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 economi c value of trophy males. To determine whether population density affec ted horn growth of males, a marked population of bighorn sheep (ovis c anadensis) 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 beginni ng to decline. Horns were measured during repeated captures of marked rams. As the number of adult ewes and yearlings increased, ram horns w ere shorter and thinner because of decreased horn growth before 4 year s of age. Some compensatory horn growth may have occurred at 5 years o f age. The effects of population density on horn growth ceased when ra ms left the nursery groups to join all-male groups. Doubling of male n umbers had no detectable effect on net annual horn growth of males gre ater than or equal to 4 years old. Spring: precipitation had no appare nt effect on horn growth of males 3-4 years old, and had a minor posit ive effect on horn base circumference for rams 5-6 years old. The prop ortion of rams 6-7 years old that attained 4/5 of a curl decreased fro m 61-73% at low density to 33-35% at high density When bighorn sheep p opulations increase to a density where intraspecific competition in nu rsery herds affects horn development of young rams, limited ewe harves ts may prevent a decrease in size of horns of mature males.