Costs of twins in free-ranging white-bellied spider monkeys (Ateles belzebuth belzebuth) at Tinigua National Park, Colombia.

A female spider monkey (Ateles belzebuth belzebuth) that gave birth to twins was studied during 13 months, and her activity budget and diet were compared to those of females with single offspring and females with no offspring to assess selective pressures that could influence litter size. We recorded qualitative information on the development and social interactions of the twins and three other single infants. Emi, the female that had the twins, had the highest proportion of resting time and the lowest proportion of feeding and moving time compared to other adult females and males. Emi also had the lowest average daily travel distance and relied more heavily on flowers and leaves than other group members who included a higher proportion of fruits in their diets. These results suggest that twins are energetically costly to spider monkeys because of the direct energy allocated into raising and carrying the twins and, also, because these costs have direct effects in the ability of a mother to obtain high-quality resources (e.g., fruits). The twins had a slower rate of physical and social development as well as a smaller body size compared to same-aged singletons by the end of the study. Thus, the extended periods of maternal care and the costs associated with rearing and carrying offspring are some of the factors that influence the evolution and maintenance of a litter size of one in most anthropoid primates.