Twinning and freemartins This sub-series contains Alexander Low's research material on freemartins in cattle, sheep, goats and pigs. Although freemartins are most common in cattle, they can occur in other animals, including sheep, goats and pigs. Low corresponded with local farmers on the topic of freemartins, from whom he was able to build up a picture of the frequency and pattern of their occurrence. 1920-1924