Territory Horse Population
In general, the Heber Wild Horse Territory’s (HWHT’s) appropriate management
level (AML) of 28 to 35 horses will be managed so that after a gather
and re-release, 28 to 35 horses remain in the
Territory area. (19,700 acres) We will attempt to retain
herd composition of about 60 percent male and 40 percent female. Age distribution will
approximate 20 percent aged between 0 and 5 years old, 60 percent between six and
fifteen years old, and 20 percent sixteen years or older. An
example is to release four males and two females in the 16+ age class, 10 males and 6 females in the 6-15 year class, and an undetermined ratio of males to
females in the 0-5 year class depending on the sex of unweaned foals being released.
A leader in the field of equine population genetics is Dr. Gus Cothran, Director of the Equine Blood Typing Research Laboratory at the University of Kentucky. In addition to blood and hair samples collected from horse breeds around the world, Dr. Cothran has been analyzing blood samples from U.S. wild horses. He has been studying the Pryor Mountain wild horse herd of southern Montana since 1991 as well as other wild horse herds on public lands in the West.
Dr. Cothran suggests that managing wild horses at low population levels leaves them vulnerable to a long range loss of genetic diversity. This is the same sort of problem which plagues endangered species around the world. But, just how small is too small? At what point do wild horse populations suffer the risk of irreparable genetic damage?
Based on his DNA analysis, Dr. Cothran now believes that the minimum wild horse and burro herd size is 150-200 animals. Within a herd this large, about 100 animals will be of breeding age. Of those 100, approximately 50 horses would comprise the genetic effective population size. These are the animals actually contributing their genes to the next generation. Dr. Cothran has stated that 50 is a minimum number. A higher number would decrease the chances for inbreeding.
(A number of variables such as an unbalanced sex ratio in favor of males would cause this minimum number to be revised upward. Unbalanced sex ratios with many more males than females occur on at least some of the wild horse and burro herd areas.)