In 1980, researchers began a study of the Craters of the Moon mule deer. Biologists collected information about the reproduction, mortality, distribution and movement of the herd. Prior to this research, it was not known how many deer lived within the monument's boundaries, if the population was increasing or decreasing, and whether the herd was healthy.

To answer these questions, researchers captured 100 deer and marked them with colored ear tags. These brightly colored streamers allowed researchers to identify each deer. They also attached radio collars to twenty deer to help track their day-to-day movements. By observing the deer over the next three years, researchers recorded 2,381 deer sightings and completed a life history on each animal. The analysis of this research provided some surprising information.

Specifically, researchers had not anticipated the unique dual migration pattern they found. The deer have a traditional fall/spring migration with movement between summer and winter ranges and a second migration that takes place during the summer. The dual migration works like this. Each year as the snow melts, more than 400 deer leave their wintering grounds at lower elevations to arrive at the monument in late April or early May. About one-half of the herd heads into the northern section of the monument where there is a good supply of herbs and shrubs. Rainfall in March and April is plentiful and small creeks, ponds or puddles provide water for this group of deer. The remainder of the herd goes to the southern part of the monument. In this area, because of the porous lava, no standing water exists. There is plenty of nutritional antelope bitterbrush, however, and when the deer eat this plant they are getting both food and the moisture they need.

When summer arrives temperatures rise and the rain stops. Springs that run year-round continue to provide water for the northern deer herd. In the southern area of the monument, however, the vegetation begins to dry out and the deer find it difficult to get enough water.

Researchers determined the exact conditions, usually in July or August, when the lack of water reaches a critical point for these deer. This happens during any 12-day period in which daytime temperatures are more than 80 degrees and nighttime lows don't drop below 50 degrees. These sustained high temperatures dehydrate plants to a point where they no longer provide enough moisture to assure the southern deer's survival. Then, the southern herd moves rapidly north. The deer population in the northern end of the monument doubles in as little as two weeks. The deer find themselves living in a situation comparable to a house with several people living in each room. They must endure these crowded conditions until rain comes and some of the deer move south again. If it doesn't rain, the entire deer herd remains in the northern end of the monument until fall. Then they return to their winter range. Scientists now see this dual migration as a specific adaptation of the deer herd to the Craters of the Moon environment--one that allows them to survive despite extremely hot summers and a marginal water supply.

Monitoring the Deer Herd

The deer monitoring program provides a yearly measure of mule deer population size, herd productivity, and overwinter fawn survival.

The information gathered allows the National Park Service to gain an understanding of year to year variation in the population and to identify long-term changes. Deer censuses are conducted in the spring and late summer each year. As soon as deer begin to arrive in the monument in April, the staff begins to document the sex and age of all deer seen. Sampling continues through the month of May and the first two weeks of June until at least 100 and preferably 200 animals have been classified.

The late summer census takes place between August 15 and September 15. Park staff conduct six early morning censuses from a vehicle along the 3.9 mile section of dirt road that runs from Highway 26 into the northern section of the monument, where most of the deer reside at that time. The censuses begin 10 minutes after sunrise and take 1-2 hours. The ranger carefully scans the terrain along the road with binoculars or a spotting scope, and records all deer visible. The deer are classified as either adult males, yearling males, females, or fawns.

Deer killed by automobiles on the highway and by hunters around the park boundary provide an estimate of population trends independent of the late summer census. Information on the date, number, sex, and age of these animals is recorded as a check on the roadside censuses.

1. Conduct an early morning walking tour on the wilderness trail. You can go as far as the lava trees, a 3 mile round trip, or if time is limited, hike the 2 mile round trip to Buffalo Caves. Ask at the Visitor Center for more detailed information about these trails. You may want to divide into small groups and hike several different trails. Groups could, for example, hike to the tree molds, the lava trees, and the North Crater Trail. Small group size is a distinct advantage for this activity, because the quieter and more attentive the students are, the more likely they are to observe deer.

2. Use the "Mule Deer Census Form" to record your data. You will classify all deer seen as adult males, adult females, yearlings, or unknown. Yearlings may be differentiated from adults based on body size and the ratio of ear length to lower jaw length. Deer with ears longer than the lower jaw are yearlings; those with ears the same length or shorter than the lower jaw are adults. Use a spotting scope or binoculars to obtain a profile view when aging the deer.

3. When data collection is complete, make the calculations on the "Work Sheet for Calculating Overwinter Fawn Survival" to determine what percentage of fawns survived the winter. Determining overwinter fawn survival is the primary objective of the spring census. For example, in the late summer of 1982, there were 94 fawns per 100 adult females and in the spring of 1983, there was a classification ratio of 73 yearlings per 100 adult females. Overwinter survival is then indexed as 73/94 x 100 or 88 percent. (This procedure will overestimate overwinter survival fawns depending on the winter mortality of adult does. The more adult does that die in winter, the greater will be the overestimation.)

4. Most groups are unable to visit the monument in September to participate in the more extensive census conducted at that time. Park staff analyze the data collected in August and September to determine an estimate for population size and relative size, sex, and age classes of deer. If you are interested in more extensive statistical analysis of the deer census data, the monument will provide you with data from the most recent fall census on request. Sample data from 1993 is included here for your use if you prefer. Complete the "Analysis of Fall Mule Deer Census Data."