Dall Sheep

Dall Sheep

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Description

Male dall sheep, or rams, are distinguished by massive curling horns.  The females, or ewes, have shorter, more slender, slightly curved horns.  Rams resemble ewes until they are about three years old.  After that, continued horn growth makes them easily recognizable.  Horns grow steadily during spring, summer and early fall.  In late fall or winter, horn growth slows and eventually ceases.  This is probably a result of changes in body chemistry during breeding season.  This start-and-stop growth of horns results in a patterns of rings called annuli which are spaced along the length of the horn.  As rams mature, their horns form a circle when seen from the side.  Ram horns reach half a circle in about two or three years and a full circle or “curl” in seven to eight years.

Species Namesake

Dall or Dall’s sheep were named after scientist William Healey Dall in 1884. Dall did not discover the sheep, but led surveys in the late 1800’s in Alaska.

Living Legacy

The challenge of sheep hunting lured naturalist Charles Sheldon to Denali in the early 1900’s. He witnessed over 2,000 sheep taken by commercial hunters, and feared the ecosystem was in danger. He joined the Boone and Crockett Club in petitioning Washington D.C. to establish the two million acre Mount McKinley National Park.

Lifestyle

Most populations occupy distinct summer and winter ranges, although some are sedentary.  Migrations are correlated with snow depth, temperature and plant phenology.  Most of the year is spent in the winter range in wind-swept areas that expose forage.  Adult males can occupy six seasonal home ranges: Pre-rutting, rutting, midwinter, late winter and spring, salt-lick, and summer.  Females usually have four ranges: Winter, spring, lambing, and summer.  Lambs inherit home ranges from older individuals and they return annually to these inherited ranges.

Food Habits

The diet of Dall sheep varies from range to range.  During summer, food is abundant and a wide variety of plants are consumed.  Winter diet is much more limited and consists of primarily dry, frozen grass and sedge stems available when snow is blown off the winter ranges.  Sheep travel to rocky areas called mineral licks in spring to eat the soil, which contains high levels of sodium and other minerals.  Since ewes and rams use licks, young rams may leave nursery groups and join other rams at this time.  This random dispersal of young rams among unrelated ram herds increases genetic diversity of the species.  Many sheep visit mineral licks during spring and often travel many miles to eat the soil at the base of these unusual soil formations.

Lifecycle

Rams establish a dominance hierarchy in the summer, in which rank is determined by horn size.  This ranking is not just for access to females, since there are no females present in the summer range, but also for social order.  Usually dominance is settled without fighting, but if there is similar horn size, as when different bands meet, dominance is settled by a fight.  These fights are often thunderous.  The two males will back off ten or twelve meters and then rush together, colliding headlong. Usually little harm results, and after several bouts the rams separate.  Mating occurs in late November and early December, with the young, called lambs, born in late May or early June.  As lambing approaches, ewes seek solitude and protection from predators in the most rugged cliffs available on their spring ranges.  Ewes bear a single lamb and the ewe-lamb pair remains in the lambing cliff a few days until the lamb is strong enough to travel.

Population Status, Threats & Conservation

Dall sheep populations in Alaska are generally considered healthy.  The remoteness of sheep habitat and its unsuitability for human use has protected Dall sheep from most problems in the past.  However, sheep numbers typically fluctuate irregularly in response to a number of environmental factors.  Populations tend to increase during periods of mild weather.  Sudden population declines may occur as a result of unusually deep snow, summer drought and other severe weather.  Low birth rates, predation and a difficult environment tend to keep Dall sheep population growth rates lower than for many other big game species.

Sources:
Dall sheep species profile, ADFG
Archive, namesake information
History of Denali Park
Gozdzik, A. 2001. "Ovis dalli" (On-line), Animal Diversity Web