Females exhibit proceptive behaviors, which in Brachyteles and Ateles are now known to correspond with their ovarian cycles. Males frequently inspect the genitalia of females by visual and olfactory means. Copulations sometimes occur with the females sitting, instead of standing as occurs in most other primates.
Female atelins typically mate with multiple partners, although the degree to which single males monopolize access to females and exclude other males from mating varies greatly. In multimale troops of red howler monkeys, the alpha male can account for 100% of all fertilizations, resulting in the genetic equivalent of a single-male troop. In woolly monkeys
and spider monkeys, high-ranking males account for most observed copulations. In muriquis, females mate with multiple partners, often one right after the other, and there is no evidence that males compete overtly with one another for access to mates.
There is no evidence of paternal or allo-parental care among the atelins. However, male howler monkeys will sometimes carry infants or position themselves between infants and extra-troop males, which may threaten infants in their efforts to take over female troops.
Reproductive seasonality varies widely across species and populations of the same species, with a tendency for more seasonal reproduction in more seasonal habitats. It is unclear whether reproductive seasonality reflects maternal condition at the time of conceptions, or the availability of food at the time of weaning. The tendency is for births to occur in the dry season when preferred fruits and new leaves are scarcest, and both conceptions and weaning to occur in the rainy season when preferred foods are most abundant.
Gestation length ranges from 6 months in Alouatta, to 7 months in Ateles, to 7.2 months in Brachyteles. Average interbirth intervals range from 2 years in Alouatta to 3 years in the atelins. Age at first reproduction for females ranges from about 4 years in Alouatta to at least 9 years in Brachyteles.
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