We offer examples of how proximate and evolutionary forms of argument may inform each other in better understanding reproductive strategy in callitrichid primates, the smallest of the anthropoid primates. In addition, we illustrate how comparative approaches, when applied judiciously, can aid in the formulation of hypotheses regarding even seemingly unique traits within a taxonomic group. In the first example, examination of the nature of genetics in cytokine systems that leads to altered ovulation number in sheep suggests some relatively simple changes could explain both the adaptation of increased ovulation number in marmosets and the subsequent decrease in ovulation number in the closely related species, callimico. In the second example, the role of body size and phylogeny in explaining the role of maternal energy constraints upon gestation and lactation is explored, leading to additional hypotheses regarding these relations in a species that is both small but also in a phylogenetic line selected for slow reproduction. Finally, the role of comparative data in the study of proximate and evolutionary explanations of "unique" human reproductive strategies is discussed.
