Dall's Sheep (Ovis dalli)

The Dall sheep, also known as the thinhorn sheep, is a wild sheep native to the northwestern regions of North America. The name “Dall” honors William Healey Dall (1845–1927), an American naturalist. There are two subspecies: Ovis dalli dalli, found in northern regions, and Ovis dalli stonei, found in southern regions and commonly called the stone sheep. Dall sheep inhabit alpine meadows and shrublands in mountainous areas.

Physical Characteristics

The Dall sheep’s coat is dirty white in color. Its thick fur, which enables survival in polar conditions, can reach up to 5 cm in thickness during winter months. Its physical structure exhibits sexual dimorphism. Males, known as ram, are larger than females and weigh between 70 and 80 kg. Females weigh between 45 and 50 kg. During winter months, when food is scarce, as with many other Arctic animals, adults may lose up to 16 percent of their body mass and lambs up to 40 percent.

The average lifespan of the Dall sheep is between 12 and 16 years. Lambs begin growing horns at around two months of age. Females have horns that are significantly thinner and smaller than those of males. Male horns are large, thick, and highly curved. Until they reach three years of age, young males’ horns are not markedly different from those of females but. After three years, they grow and curl dramatically. An adult male in the herd can be immediately identified by his large horns, which grow continuously from spring through autumn. Age can also be determined by examining the characteristic rings (annulus) on the horns.

Diet

During winter months, when food is scarce, the Dall sheep’s diet consists largely of dried and frozen grasses, lichens, and sometimes mosses. During Summer months, it consumes abundant and varied plant. One of the most important nutritional sources for this species is mineral-rich rocks. Especially in spring, herds visit areas with abundant mineral-rich rock formations and lick the rocks and soil. Like certain deer species, elephants, squirrels, and foxes, sheep require phosphorus, sodium, calcium, iron, and zinc, particularly for bone and case tissue development, which they obtain from these mineral deposits. When herds meet, lambs sometimes join different groups. This behavior is believed to contribute to the genetic diversity of the Dall sheep.

Reproduction

Lambs are typically born at the end of May or during the first weeks of June. As lambing approaches, ewes isolate themselves among rugged cliffs to find safe areas protected from predators. Although a lamb begins feeding on vegetation about one week after birth, weaning usually occurs around the beginning of October. A ewe’s first lambing occurs at three or four years of age, and she typically gives birth to one lamb per year. The most critical period in a lamb’s life is its first 30 to 45 days, during which it is most vulnerable and mortality rates are highest time.

The necessity of living in herds makes maintaining social order and hierarchy essential for the survival of the Dall sheep. Social rank and dominance within the herd are established through several different behavior, including head-to-head collisions. Rams charge at each other from a distance and collide with their horns. Other displays of dominance include forcefully stamping their hooves on the ground and attempting to mount one another. Similar displays occur among females when competing for food or resting areas. In lambs, these same behaviors are part of their play.

Population fluctuations in the Dall sheep are seasonal and influenced by a range of environmental factors. Under favorable polar climate conditions, the population remains relatively stable. However, during periods of summer drought, unusually thick snow layers, or other severe weather events, population declines may occur. The Dall sheep population is also affected by the numbers of its natural predators: wolves, coyotes, and golden eagles. Despite all these fluctuations, there is no cause for concern regarding a significant population decline in the Dall sheep, and it is classified as “minimum” on the International Union for Conservation of Nature’s Red List.