Wildlife Dimension of Elk Management in Yellowstone

Elk and Brucellosis
Natural History of Elk
Herd Structure of Elk


The Issue

When it comes to elk management, one of the many challenges park managers face is Brucellosis.    In the 1930’s it was first discovered that elk herds in Yellowstone National Park and at the National Elk Refuge in Wyoming carry the disease.  The problem lies in the fact that ungulates in the Greater Yellowstone area (primarily bison and elk) pose the risk of spreading Brucellosis to cattle when they seasonally migrate outside the park to seek better forage.  Although the transferring of the bacteria from wildlife to cattle has been shown to occur in a laboratory setting, there have been no documented cases of it actually occurring in nature.

What is Brucellosis?

Brucellosis is a bovine-related disease that is caused by transmission of the bacteria Brucella abortus.  It is transmitted by the ingestion of reproductive fluids from an infected female, and causes females to spontaneously abort their fetuses.  In the Greater Yellowstone area, it is carried by both bison and elk.  Both species experience the same effects of the disease, but at different rates.  In studies done by the Wyoming Game and Fish Department, it was found that when inoculated with the disease, elk experience a 50%-70% abortion rate while bison experience a 100% abortion rate.  Because transmission is more likely to occur from late pregnancy to several weeks after birth, they also experience different periods of possible transmission.  For elk, the period of possible transmission is between February and mid-June.  For bison, that period is slightly longer.  For detailed information on the effects of Brucellosis, click here.


Transmission at the Feeding Grounds

It is believed that in Wyoming, Brucellosis is limited to the elk in the western part of the state, near the feeding ground complex.  There are 23 elk feeding grounds in the state of Wyoming that feed an average of 22,000 elk.  Feeding grounds are where elk migrate to in the winter season when forage is limited and where they are supplemented with food from park officials.  The general purpose of feeding elk in a time when food is generally limited is to reduce winter mortality and increase the hunting stock.  Elk at these grounds are highly concentrated and are therefore at increased risk of transmitting Brucellosis, especially since females are late in their pregnancy during winter.  Therefore, feed ground elk have a high rate of infection.  The disease has been detected by blood tests in 18 of these feeding grounds (equaling about 18,000 elk).  At highly infected feeding grounds, elk herds may lose up to 12% of their reproductive potential.  To make matters worse, it didn’t take long for bison to discover the feeding grounds.  Bison commonly feed at the grounds, thus increasing their chances of getting infected with the disease.


Why are bison more of a concern than elk?

Although bison and elk both carry Brucellosis and both can transmit the disease to cattle, bison have become more of a concern.  More time, money, and effort has been put into preventing infected bison from entering cattle grounds and ranches surrounding the Greater Yellowstone area.  A few reasons for this are:

  • Bison have a longer period of possible transmission because of longer breeding and calving periods.
  • Bison have a higher abortion rate than elk when artificially inoculated.
  • Bison herds experience more contact with each other than elk herds, especially during pregnancy, birthing, aborting, and calving.  Brucellosis is more readily transmitted.
  • Vaccines are less effective in bison than in elk.
  • Elk have a higher economic value (due to hunting)
  • Some people may think that because bison look more like cattle than elk do, they have a higher chance of spreading it to them.


The livestock issue

Cattle ranchers surrounding the Greater Yellowstone area are concerned that bison and elk will transmit Brucellosis to cattle when they wander outside the park.  Usually bison and elk do not come in close contact with cattle, but because the disease is transmitted through reproductive fluids, they can still pose a risk.  Fluids containing the bacteria can last for several days and even though the infected female may not be present, she may have left behind fluids on the ground or hay for a cow to later ingest.  However, transmission of Brucellosis to cattle is highly unlikely and there have been no documented cases of it occurring.  But this is not to say that it cannot happen.  Among cattle the disease is transferred more readily.  Therefore, infection of just one cow may lead to infection of the entire herd, which could have serious impacts for not only the individual producer, but also the entire livestock industry for that state.  A state’s loss of Brucellosis-free status can have serious economical consequences.


What is Being Done?

A vaccine has been developed for preventing Brucellosis transmission in elk.  The vaccine is injected via darts to female elk at the feeding grounds, and works to reduce abortions in elk and to reduce transmission rate as well as increase individual resistance to the bacteria.  Calving success for a vaccinated elk is double that of an unvaccinated elk.  However, it will take several years for the vaccine to significantly reduce Brucellosis in elk herds.  This is because the vaccine has no effect on animals that are currently infected.  Because the lifespan of an elk is relatively long, it may be several years before all infected individuals are eliminated from the population.  The goal of the vaccination program is to obtain at least a 90% vaccination rate.  Expected results would be increased calf production in previously infected feed grounds and also significant reductions in transmission of Brucellosis to cattle.


An effective vaccine has yet to be developed for Brucellosis control in bison.  To find out what’s being done to prevent bison sources of transmission to cattle, click here.

Natural History of Rocky Mountain Elk

Elk are the largest members of the deer family who are genus Cervus and species elaphus. A full grown male, elk referred to as a bull is comparable in size to a horse measuring five feet tall and weighing up to eleven hundred pounds.  Female elk are referred to as cows and though may be comparable in size to bulls, usually are shorter and weigh less.

Elk are ungulates, which is the Latin word for hoof and referrers to animals with hoofs.  Hoofed mammals are divided into two different orders, classified by their number of toes.  The Artiodactyls are those with an even numbered of toes such as elk, deer and bison, where the Perrissodactyla are those having an odd number of toes such as horses and elephants. 

a)       Rocky Mountain Elk Life Cycles

Elk are mammals driven by instinct.  Instinct is something that compels a species to partake in activities that eventually promote their survival and reproduction.  Instincts that promote elk’s reproductive fitness include seasonal movements, choice of calving grounds, the selection of social groups and areas for feeding and resting.  With a life, which is estimated between eight and twelve years, elk are part of many processes that through instinct propagate their species survival.




Each spring, increasing daylight hours elevates the hormone testosterone in male bulls. The increase of testosterone in their bodies prepares their bodies for the upcoming mating season.  One of the first physical signs that reveal the bulls preparation for the mating season is the growth of antlers, which begin bony bumps on the male elk’s skulls. Antlers grow succeeding larger as a bull matures. The antlers complete their growth and in September.  At this time the velvet, which supplied the growing antlers with blood and nutrients dries up and is rubbed off against trees and brush.  By rubbing off of the velvet, the antlers become ready for display.  A second year bull usually has un-branched antlers called prongs, where a mature bull can have eight or more branches on his antlers, with a spread of four feet and weighing up to forty pounds.  Intense feeding coincides with this time of year as fat reserves are reestablished and migration toward the more nutritious food in the higher elevations begins. 


Cows often begin their migration before they give birth, which generally coincides with when the most nutritious food is available.  The young are born in mid May through early July, depending on where the elk live.  When a cow is ready to give birth, she leaves the herd and rejoins in a couple weeks once the calf is strong enough to keep up with the herd. During the spring and early summer, as new shoots, leaves and buds of many plants emerge elk can move appreciable distances in search of the new highly nutritious plant growth.

The Fall

            The Rut and Harem Structure

In Autumn after a summer of intense feeding the elk mating season referred to as the Rut begins. At this time the animals move to lower elevations, segregate into separate groups and establish mating groups called Harems.  A harem is usually established during the months of September through October and mature bulls move in among groups of cows and calves. Harems are usually smaller than the large cow and calf herds of the summer as well as lack the male yearlings who are driven off by more mature bulls. Younger bulls are able to bread by their second summer and often try to enter the harems, but seldom get a chance due to competition by bigger bulls.  The biggest bulls in peak physical condition ranging from about six to eight years old are the individuals who usually take charge of the harems.

Bull Dominance

To show their dominance  elk also bugle.  A bugle is a high pitched whistling followed by grunts that allow a bull to let his presence be known.  As a bull gets older, his bugle gets louder and more intense.  This allows a bull’s size to be recognized through his bugle and solicits his presence. Bulls also show off their antlers and bodies to each other, sizing up each other’s fitness and ability to defend their right of mating.  Bulls also engage in strength battles with each other to prove dominance and strength by locking antlers and shoving each other back and forth. Young males usually retreat upon an encounter with a mature bull, since their chances of winning such a battle are slim. On the other hand, bulls equal in size typically confront one another and display their dominance by bugling, thrashing the ground with their antlers and finally ensnare each others antlers and impel each other. Fighting is a show of strength, not a battle to the death, and is done for the ultimate reason of mating. 


Once a bull has shown his dominance, and has control of a harem he anticipates receptive cows. A cow is receptive for mating less than twenty-four hours, so when she is estrus, or in heat, the bull elk must be alert and ready since the cow won’t be willing to mate again until her second, third or fourth estrus cycle, which are twenty days apart. Once the mating is complete, the rut comes to an end.  At this time harems disband and the cows regroup. Bulls of all ages once again gather together in bachelor groups and dispersing from the females until the next spring.  For the remainder of the fall, both sexes resume eating while they prepare for the coming winter.

The Winter

            Food Limits

Winter is a time of limited food resources and harsh weather conditions. During this time elk herds devour exposed vegetation, which often is limited by the depth of snow.  When the snow gets too deep and the weather becomes too harsh fat reserves gained in the spring and summer are used to survive.

            Winter Mortality

The mortality of herds is usually large this time of year, but is often dependent upon the severity and length of the particular winter. The extreme conditions of winter usually eliminates the weak, young old and malnutitoned individuals first, leaving the healthier ones for the coming year.

b) Movement and Migration:

Elk occur widely but not uniformly throughout habitats.  The presence, absence and abundance of elk in various locations are determined by many factors.  Among the more important factors of elks location are the amount of moderately level and low-elevation habitats present, the availability of preferred food plants, the timing and severity of storms and the depth and persistence of snow.  Elk distribution also tends to change over time, as vegetation changes due to natural disturbances such as floods and fire, as well as human disturbances.  Though elk’s movements can vary from year to year due to the above factors, elk typically follow a seasonal pattern of movement inhabiting lower elevations in the winter and higher elevations in the summer.

Summer Migration

Lengthening days, warming temperatures and melting snow are all signals that trigger the start of the spring movement for elk.  As the snow begins to melt in the lower elevations, elk progressively migrate to higher elevations discovering the newly exposed fresh vegetation.  The migration from the lower elevation to the higher elevations that transition the elk from their sparse winter diet into their intense summer feeding, varies in length.  The length of the migration depends on where the elk live and the present food potential in their current location.  The actual migration can range from a few days to a couple of months.  During this time elk eat an extraordinary amount of food, as they replace their depleted fat reserves from the previous winter.  Consuming up to fifteen pounds of vegetation daily an elk spends the majority of its time eating while present on their summer feeding grounds.

Winter Migration

In autumn after a several months of intense feeding in the higher elevations, falling temperatures, snow piling up in the high country and more frequent storms signal the elk to leave their summer range in the high elevations and begin their decent to the lower elevations.  In the lower elevations the elk encounter their winter range.  The winter range is where the elk reside until warmer temperatures call them back to the higher country.  On the winter range, the elk try to locate areas with less snow or no snow at all where food is easily accessible.  In the winter elk usually eat less and rest more.  Two popular theories explain this behavior.  One theory consider less movement and more rest as a means of conserving energy, while others believe that since the food consumed in the winter is tougher for rest is required for digestion.

c) Habitat Requirements:

Animals require food, water, shelter and space to suit their specific needs.  Elk also require these things, which will be discussed below.


An elk, like any member of the deer family is an herbivore.  An herbivore is one whose diet is composed entirely of vegetative material.   Though elk only consume vegetation, they are quite selective of the parts that are taken from each plant.  Most elk consume the entire growth of grasses and herbs, but only select the fronds and leaf tips of ferns, the fallen leaves of such trees as red alder, and the soft terminal branching and needles of a hemlock tree.  Broad descriptions of the vegetative material elk consume include various grasses, leaves, shrubs, trees, twigs and bark. These materials, which are often tough and fibrous, require a special type of stomach to digest, process and extract the essential nutrients within.  Elk have a multi-chambered stomach that digests, processes and extracts these essential nutrients within by having the food they eat go through various phases of digestion, which turns their food into the nutrition they require for life.  Though nutrition is key, it is also critical that elk have water.  Elk have many uses for water including cooling themselves off and aiding digestion.  Elk usually are drawn to wherever water is accessible, which can be at various water hole, lakes, creeks and springs.  If no water is available, elk become very resourceful in obtaining the precious life juice.  In the wintertime when water is scanty because of the freezing temperatures, elk obtain water by eating snow.


An elk with plenty of food and water are not complete.  Elk need shelter.  During winter storms and flooding, elk often move away from the colder and more hazardous river bottoms to the denser and more protective forest canopies.  This type of shelter protects elk and other animals from the elements, providing a break from the wind, holding warmth in and catching much of the snow before it hits the ground.  While protecting a herd from the elements, a shelter also gives elk a place to avoid and hide from predators, allowing a greater survival rate of individuals from being consumed. 


Like every animal, elk require room to roam about away from particular confined parameters.  Elk usually ramble around a home range, which in the summer time consists of about two square miles, and in the winter about eight square miles.  Though these are average parameters elk are capable of trekking more than fifty miles in a day and the distance they travel is usually not the limiting factor.

c) Natural Predators

As a staple of many predators diet elk can be consumed at various rates depending upon the ecosystem. Elk’s main predators include black bears, grizzly bears gray wolves, cougars, and mountain lions.  If approached by a predator a bull or cow may become aggressive with the individual and attempt to ward them off.  This behavior may be effective for certain predators such as wolves, who will probably pick a weak or sick individual upon an encounter with an aggressive elk.  If on the other hand a grizzly desires an elk, the elk has no chance and can be easily pursued and destroyed.  


Rocky Mountain Elk Federation

Herd Structure of Yellowstone’s Elk

There are eight elk herds, totaling approximately 120,000 elk, in the Greater Yellowstone area.  They are usually named after the geographic area they winter in.  The elk herds are named as follows:

  • Northern Elk herd
  • Clarke’s herd
  • North Fork Shoshone herd
  • Carter Mountain herd
  • Jackson herd
  • Sand Creek herd
  • Madison-Firehole herd
  • Gallatin herd

The elk herds of Yellowstone are among the largest in the world.  The smallest have approximately 650 animals (Madison-Firehole herd) and the largest have upwards of 22,000 (Northern Elk herd).  Most elk herds are migratory; that is, they change their location (range) depending on the season.  In winter they occupy low elevation ranges where there is less snow.  As the snow melts in the spring, they migrate to their higher elevation summer ranges.  As the map shows, most elk herds summer inside Yellowstone National Park (YNP).  It is estimated that there are over 30,000 elk that summer inside YNP, and 15,000-22,000 that winter inside YNP.

The Northern elk herd has been the basis of the controversial overgrazing issues that YNP managers face.  This herd is the largest of all in the Greater Yellowstone area, weighing in at 15,000-22,000 members.  Their winter range (or “Elk Northern Range”) spans about 153,000 hectares, of which 65% lies within YNP.

What Makes up a Herd?

Within a herd, there are several subgroups, characterized as the following:

  • Cow/calf herds – In the summer, after calves have been born, several calves may form smaller groups centered around a single cow (adult female).  This gives cows the advantage of having time to eat while leaving their calves with a “babysitter” cow.
  • Harems – In the autumn, bulls (mature males) will join a group of cows and calves, creating a harem.  Harems are generally smaller than cow/calf herds.  Yearling males do not participate in harems, as they are chased off by mature males.  A bull will have 20-30 cows in his harem and must compete with other males to have access to his harem.
  • Bachelor groups – After the mating season ends, the harem separates.  The cows (likely now pregnant) regroup and bulls of all ages (including the yearlings) gather to form bachelor groups.  Both cows and bachelor groups begin to migrate to their winter range, although they often occupy different areas of the winter range.

Herd Stability

The concept of natural regulation is based largely on ecological terms such as equilibrium and stability.  Fluctuations in elk populations can occur from variations in winter severity, quantity and quality of available forage, emigration, hunting, and predation.  A population is defined as stable if it experiences these fluctuations and still returns to a stable point, or equilibrium.  For example, if a severe winter results in die-off of a large proportion of an elk herd, and that herd is able to increase reproduction back to equilibrium numbers, then it is stable.  Furthermore, stability can be either local or global.  A population is locally stable if it returns to equilibrium after a small perturbation.  It is globally stable if it returns to equilibrium after large perturbations in population numbers.

The Northern Elk Herd

It has been suggested that the elk numbers on the northern range are globally stable.  They experience yearly variations in winter severity, causing substantial fluctuations in abundance.  After large deviations from equilibrium, this herd always returns to its population equilibrium.  In addition, the concept of a dynamic equilibrium has been used to describe elk on the northern range.  This means that equilibrium points are not constant through the years; they can change over time.  Equilibrium points can change depending on climate or environmental influences.  For example, an elk population’s equilibrium point after a severe winter may be lower than after a mild winter.

Whether or not the Northern Elk herd is globally stable or if they experience a dynamic equilibrium has sparked much debate among wildlife managers in YNP.  Understanding how elk populations function is critical in deciding how much of a role humans must play in managing the herds to reassure their persistence to future generations.