Rams resemble ewes until they are about 3 years old. After that, continued horn growth makes them easily distinguishable. Their 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 the rut. This pattern of horn growth results in rings called annuli that are spaced along the length of the horn. A sheep’s age can be accurately determined by counting these annuli. Researchers have recorded rams as old as 16 years and ewes as old as 19 years. More typically, a 12-year-old sheep is considered very old. 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, three-quarters of a circle in four to five years, and a full circle or "curl" in seven to eight years. Dall rams normally do not breed successfully until they approach dominance rank (at full curl age and size).