Horn Growth in Mountain Goats (Oreamnos americanus)

Abstract

Horns of bovids are important social organs, their growth is often indicative of population characteristics and habitat quality, but little is known of the factors affecting their growth in individuals. We studied horn growth of 135 (51 males, 84 females) marked mountain goats (Oreamnos americanus) in Alberta, Canada, for 9 years. In both sexes, horn growth was quadratic during the first 5 years of life and not significant after 5 years of age. Goats completed 93% of horn growth by 3 years of age. Horns of males grew more than those of females during the first 1.5 years of life. Horns of females grew more than those of males during the third year. Although males maintained longer horns than females because of their longer first increment, adult males had shorter horns than females for a given body size. Males had thicker horns than females at all ages, absolutely or relative to body size. Horn growth early in life was correlated negatively with later growth. Annual growth increments of horns of females aged 4–5 years were affected negatively by lactation. Horn growth was not affected by total rainfall during the vegetation-growing season. Total length and basal circumference of horns were correlated positively with mass, chest girth, and hind-foot length in both males and females. While sexual dimorphism in body mass of mountain goats increases until at least 5 years of age, most dimorphism in horn growth is achieved by 2 years of age. Therefore, body mass of mountain goats may be a more important factor than horn size for male mating success.