Goeldi and the dwarfs: the evolutionary biology of the small New World monkeys

Abstract

The New World monkeys (Platyrrhini) have traditionally been divided into two main families containing the true monkeys (Cebidae), and the marmosets and tamarins (Callitrichidae), although alternative schemes have been proposed in recent years. Some authors have regarded the marmosets and tamarins as primitive, but others have proposed that they are specialized dwarf forms. A particular place is occupied by Goeldi's monkey ( Callimico goeldii), which has been variously classified (e.g., in the Cebidae, in the Callitrichidae, and in its own family Callimiconidae). Callimico possesses a mosaic of morphological features typical either of marmosets and tamarins or of other New World monkeys, and is hence of particular interest for studies of platyrrhine evolution. Phylogenetic relationships of Goeldi's monkey to other New World monkeys are reviewed on the basis of morphological, chromosomal and biochemical evidence. It is confirmed that Callimico, marmosets and tamarins clearly form a monophyletic group. As proposed by Ford (1980), Rosenberger (1984) and Sussman & Kinzey (1984), the evolution of this group involved reduction in body size (dwarfism), accompanied by additional developments such as reduction of the molar tooth row, secondary redevelopment of claws and (in marmosets and tamarins) the emergence of twinning. This interpretation is confirmed by chromosomal banding patterns and by biochemical comparisons, although the latter have yielded conflicting results regarding the pattern of relationships among the clawed New World monkeys. Information from Callimico (which does not twin) is used to test two alternative hypotheses proposed to account for twinning: (1) the avoidance of obstetric problems associated with birth of a relatively large infant; and (2) increase in reproductive output associated with small body size and unpredictable ecological conditions.