Relative allocation to horn and body growth in bighorn rams varies with resource availability

Abstract

Males may allocate a greater proportion of metabolic resources to maintenance than to the development of secondary sexual characters when food is scarce, to avoid compromising their probability of survival. We assessed the effects of resource availability on body mass and horn growth of bighorn rams (Ovis canadensis) at Ram Mountain, Alberta, Canada over 30 years. The number of adult ewes in the population tripled during our study, and the average mass of yearling females decreased by 13%. We used the average mass of yearling females as an index of resource availability. Yearling female mass was negatively correlated with the body mass of rams of all ages, but it affected horn growth only during the first three years of life. Yearly horn growth was affected by a complex interaction of age, body mass, and resource availability. Among rams aged 2–4 years, the heaviest individuals had similar horn growth at high and at low resource availability, but as ram mass decreased, horn growth for a given body mass became progressively smaller with decreasing resource availability. For rams aged 5–9 years, horn growth was weakly but positively correlated with body mass, and rams grew slightly more horn for a given body mass as resource availability decreased. When food is limited, young rams may direct more resources to body growth than to horn growth, possibly trading long-term reproductive success for short-term survival. Although horn growth of older rams appeared to be greater at low than at high resource availability, we found no correlation between early and late growth in horn length for the same ram, suggesting that compensatory horn growth does not occur in our study population. Young rams with longer horns were more likely to be shot by sport hunters than those with shorter horns. Trophy hunting could select against rams with fast-growing horns.