Influence of trophy hunting and horn size on mating behavior and survivorship of mountain sheep

Abstract

We conducted a study of the effects of horn sizes and trophy hunting on mating behavior and survival of rams in hunted and unhunted populations of Dall sheep (Ovis dalli), Rocky Mountain bighorn (O. canadensis canadensis), and desert bighorn (O. c. nelsoni) sheep. Mating success was positively correlated with horn size in Dall sheep (P = 0.03) and Rocky Mountain bighorns (P = 0.05), but not in the desert bighorn (P > 0.05) taxa. Group sizes, rams per rut group, and competition between rams were lowest in desert bighorn sheep. There were indications of greater harassment of ewes by young rams in trophy-hunted populations. In hunted populations, compared with unhunted, ewes ran away more often from approaching rams, ewes moved farther away from courting young rams (P = 0.003), younger rams performed fewer courtship displays (P = 0.042) and more aggressive displays to ewes, and sheep interacted 27% more of the time. Ram-to-ewe interaction times per individual ewe did not differ for any of the taxa (P > 0.05), and, apparently as a consequence of this, we found no discernable effects of trophy hunting on survivorship of ewes, ewe fecundity, or recruitment of young (P > 0.05). There were greater energy expenditures by young rams in the heavily hunted Dall sheep population versus the paired Dall sheep unhunted population, but not in the lightly hunted Rocky Mountain and desert bighorn populations when compared with unhunted populations. This was consistent with evidence for depressed survivorship of rams too young or too small to be hunted (approximately ages 4-6) in the heavily hunted Dall sheep population (P = 0.0001), but not in the bighorn sheep populations (P > 0.05).