THE EVOLUTIONARY SIGNIFICANCE OF MOUNTAIN SHEEP HORNS.

Abstract

In proportion to body size, North American wild sheep (Ovis canadensis and Ovis dalli) carry the largest horns among ruminants. Their horns have been recorded as exceeding 51 inches (130 cm.) in length, and 16 inches (40.6 cm.) in circumference at the base (Anonymous, 1964). Old males may bear eight to twelve per cent of their body weight as horns. The horns of rams grow substantially in length and mass each year. This increase is greatest in the early years of life and smallest in later years. The horns of females remain quite small throughout life and rarely exceed nine inches (23 cm.) in length. The huge horns of rams invite enquiry as to what selection processes brought about and retain such disproportionate organs. Several functions can be proposed for ram horns. Horns act as weapons.-This implies that horns can inflict damage on opponents when used in fighting. This I have found. The tiny horns of mountain goat (Oreamnos), however, also serve as weapons (Geist, 1964). Hence this proposed function alone explains little about the evolution of sheep horns. Horns act as a shield.-During fighting rams attempt to catch the opponents' head blows with their horns (Fig. 1). There is little doubt that sheep horns function in this manner. Other bovids, however, also attempt to catch an opponent's attack with their horns, i.e., Bos and Bison. These cattle carry only small, curved horns. Again, this function explains by itself little about the evolution of mountain sheep horns. Horns act as shock absorbers.-During their frequent social interactions, rams collide head on. They may run on their hindlegs at each other and then jump into the clash. Since the rams may weigh about 220--250 pounds each, the collisions must be frightful. It appears quite reasonable hence, that the heavy horns function in absorbing the heavy impact. This appearance is deceptive. A number of Artiodactyla clash head on during fighting, or deal out blows with their heads. None carry horns to match those of sheep males. Though female sheep fight very similar to rams, they carry only tiny horns. The wart hogs (Phacochoerus) and giant forest hogs (Hylockoerus) in fighting clash head on after a run at each other (Mohr, 1960; Fradrich, 1965). They possess no horns at all. Bison bulls may clash head on with considerable force (McHugh, 1958). They have only tiny horns. Giraffes (Giraffa) deal out severe blows with their heads, but carry only little ossicones on the skull. All these species (including sheep of both sexes) possess however one feature in common. They have a pneumated skull with a double roof of bone over the brain. Since there is no correlation between horn size and the mode of fighting: I reject the shock-absorption function. Horns act as display organs.-This is an old, but unverified concept of obscure origin. Beninde (1937) stated it clearly. He proposed that large-antlered deer could intimidate small-antlered opponents with their horn size, and hence gain social advantages without fighting. This hypothesis was never carried through to its various logical conclusions and hence never verified or rejected. If the horns of mountain sheep have been selected for as display or intimidation organs, then the following predictions must be inherent: