In the 1960's long-term studies of primate behavior often used as subjects tamarins, small monkeys that were thought ideal because they require only small cages, breed frequently, and grow quickly. Field studies were not used because they were costly and difficult, Tamarins were kept caged in male-female pairs, because otherwise, serious fights erupted between unrelated females. On the basis of the fact that breeding occurred, tamarins were viewed as monogamous.