Both genera have a polygamous mating system with promiscuous mating. Single births are the rule for both genera. Squirrel monkeys have a yearly reproductive cycle with a 2-3 month breeding season and a corresponding 2-3 month birthing season. The birth season (approximately 5.5-6 months after the breeding season) is correlated with the wet season and food abundance in their specific range. Unique among primates is an annual change in the males' physiology that occurs just prior to the breeding season. Adult males increase in body weight 10-30% (primarily in the upper body), and the testes double or triple in size, accompanied by the onset of spermatogenesis. These males are referred to as "fatted males." During the breeding season females have an estrus cycle of 12-14 days. The gestation period varies from 155-180 days. Females generally reach sexual maturity at three years with males not reaching full sexual maturity (evidenced by attaining the fatted stage) until 5-6 years. The ratio of infant size to mother size is the largest for any mammal, approaching 1:6. Infant birth weight is 3.2-3.9 oz (90-110 g).
Some species of capuchins are reported to be seasonal breeders, while others breed year round with a peak of births in a certain season. For many capuchin species paternity is often unknown as females have been seen to mate with more than one male. Estrus females follow and solicit mating from males, and males rarely fight over access to females. Female Cebus apella tend to groom and breed only with the dominant male, which is thought to create a bond so that he will protect the infants that he has sired. Females reach sexual maturity at 4-5 years; males do not reach full maturity until 8-10 years of age. Female estrus cycles are 18 days for Cebus apella, and the gestation period for Cebus varies from 149 to 168 days. Infants at birth are about 8.5% of the mother's weight, about 8 oz (220 g).
Both infant squirrel monkeys and capuchins are dependent on their mothers for parental care. An infant squirrel monkey clings to its mother's back from day one. It rides in the middle of her back with its head turned to one side, clutching her fur tightly with hands and feet and wrapping its tail tightly around her body. When nursing, the infant crawls forward and positions its head under her arm, rooting around under her armpit until it finds the nipple. Infants ride this way for several months. At 3-4 weeks other animals attempt to carry the infant and the mother may allow older females without young to carry the infant, but the mother always maintains visual contact. Juvenile females are occasionally allowed to carry infants, but the mother always remains nearby, ready to retrieve the infant at the first sound of trouble. It is thought that this is the way in which young females learn mothering skills. Infants begin to be weaned at five months and are independent from the mother by 11-12 months, just prior to their mother giving birth during the next birthing season. The youngster still maintains a close relationship with its mother and often travels nearby her and the new infant. Capuchin infants initially
cling to the mother's underside or across her shoulders, and at about six weeks align their bodies along the mother's back. As infants and juveniles, both genera return to the mother for protection and security when stressed. At 2-3 months they begin to explore their environment and develop social relationships with other group members and begin to play with similar aged infants. Social play helps animals learn the subtleties of proper social behavior and the control of aggressive responses. It also helps to develop sexual behavior and to integrate the young into the group.
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