The yakalo is a cross of the yak (Bos grunniens) and the American bison (Bison bison, known as a buffalo in North America). It was produced by hybridisation experiments in the 1920s, when crosses were made between yak bulls and both pure bison cows and bison-cattle hybrid cows. As with many other inter-specific crosses, only female hybrids were found to be fertile (Haldane's rule). Few of the hybrids survived, and the experiments were discontinued in 1928.