A comparative perspective on the evolution of tamarin and marmoset social systems

Abstract

Tamarins and marmosets (callitrichids) present an unusual opportunity for study of the determinants of primate social systems, because both the mating and infant care patterns of callitrichids are variable, even within individual populations. In this paper, I briefly describe three characteristics of callitrichid social systems that distinguish them from most other primates: extensive male parental care, helping by nonreproductive individuals, and variable mating patterns. I then discuss the evolution of these characteristics and of the frequent twinning exhibited by callitrichids. I suggest that an ancestor of modern callitrichids gave birth to a single offspring at a time, mated monogamously, and had significant paternal care. The idea that males of this ancestral form must have provided paternal care, even though only single infants were born, derives from a comparison of litter/mother weight ratios in modern primate species. Twinning perhaps then evolved because of a combination of dwarfing in the callitrichid lineage, leading to higher litter/mother weight ratios, and a high infant mortality rate, and because the extensive paternal care already present facilitated the raising of twins. I propose that the helping behavior of older offspring may have coevolved with twinning, because helpers would have increased the chances of survival of twins, and the presence of twins would have increased the benefits of helping. Finally, the high costs of raising twins and the variability of group compositions, especially the fact that some groups would not have had older offspring to serve as helpers, may have selected for facultative polyandry in saddle-back tamarins ( Saguinus fuscicollis ) and perhaps in other callitrichid species. Both helping and cooperative polyandry have been extensively studied in bird species, and I apply some of the conclusions of these studies to the discussion of the evolution of callitrichid social systems.