When threatened, a yak herd forms a protective circle with calves in the center and adults facing outward with their horns.

The domestic yak (Bos grunniens) and its wild ancestor (Bos mutus) have evolved sophisticated social structures to survive the harsh Tibetan Plateau. Research published in journals like 'Mammalia' highlights that yaks exhibit high levels of gregariousness, especially when facing their primary natural predator, the Himalayan wolf. This defensive formation is a classic example of 'selfish herd' theory and cooperative protection, where the physical mass of the adults acts as a living wall.The adult yaks possess impressive weaponry, with horns that can reach up to 90 centimeters (35 inches) in length. During a defensive stand, they lower these horns to chest level, targeting the vital organs of any lunging predator. This behavior is most common during the calving season, which typically occurs between April and June. Observations by zoologists like George Schaller have documented that wild yaks are significantly more aggressive than domestic ones, often charging predators that get too close to the perimeter.Beyond physical barriers, yaks utilize acoustic and visual signals to maintain the formation. They produce low-frequency grunts and snorts that serve as alarm calls to tighten the circle. The sheer size of an adult yak, which can weigh between 300 and 1,000 kilograms (660 to 2,200 pounds), makes the formation physically immovable for most solitary hunters. This evolutionary trait ensures the survival of the next generation in one of the world's most extreme environments.