The caribou, known as reindeer in Eurasia, is an ungulate of the Arctic and subarctic regions. While these animals may appear quiet, their acoustic environment is complex, involving both vocal sounds and unique mechanical noises generated by movement. Understanding this sound repertoire is important for appreciating how these herd animals maintain cohesion and communicate across their expansive habitats.
Caribou Vocalizations: Communication and Context
Caribou use various sounds created through their vocal cords and nasal passages to communicate within their herds. The most common sound is a simple grunt, which serves as a general-purpose call for maintaining contact and cohesion, especially during migration or when grazing. Females use grunting to communicate with their calves, who respond with high-pitched bleats or mews. This individualized mother-calf vocal recognition is established early and is a component of their social structure.
Snorts and hisses are used as signals of alarm or irritation, indicating a perceived threat or discomfort to other nearby animals. These abrupt, forceful exhalations through the nostrils can alert the immediate group to danger, prompting collective vigilance or a sudden escape. The most dramatic vocalizations are the deep, guttural bellows produced by the adult males, or bulls, during the autumn rutting season. These loud calls are a form of acoustic advertising, used to challenge rival males and attract receptive females to establish dominance.
The intensity and depth of the male’s bellow is amplified by a specialized anatomical feature. Bull caribou develop an inflatable laryngeal air sac in their neck region during the rut. This large sac acts as a resonance chamber, changing the frequency of the vocalizations to make them deeper and louder, signaling the male’s size and fitness to competitors and mates. The rutting call is often accompanied by a specific posture, where the male lowers and extends his head and neck to visually emphasize the swelling throat region.
The Distinctive Sound of Movement
Caribou produce a unique, loud clicking sound with every step they take, a mechanical noise often heard before the animal itself is seen. This characteristic sound originates from the feet and ankle joints, not the knee, as is commonly believed. Specifically, the sound is generated by tendons snapping over the small sesamoid bones located near the fetlock joint as the animal shifts its weight.
The clicking is loud and can be heard from many yards away, especially when a large group is moving together. This noise acts as an acoustic beacon, helping herd members locate one another when visibility is compromised, such as during blizzards or fog. The sound helps maintain the integrity of the herd by marking the direction and speed of the group’s movement.
Beyond the steady clicking of movement, caribou also create noise with their antlers, especially during the breeding season. The sound of clashing and rattling antlers is a common acoustic signature of the rut, occurring when males spar to determine hierarchy. These loud, percussive sounds are a display of strength used to assert social standing. The scraping of antlers against brush or the ground is also a regular sound as caribou rub their growing antlers to remove velvet or mark their territory.
Variation in Caribou Sounds
The sounds produced by caribou vary based on the age, sex, and physical condition of the animal. Sexual dimorphism in vocalization is most apparent during the rut, when the large laryngeal air sac allows the mature bull to produce calls that are louder and deeper than those of cows or younger males.
This air sac in the bull continues to grow until the age of about six years, allowing for increasingly deeper and more powerful rutting calls over time. The mechanical clicking of the feet also varies with age, as young calves do not produce the sound until they are several months old. This delay is likely due to the size and development of the sesamoid bones and tendons.
The environment itself influences the effectiveness of caribou communication, requiring adjustments in volume. Sounds travel differently across the open expanse of the tundra compared to the denser, sound-absorbing environment of the boreal forest. Herd animals in open country may rely on lower-frequency sounds that travel farther. Those in forested areas may use louder or more frequent calls to overcome the acoustic dampening effects of trees and undergrowth. Differences in subspecies, such as woodland versus barren-ground caribou, may also exhibit slight variations in their core calls due to adaptations to their specific habitat types.