Horn aberrations in Dall's sheep (Ovis dalli) from Yukon Territory, Canada

Horn aberrations in Dall's sheep from the Kluane Lake area of Yukon Territory were of two basic types. In the more prevalent type, the horn was severed at an annual growth check and the terminal segment of the remaining portion formed a short conical protuberance. The second type had characteristics of the first, but in addition, underwent extreme twisting or torquing during subsequent growth. Sheep with aberrant horns represented 2% of the Kluane Lake population or 14% of harvestable rams (greater than or equal to 6 yr old). Deformed horns, occurring in ewes and rams, were generally observed in animals 6 yr or older. Aberrations followed necrosis of the terminal segment of the horn core: cavitation of the sheath between successive periods of growth or the sequestering of portions of necrotic core within the sheath resulted. All sheep examined (eight) had a normal complement of 54 chromosomes.