers (2,594 miles) of snow tracking along wolf trails; and 750 carcasses of ungulates killed by wolves. Also, 4,300 snow cores and more than 24,000 hours of wind data were collected to model spatial and temporal dynamics of the snow pack.
Prior to wolf restoration, the probability of an elk dying was related to its age, body condition, and snow pack. The primary cause of death was starvation, with younger and older elk more likely to die than elk in the prime of their life (3-9 years old) that have uniformly high survival rates. Elk rely on their teeth to obtain and break up plant materials, which are further broken down by microbes in their four-chambered stomachs to obtain energy and protein. Teeth wear with age, so older elk become less efficient at obtaining nutrients and accumulating the fat and protein reserves needed to survive winter when the availability of nutritious foods is low. This is especially true in the Madison headwaters region where high concentrations of silica in the soils and fluoride in the waters accelerate tooth wear—thereby leading to a shortened life span compared to elk in other areas. In addition, calves are smaller in body size, and as a result have smaller stores of fat and protein to metabolize during winter when forage was scarce. Deep, prolonged, or hard snow conditions also increased the risk of starvation of young and old elk by limiting access to forage under the snow and requiring more energy for them to forage and move about the landscape. As a result, the proportion of elk in the population dying from starvation each winter varied among years depending on winter severity. However, elk that frequently used geothermal areas (where heat from the interior of the earth reduced or eliminated snow pack) were less vulnerable to starvation.
Wolves recolonizing the Madison headwaters area strongly preferred elk as prey and killed comparatively fewer bison, even though bison were more abundant than elk from midwinter through spring. Bison kills were more frequent during late winter when animals were in poorer condition. The wolves' preference for elk probably reflects the formidable challenge of killing bison, which form groups to aggressively and cooperatively defend themselves and their young. In contrast, elk do not use group defenses and generally flee when attacked. Wolves strongly selected calves and older elk, which are the age classes most vulnerable to starvation mortality during winters of average to severe snow pack. However, the survival of elk calves was lower and less variable among years after wolf numbers increased, suggesting predation limited the recruitment of animals into the breeding population. The survival of adult female elk was 5-15% lower following wolf recolonization, primarily in the middle to older age classes. The diets and nutrition of elk remained similar to those prior to the arrival of wolves. Elk pregnancy rates remained high, but elk abundance decreased rapidly as breeding females were killed and wolf predation on calves consistently reduced recruitment to low levels. As elk numbers decreased due to wolf predation, wolf kill rates remained high and wolf numbers continued to grow. As a result, predation removed a higher portion of the elk population each year until elk became scarce. Thereafter, wolf kill rates decreased, strife among packs increased, wolf numbers declined, and packs began to hunt elsewhere for most of the year.
After wolves established in the Madison headwaters, the probability of an elk dying was strongly influenced by factors other than its physical condition, including characteristics of the landscape and weather that increased its susceptibility to predation by wolves. Elk at higher elevations with deeper snows were more likely to be killed by wolves, as were elk in thermal areas or meadows where they could be chased into habitat boundaries of deeper snow or burned timber with down-fall that impeded their escape. Conversely, elk on steep slopes with shallow snow and good visibility, or in areas where they could quickly escape to deep, swift, and wide rivers after encountering wolves, were less vulnerable to predation. As a result, in less than two decades, elk went from being numerous (~400-600 individuals) and broadly distributed throughout the Gibbon, Firehole, and Madison drainages during winter to scarce (less than 25 individuals) and constrained to relatively small refuges in the Madison drainage where they were more likely to observe approaching wolves and escape if detected and attacked. Wolves killed nearly all of the elk in the Firehole and Gibbon drainages where susceptibility to predation was high. Many of these elk were strong and in good condition, but were caught in "terrain traps" where they were unable to flee effectively. Wolves also substantially lowered adult survival and limited recruitment in the Madison drainage;but less than two dozen elk persisted in areas with shallower snow bordered by the swift, deep, and wide Madison River. Encounters with wolves remained high in these areas, but adult elk were sometimes able to flee to nearby refuge habitat.
Ultimately, this study demonstrated how behavioral, physical, and environmental factors interact to influence the vulnerability of elk to predation by wolves and, in the end, revealed wolves can have a dramatic effect on the abundance and distribution of elk across the landscape. While the Madison headwaters study may represent what could be considered a "worst-case" scenario with respect to the impacts of wolf restoration on elk, the processes documented in this study are similar to those documented in other wolf-elk systems throughout the Greater Yellowstone Ecosystem by other research teams. Integrating the results from this impressive body of scientific work, we conclude the impacts of wolf restoration can be substantial for elk herds spending winter in forested, mountainous environments where elk are quite vulnerable to predation due to a heterogeneous landscape with deeper snow pack. Predators tend to be more diverse and numerous in these areas due to lower susceptibility to human harvest and less conflict with livestock production. Conversely, the impacts of wolf restoration can be modest for elk herds spending winter in open, low-elevation valleys where elk are less vulnerable to predation due to a more homogeneous landscape with shallower snow. Also, predators tend to be less numerous in these areas due to high susceptibility to harvest and culls after livestock depredations. Over time, higher survival and recruitment in lower elevation valleys should lead to an increased proportion of elk spending winter in these areas. Indeed, a review of migratory elk populations throughout the Greater Yellowstone Ecosystem indicates broad-scale distribution shifts are occurring, with a higher portion of elk spending winter on lower-elevation ranges.
Certainly, many factors other than wolves, including human harvests, drought, and predation by bears and mountain lions, have had substantial effects on elk populations living in the Greater Yellowstone Ecosystem. However, the restoration of an additional top predator was a transformational event that eventually facilitated and maintained a substantive decrease in elk numbers and many other indirect effects to decomposers, other herbivores, predators, producers, and scavengers throughout the ecosystem. As a result, this bold restoration effort also led to a substantially improved understanding of the role of apex predators in terrestrial communities.
Pelican Valley & Mollie's Pack
by Douglas W. Smith, Travis Wyman, Daniel R. Stahler, & Daniel R. MacNulty
Unlike the northern range, wolf work in the interior can be tough. So tough, it was originally envisioned as aerial monitoring only, which is how most wolf studies accomplish the task of remote study. Bob Garrott and his team had successfully mastered ground data collection in the Madison-Firehole river drainages, but work elsewhere seemed infeasible. Most of these other locations were far from roads.
Then in 1998, the idea of working in Pelican Valley came up—a long famous place and what some would call the "heart" of Yellowstone. Situated in the middle of the park and vital to much wildlife, it certainly fits. A pack of wolves lived there, named Mollie's pack (after the late Director of the U.S. Fish and Wildlife Service who held her ground on wolf reintroduction despite criticism), and they seemed unique. Initially there were many hurdles to overcome; one was the uncertainty of success and, perhaps more importantly, some significant safety issues. The plan would entail camping out for two weeks in winter without a shelter and observing from a high point above the valley or an observation point (OP). Before that though, the first task was to see if the wolves were even in the valley enough to make observation worthwhile. A quick plotting of radio locations revealed wolves were in the valley a significant portion of the time, especially in late winter. We decided we might just have a project!
Clearing this new research with rangers and administrators took time. The administration's desire was for us to use Pelican Springs cabin, but if we did we would not be able to see the valley. A daily ski across the valley would disturb any wildlife we wished to observe. The decision was made to camp at the OP above the valley and stay put–no wildlife disturbance. In 1999, with scant winter equipment, we did just that. It proved to be a wise decision.
Once the hardship and struggle of hauling two weeks of gear across Pelican Valley was accomplished, with subpar equipment (especially sleds) and up a large hill, major scientific insights followed. At first we just watched and gathered behavioral data.
Quickly, we realized there were few elk, and later no elk due to the harsh environment, so Mollie's pack wolves had adapted to eating only bison in the winter. Quickly the story became about wolves and bison. Formidable prey compared to elk, killing bison presented a different challenge to the wolves. Several bison kills were witnessed, and a few were filmed, wetting the appetite to learn more and how their strategy differed from killing elk. Bison commonly stand their ground, whereas elk commonly flee–a major difference we noticed right away. Wolves facing a 1,000-2,000 pound animal presented a unique set of problems; taking the bison head on was out of the question. Wolves would have to work the environment to their advantage. Watching and waiting for the right moment to attack was critical. Wolves seem to have all the time in the world, so they were never in a hurry and waited. When they decided to attack, they chipped away: attack, wound, and wait; attack, wound, and wait...Using this strategy, some kills took up to nine hours. The wolves also had to use terrain to their advantage. Wind-blown hills had no snow and the bison favored such terrain for better footing; between the hills were troughs that collected snow, so the wolves favored these areas for attack as the snow hampered bison defense.
Confrontation between bison and wolves was stunning to watch;rarely observed nature in action. Pressuring bison for hours, wolves gradually drove them into deep snow and then jumped on them, many wolves at times, hanging from muscle and hide by their teeth. Once on firm ground, the bison shook the wolves off like water droplets, finally swinging their horns at them. Seemingly undeterred, the wolves waited for their next chance, or inexplicably left the bison, sensing an unseen cue or sign that made them abandon the effort.
At times, persistence paid off and a kill was made. But then another problem cropped up: who gets the spoils? This time of year a large bison carcass is a food bonanza. Every critter far and wide came in to grab what they could: weasels, foxes, coyotes, ravens, eagles, magpies, and grizzly bears. Once bears arrived it was over for the wolves. The carcass now belonged to them. Virtually every documented carcass in Pelican Valley from March through October attracted grizzly bears. It was not a matter of if but when, and the wolves had to grab as much meat as they could before the bears moved in. Up to 24 bears have been observed on one wolf kill at the same time. In March during our study, these carcasses became small "eco-centers" and most of the action in the valley occurred here.
Through time, our science became more sophisticated with fixed locations to observe from at regular intervals throughout the day, in addition to opportunistic observation of behavioral interactions. These observations indicated bison organized themselves differently when wolves were present in the valley versus when they were gone. Bison stayed closer to areas of good footing when wolves were around, and straying into riskier areas to forage when wolves were absent. Eventually the bison cow/calf groups left, probably because of wolf pressure, leaving about 40-80 hardy bulls for the wolves to deal with. So the valley changed, but in a vigorous way, and in fact gained some with the addition of wolves as they provided the carcasses that life hinged on in late winter.
Of course, we changed too. We purchased better equipment, especially sleds and light teepees that made living there for two weeks tolerable. We also dug into the snow and made caves to sleep in, and other years cut snow blocks with a saw to make an igloo. Crawling in either shelter, you could escape the near-constant roar of the wind or at night be oblivious to a foot of overnight snow that collapsed tents. For 16 straight years we managed the storms and wind that made Pelican Valley famous; and like with all things, we told stories, building memories that grew into a fondness for the place. After these years of study, it was felt our objectives had been achieved, so we turned things back to the valley, to the animals and plants that endure this harshness in the heart of Yellowstone.
Wolf Management: Den Closures, Habituation, & Hunting
by Douglas W. Smith, Kira A. Cassidy, Daniel R. Stahler, Erin E. Stahler, & Rick McIntyre
Although wolf reintroduction to Yellowstone National Park (YNP) was a very deliberate management action, and initially almost all of our work was management related, most of the wolf program today is monitoring and research. One reason for this is that there is almost no human safety threat posed by wolves. Why this is so is not entirely clear, but wolves seem to be naturally wary of people, or perhaps centuries of persecution have made them this way. Wolves are also less interested in human foods than other carnivores because they do not eat daily and are accustomed to the feeling of hunger. Therefore, it does not drive their behavior. Wolves commonly go days and sometimes a couple weeks without eating, so they do not become desperate for a meal. Wolves will feed on garbage, but when doing so are usually still wary of people (until conditioned). Overall, wolves are probably the least dangerous large carnivore. This does not mean we are not alert to the occasional wolf that may have received human food and is gradually losing its fear of people. Rather our management is not dominated by human-wolf interactions. Mostly we are focused on the flip-side, managing wolves so they are adequately protected from people—the other side of the National Park Service mandate. With Yellowstone being probably the best place in the world to view free-ranging wolves, much of our wolf management is geared toward people.
Protection of Wolf Dens & Rendezvous Sites
First and foremost are dens. Research has shown that wolves can be sensitive to human disturbance in the first six weeks after pups are born (Frame et al. 2007). Studies that have experimentally disturbed wolves during this time period found that sometimes the den will be relocated (Frame et al. 2007). Any time young pups are moved there is a risk of mortality, so this is the time period we try to protect wolves the most. The original Federal Special Regulations recommended protecting areas around dens until June 30. After this date, pups are mature enough to withstand disturbance and den relocation. In Yellowstone, we have only used this date as an approximate guideline because some circumstances are unique to a park. For example, we keep a den in Lamar Valley and a rendezvous site (the above ground site that wolves use after a den) in Hayden Valley, both popular viewing areas, closed for longer not only to protect wolves but also to allow for visitor enjoyment—a key national park policy objective. If we opened these areas, many people, with no ill intent at all, would approach the wolves hoping to see or photograph one, especially a pup, which would displace the wolves and make them less visible afterward. Despite our protection, many people mistakenly walk into the Hayden Valley rendezvous site. Packs that use this area have low pup production. The correlation between few pups and high disturbance is a concern. This possible relationship has caused us to keep the area closed after the recommended June 30 deadline, wanting to err on the side of resource protection. Finally, remote dens are left unmanaged mostly because it is unlikely they will be disturbed.
Although the risk of human injury from a wolf is almost zero, it is not actually zero, so another management activity is to keep wolves and humans apart. Our best tool for this is enforcing the park regulation that people must stay 100 yards from a wolf (or bear), and if the wolf moves closer, then the person must maintain this distance. This will keep wolves and people safe and prevent habituation.
When wolves and people do interact, Mark McNay, formerly of the Alaska Department of Fish and Game, summarized the outcomes for North America during the 20th century (McNay 2002). He found only 19 cases of aggression of non-rabid wolves toward people from 1900- 2001; these encounters excluded 20 incidents involving dogs or defensive behavior (protection by wolves of other wolves). There were no fatalities. Since 1969, there were 18 aggressive encounters and 11 of them involved habituated wolves (McNay 2002). Clearly, habituation needs to be prevented. How is this done? Keeping wolves and humans apart is one way, but keeping human food from them (similar to bears) is another.
Since McNay's study there have been three fatalities in North America, but the circumstances were similar. Wolves lost their natural fear of humans through exposure. It appears for wolves to attack humans they must first become familiar with them, lose their natural fear, and then attack, although this is very rare. This is not the case with other carnivores which may attack a person on their first encounter. In YNP, we have removed two wolves proactively because they had probably obtained human food and were exhibiting inappropriate behaviors (e.g., closely approaching humans). One wolf chased a person on a bicycle and a motorcycle. Another wolf walked up to several people and closely inspected anything they had in their hands (i.e., thinking it was food). In another situation this wolf tore apart a back-pack looking for food. Unfortunately, aversive conditioning did not work, so we removed them.
Before removal, park staff try everything they can to discourage habituated wolves (YNP 2003). Typically this means aversive conditioning. Confused with hazing, which is opportunistic negative reinforcement, aversive conditioning targets individuals. We start gently and escalate if there is no response--yelling, horns, and sirens first, graduating into cracker shells and ending with nonlethal bean bags or rubber bullets fired from a shotgun (YNP 2003). Initially, this was recommended against. We subjected these methods to professional review before we formulated our policy, and some comments were "don't bother with aversive conditioning, it doesn't work, just kill the wolf." This has been done in other places because wolves are so common and removing a tame one will have little impact on the population. This is true, but in a park setting we chose to respond differently. And to some people's surprise, we have found aversive conditioning to be effective.
Since wolf reintroduction, 55 wolves in 127 incidents have exhibited habituated behavior (this is different from McNay's "aggressive" category). Thirty-eight of these wolves were aversively conditioned 76 times; 49% of these actions immediately changed the wolves' behavior. Another 42% were probably successful, but not clearly so; because in eight cases we did nothing and the wolf never approached a person again. Finally, in 13 incidents the wolf either died or disappeared within six months of the incident. This is strong evidence that aversive conditioning does change habituated wolf behavior.
Where was habituation most common? Of the 127 events, 102 (80%) were on park roads, 14 (11%) in developments, and 11 (9%) were in the backcountry. Clearly the roads are a hot spot, and this is where the park has focused outreach and staff to avoid human-wolf contact. Most roadside encounters were in the spring/summer (May, June, and July; figure 1). This time period is also when pups become yearlings and many habituated wolves are young. Young wolves (yearlings in particular) have a lot of free time since older adults typically hunt and care for pups. Although yearlings do care for pups occasionally, they have less investment in pups so they take care of them less. They also explore and range widely and have a strong curiosity—which can lead them to humans. Some have compared this to human teenager behavior. The lesson is to be alert to young wolves in spring. Further, 54% of our habituated events have been confined to four packs, all of them road-adjacent packs: Canyon (23 incidents, 18%; pack lives in Hayden Valley area), Lamar Canyon (22 incidents, 17%), Druid Peak (13, 10%; Lamar Valley area combined with Lamar Canyon is 27% of incidents), and Hayden Valley (11 incidents, 9%; with Canyon, this is 27% of the incidents in the Hayden Valley area). Lamar and Hayden valleys are both open valleys with roads where visitors commonly encounter wolves. Arguably, these two valleys have more wolf-human proximity than any other location in North America. Certainly there are other places where wolf-human contact is more acute, but there might not be any other place where year-in and year-out wolves and humans coexist as much. Importantly, no one has been hurt, some aversive conditioning has occurred, and it has mostly been successful. Only one wolf (restricting the area to only these two valleys) has been removed.
Wolf Hunting Outside of Yellowstone
The last wolf management issue of concern is packs that primarily live in YNP, but wander outside of the park during the hunting season. Some of these wolves are legally harvested (figure 2). This is a difficult issue because the wolves are not aware of the boundary or the differing management objectives. These objectives are not mutually exclusive, but they are not the same. A gradual transition of regulations from inside the park to outside the park was necessary. Also, park wildlife that spend most of their time inside YNP are not conditioned to human hunting and are less wary and possibly more vulnerable to human take—wolves being one example. Many compare wolves to elk that are also cross-boundary, but migratory elk spend about half the year outside the park so they learn to be wary. To accomplish this goal the states have created small hunting units with quotas next to the park. Montana has created two special hunting districts north of Yellowstone that limit the number of wolves that can be taken. Wyoming has also created relatively small hunting units next to YNP which allows for precise control of harvest. Both of these actions have limited the harvest of wolves that primarily live within YNP (figure 2). This regulated hunting will ensure that human-take outside of YNP will not impact the wolf numbers inside the park. These actions do not control what wolves will get harvested; but it does reduce the chances that a commonly observed wolf, cherished by the public, will be removed. It also preserves the social fabric within wolf packs by not removing too many wolves of high social rank, thereby preserving the natural functioning of the pack and population dynamics. Overall, having wolves protected within YNP and harvested in a sustainable fashion outside the park is good for wolves in the long run. Such a mosaic of management practices protects wolves in some areas and limits them in areas of human conflict, which may reduce human dislike of wolves. Wolves are a polarizing issue for the public. Controlling problem wolves and hunting some of the others enhances acceptance of having them on human dominated landscapes. This is a foundational premise for all state agencies, and although questioned by some social scientists (Treves and Bruskotter 2014), has quelled some of the controversy over wolf restoration to the West. In the park, our mission is balancing wildlife protection with human enjoyment. It has taken some time; but we have achieved the proper balance for wolves to function as they should, and for people to observe and enjoy seeing wolves without harming them in a natural and wild setting.
Frame, P.F., H.P. Cluff, and D.S. Hik. 2007. Response of wolves to experimental disturbance at homesites. Journal of Wildlife Management 71:316-320.
McNay, M.E. 2002. Wolf-human interactions in Alaska and Canada: a review of the case history. Wildlife Society Bulletin 30: 831-843.
Treves, A., and J. Bruskotter. 2014. Tolerance for predatory wildlife. Science 344:476-477.
Yellowstone National Park. 2003. Management of habituated wolves in Yellowstone National Park. Internal Report. Yellowstone National Park, Mammoth, Wyoming, USA.
by Douglas W. Smith, Daniel R. Stahler, Matthew C. Metz, Kira A. Cassidy, & Erin E. Stahler
Winter study is the term used to describe our winter wolf research program. The study was mostly adapted after the winter wolf research project in Isle Royale National Park in Lake Superior, Michigan (Allen 1979) and partly after another project in the Brooks Range of Alaska (Dale et al. 1995). Historically, most wolf work has been done in the winter: packs roam together (unlike summer), leave easily observed tracks, and the white, snowy background make wolves easy to see from an airplane. Planes equipped with skis could land on lakes making huge swaths of otherwise inaccessible country accessible. The Isle Royale research program established a winter study starting in 1959 which continues to this day, and named it simply "Winter Study" (Allen 1979). Researchers flew out to the island in early January (over Lake Superior and sometimes open water) and stayed for almost two months. They flew every day, weather permitting, looking for wolves, counting moose, and searching for moose killed by wolves. The results included annual estimates of wolf and moose numbers plus wolf predation rate–the proportion of the moose population killed by wolves annually (Mech 1966, Peterson 1977, Vucetich et al. 2011). This work was the foundation for Yellowstone's winter wolf research.
Yellowstone is not Isle Royale. We are not an island, and we live on site year-round. There may be other times of year when data on wolves should be gathered. When designing our study we looked at another study in the Brooks Range of Alaska, where like Isle Royale they tried to string together many consecutive days flying, but varied the season and flew 30 consecutive days (weather permitting; Dale et al. 1995). The published results provided a good picture of how many and what kind of prey the wolves killed. We combined the two strategies: fly every day possible for two 30-day periods in early and late winter. We chose early and late winter because it was well known that wolf kill-rates change through winter, and book-ending winter would give us a good picture of the entire winter. So, since 1995 the Yellowstone Wolf Project has studied wolves for two, 30-day periods from mid-November through mid-December and again during the month of March. This has been our foundational research program and has generated some of our most important data. We have consistently done it every year, the same way; so data gathered are comparable, and changes can be tracked through time effectively.
This annual program has become a tradition for the Wolf Project, an annual ritual marking the passing of the seasons and years. Several hundred technicians have participated; the work is grueling, but some say it's the best experience of their lives. The work begins at first light to last light every day with one day off per week, and in March that can make for some very long days. One other difference for Yellowstone is we combine aerial surveys with ground surveys. Because Isle Royale and the Brooks Range are so remote, they have no road system, but we do. Using the road system, our design has three groups of volunteers, each assigned to a pack for 30 days. Their job is to be experts for that pack by keeping them in sight from the road. Simultaneously, when the weather permits, an airplane flies over all packs. This allows us to get a subset of data from three packs monitored both ways and monitoring by air only for the rest. In this way we can estimate what we are missing from air-only packs.
Importantly, winter study is fun. The park empties out, and it seems the wolves come alive. Winter is either moving in or out, and it's a wonderful time to be out every day. We have expanded into summertime studies now. As with time, all things change, and wolf research has too; but it would be hard to replace the tried and true winter-work, and we're happy for that.
Allen, D.L. 1979. Wolves of Minong: their vital role in a wild community. Houghton Mifflin Company. Boston, Massachusetts, USA.
Dale, B.W., L.G. Adams, and R.T. Bowyer. 1995. Winter wolf predation in a multiple ungulate prey system, Gates of the Arctic National Park, Alaska. Pages 223-230 in Ecology and conservation of wolves in a changing world. L.N. Carbyn, S.H. Fritts, and D.R. Seip, editors. Canadian Circumpolar Institute, University of Alberta, Edmonton, Alberta, Canada.
Mech, L.D. 1966. The wolves of Isle Royale. Fauna of the national parks of the United States: Fauna Series 7. Washington, D.C., USA.
Peterson, R.O. 1977. Wolf ecology and prey relationships on Isle Royale. National Park Service scientific monograph series, number eleven. Washington, D.C., USA.
Vucetich, J.A., M. Hebblewhite, D.W. Smith, and R.O. Peterson. 2011. Predicting prey population dynamics from kill rate, predation rate and predator-prey ratios in three wolf-ungulate systems. Journal of Animal Ecology 80:1236-1245.
Piece by Piece: Wolf Project Sample Collections Go Beyond Ecology
by Kira A. Cassidy, Deb Guernsey, Blaire Van Valkenburg, Quinn Harrison, Brenna Cassidy, & Erin E. Stahler
Looking closely at a wolf skull can provide a glimpse into the life history of Canis lupus. The large teeth draw your eye first, especially the four canines—perfectly aligned, built for grasping prey and tearing muscle and tendon. A maze of nasal bones leads up to front-facing eye sockets—empty spaces once home to the hardwiring of a wolf's strongest senses: olfaction and vision. And then the sagittal crest, that spike at the skull's peak, that provides purchase to large masseter muscles for chewing through a femur to get the marrow, or holding on to a running, kicking elk or bison.
On top of those features evolution has bestowed upon the wolf, each individual also has a story to tell. Injuries, tooth wear, and infection (if manifested in the skull) can give details about the hardships a wolf encountered during its life. Following advice from Ron Nowak (a biologist specializing in wolf morphology who helped streamline the Canis lupus sub-species debate) the Yellowstone Wolf Project added wolf skulls to the list of biological samples collected. The collection has expanded our knowledge about gray wolves over the last two decades and continues to grow in size and depth.
These skulls are being used to assess rates of tooth wear and tooth fracture to compare with wolf populations from the past and other carnivores such as lions. During times of food stress, carnivores tend to ingest more of a carcass by chewing and consuming bones. This practice leads to more rapid tooth wear and breakage; measuring it can help detect changes in carcass consumption in the last 20 years. When wolves were first introduced to Yellowstone, elk were very abundant. Consequently, wolves did not have to put extra effort into chewing bones to obtain nutrition. However, the number of elk has decreased and as a result the wolves may be finishing carcasses more completely and wearing their teeth more heavily. By tracking shifts in wolf tooth wear patterns through time, we can gain insights into how this large carnivore is affected by changes in prey availability. The effort to clean and process wolf skulls and teeth is incredibly time consuming and often underappreciated. Paleopathologist Sue Ware, for example, has cleaned, measured, and examined the skulls for evidence of cause of death, injuries, and abnormalities, spending countless hours in the lab processing and taking measurements.
In addition to skulls we also collect other samples—on both live and dead wolves—with the goal of answering specific biological questions. Genetic samples (either through whole blood collection or tissue samples) are sent to the University of California at Los Angeles, and have been used to construct a detailed pedigree of Yellowstone wolves. This information has been used to test the genetic health and viability of wolves in the northern Rocky Mountains (vonHoldt et al. 2008, 2010), to explore the process of domestication (Anderson et al. 2009, Janowitz Koch et al. 2016), and to investigate the effects of genes on heritable behaviors and traits (Hedrick et al. 2014, Schweizer et al. 2016).
Blood samples can reveal exposure to diseases by testing the serum for specific antibodies. These tests confirmed canine distemper virus (CDV) outbreaks coinciding with years of low pup survival and have provided insight into other, less fatal diseases, such as canine parvovirus, canine adenovirus, and canine herpesvirus (Almberg et al. 2009). In addition to serum evidence to the wolf's past, we collect whisker samples (1-2 per wolf during either capture or at death). These samples are analyzed to measure the isotopic signature of the wolf's diet from the previous six months. The species wolves prey on each have their own isotopic signatures based on the carbon:nitrogen ratio in their diets and pass this on to the wolves. This information has been an important addition to prey-selection work, especially with remote packs observed infrequently, as some packs seem to supplement their mainly elk-based diet with different species (e.g., deer, bison, and beaver).
Wolf scats have been collected to answer specific research questions related to prey-selection (Trejo 2012), disease exposure (Almberg et al. 2009), and genetic analysis (Ausband et al. 2010). These projects have helped hone data collection and analysis methods, including prey fur identification in a laboratory setting and viral stability in different climatic environments. During capture operations we take a variety of measurements of the wolf's body, including length, chest and neck circumference, and weight. These measurements have been used to model body mass changes between the sexes by age (MacNulty et al. 2009, Stahler et al. 2013).
With changes in the northern range elk herd size and composition, it has been essential to not only collect samples from wolves but also their main prey species. We visit many ungulate carcasses (killed by wolves and other causes of death) to collect a variety of samples. A tooth is used to determine the age of the prey and has helped confirm wolves often target the very oldest cow elk and youngest calves. Prey condition changes throughout the year with almost all elk in excellent shape in the late summer and fall as evidenced by high fat content in their femur marrow; however, by late winter many elk are in much poorer condition. By collecting marrow samples all year, we can track the seasonal health of the herd to map the average and compare with those individuals killed by wolves. Collected samples from prey killed by wolves often have lower fat content than would be expected for that time of year.
Some samples can even be used to back-fill historic data on the health of the elk herd (Wright et al. 2003). We collect a metatarsus from each elk, as it is one of the last long bones to develop while the individual is still in utero. The development of this bone correlates well to the health of the elk's mother and can be affected by the mother's age but also the weather patterns and snow depth during her pregnancy and the previous summer's forage quality. To date, the Wolf Project has collected over 2,600 elk metatarses.
Many of the samples collected make their way from the field to holding freezers and then on to different labs throughout the country. However, the wolf skulls are housed in the park's Heritage and Research Center Museum. Given Yellowstone National Park's 144 years, and the ecosystem's thousands-of-years-long chronicle with wolves, it only seems fitting that the wolf skulls collected in Yellowstone represent this historic spirit. Over 160 wolf skulls are currently preserved in the museum for everyone from students, researchers, and artists to enjoy. Each of the thousands of samples collected is unique and represents a concerted, sometimes exhausting, effort put into the collection process, with the end goal to advance the scientific knowledge and to reveal the intrinsic value of Yellowstone wolves, past and present.
Almberg, E.S., L.D. Mech, D.W. Smith, J.W. Sheldon, and R.L. Crabtree. 2009. A serological survey of infectious disease in Yellowstone National Park's canid community. PLoS ONE 4:e7042.
Anderson, T.M., B.M. vonHoldt, S.I. Candille, M. Musiani, C. Greco, D.R. Stahler, D.W. Smith, B. Padhukasahasram, E. Randi, J.A. Leonard, C.D. Bustamante, E.A. Ostrander, H. Tang, R.K. Wayne, and G.S. Barsh. 2009. Molecular and evolutionary history of melanism in North American gray wolves. Science 323:1339–1343.
Ausband, D.E., M.S. Mitchell, K. Doherty, P. Zager, C.M. Mack, and J. Holyan. 2010. Surveying predicted rendezvous sites to monitor gray wolf populations. The Journal of Wildlife Management 74:1043-1049.
Hedrick, P.W., D.R. Stahler, and D. Dekker. 2014. Heterozygote advantage in a finite population: black color in wolves. Journal of Heredity 105:457–465.
Janowitz Koch, I., M.M. Clark, M.J. Thompson, K.A. Deere-Machemer, J. Wang, L. Duarte, G.E. Gnanadesikan, E.L. McCoy, L. Rubbi, D.R. Stahler, and M. Pellegrini. 2016. The concerted impact of domestication and transposon insertions on methylation patterns between dogs and gray wolves. Molecular Ecology. doi:10.1111/mec.13480 MacNulty,
D.R., D.W. Smith, J.A. Vucetich, L.D. Mech, D.R. Stahler, and C. Packer. 2009. Predatory senescence in ageing wolves. Ecology Letters 12:1347-1356.
Schweizer, R.M., B.M. vonHoldt, R. Harrigan, J.C. Knowles, M. Musiani, D. Coltman, J. Novembre, and R.K. Wayne. 2016. Genetic subdivision and candidate genes under selection in North American grey wolves. Molecular Ecology 25:380-402.
Stahler, D.R., D.R. MacNulty, R.K. Wayne, B. vonHoldt, and D.W. Smith. 2013. The adaptive value of morphological, behavioral, and life history traits in reproductive female wolves. Journal of Animal Ecology 82:222-234.
Trejo, B.S. 2012. Comparison of two methods used to characterize the summer diet of gray wolves (Canis lupus). Thesis. Humboldt State University, Arcata, California, USA.
vonHoldt, B.M., D.R. Stahler, D.W. Smith, D.A. Earl, J.P. Pollinger, and R.K. Wayne. 2008. The genealogy and genetic viability of reintroduced Yellowstone grey wolves. Molecular Ecology 17:252-274.
vonHoldt, B.M., D.R. Stahler, E.E. Bangs, D.W. Smith, M.D. Jimenez, C. M. Mack, C.C. Niemeyer, J.P. Pollinger, and R.K. Wayne. 2010. A novel assessment of population structure and gene flow in grey wolf populations of the Northern Rocky Mountains of the United States. Molecular Ecology 19: 4412- 4427.
Wright, G. 2003. An analysis of the northern Yellowstone elk herd: population reconstruction and selection of elk by wolves and hunters. Thesis. Michigan Tech University, Houghton, Michigan, USA.
Last updated: July 6, 2016