The mountain goat (Oreamnos americanus) is an iconic North American mammal that navigates the most extreme alpine terrain with remarkable agility. This animal’s impressive ability to thrive in high-altitude environments is directly linked to its powerful physical structure and substantial body mass. Understanding the weight of a mountain goat provides insight into the adaptations that allow this species to survive in a challenging ecosystem.
Average Weight of Mature Mountain Goats
The weight of a mountain goat varies significantly based on sex, a phenomenon known as sexual dimorphism. Mature males, called billies, are notably heavier and more robust than mature females, or nannies. A typical adult billy averages around 150 pounds, but individuals can reach a maximum weight of up to 300 pounds.
The larger mass in males provides an advantage during the rutting season, where dominance is established through physical displays. Mature nannies are considerably lighter, generally averaging about 125 pounds, with their weight maxing out around 250 pounds.
Coastal populations, which typically benefit from a more consistent and richer food supply, often exhibit weights at the higher end of the range. Billies from these areas can weigh 275 to 300 pounds, sometimes 100 pounds more than their counterparts in the Rocky Mountains. This variation highlights how the availability of high-quality forage directly influences body mass.
Key Factors That Influence Weight
A mountain goat’s mass is not static but fluctuates dramatically throughout the year in response to environmental and biological demands. The most pronounced change is seasonal, with goats gaining substantial fat reserves during the late summer and fall in preparation for winter. This period of increased feeding is necessary to build the insulating layer and energy stores needed to survive the harsh winter months.
Age also plays a substantial role in determining an animal’s weight, with full maturity generally reached around five to six years. Newborn kids weigh only about 35 pounds, and yearlings typically weigh between 60 to 70 pounds. Beyond age, a nanny’s reproductive status significantly impacts her weight, as females carrying or nursing young require more energy.
Contextualizing Weight with Physical Dimensions
The substantial weight of a mature mountain goat is supported by a compact, muscular physique built for climbing. An adult goat stands roughly 31.5 to 43.3 inches tall at the shoulder, a measurement often exaggerated by the thick, shaggy white coat. The overall body length, measured from head to rump, typically ranges from 55 to 70 inches.
Billies often appear stockier than nannies, possessing a noticeable muscle mass, particularly around the neck and shoulders. This musculature gives males a pronounced hump above the shoulders, contributing to their imposing presence. In contrast to the heavy adults, a kid stands only about 20 inches at the front shoulder. The black, pointed horns, which are present on both sexes and can measure up to 11 inches long, contribute very little to the overall weight but complete the animal’s powerful profile.