A yak's heart is so large that it occupies nearly 25% of its chest cavity.

Domestic yaks (Bos grunniens) and their wild counterparts live at elevations between 3,000 and 5,500 meters (10,000 to 18,000 feet) across the Tibetan Plateau. At these heights, oxygen levels are roughly 40% lower than at sea level. To compensate, yaks have evolved unique physiological adaptations, including a heart that is significantly larger relative to their body size compared to low-altitude bovines.Research published in journals like 'Nature Genetics' highlights that yaks possess specific gene mutations, such as in the ADAM17 and HIF-1 genes, which help them manage hypoxia. Their massive hearts work in tandem with exceptionally large lungs and a high concentration of red blood cells. This combination ensures that despite the thin atmosphere, their tissues receive a constant and robust supply of oxygen.A typical yak heart weighs about 0.5% to 1% of its total body mass, whereas a standard beef cow's heart is much smaller in proportion. Furthermore, yaks maintain a high cardiac output without developing pulmonary hypertension, a condition that often kills other cattle breeds at high altitudes. This evolutionary specialization allows them to carry heavy loads across treacherous mountain passes where most mammals would suffer from altitude sickness.