Effects of population density on horn development in bighorn rams

Trophy hunting is a management goal for many populations of ungulates and has important implications for conservation because of the economic value of trophy males. To determine whether population density affected horn growth of males, a marked population of bighorn sheep (ovis canadensis) in Alberta, Canada, was studied for 27 years. For the first 9 years, population density was kept stable by removing adult females; afterwards, the numbers of ewes and yearlings tripled before beginning to decline. Horns were measured during repeated captures of marked rams. As the number of adult ewes and yearlings increased, ram horns were shorter and thinner because of decreased horn growth before 4 years of age. Some compensatory horn growth may have occurred at 5 years of age. The effects of population density on horn growth ceased when rams left the nursery groups to join all-male groups. Doubling of male numbers had no detectable effect on net annual horn growth of males greater than or equal to 4 years old. Spring: precipitation had no apparent effect on horn growth of males 3-4 years old, and had a minor positive effect on horn base circumference for rams 5-6 years old. The proportion of rams 6-7 years old that attained 4/5 of a curl decreased from 61-73% at low density to 33-35% at high density When bighorn sheep populations increase to a density where intraspecific competition in nursery herds affects horn development of young rams, limited ewe harvests may prevent a decrease in size of horns of mature males.