Monogamous primates living in small family groups often exhibit delayed patterns of reproduction. In many species, females will not conceive until after they have left the family group into which they were born and become established within a new group. There are a number of theories as to why this may happen, and through what means reproduction is suppressed. In some species, this lack of reproduction can be explained by either avoidance of inbreeding or alloparenting, both of which limit the reproductive options for some individuals in order to benefit the survival of the group as a whole. However, in owl monkeys, sub-adults living in their birth group have not been observed to help raise the younger group members, which negates the possibility of alloparenting. Furthermore, in situations where the breeding male in the group has been replaced (due to death, displacement by an outsider, or by keepers in a captive breeding group) and is not genetically related to young adult females, breeding still fails to occur, making the inbreeding avoidance hypothesis unlikely.
A 2017 paper by Margaret Corley et al., examined pregnanediol-3-glucuronide (PdG) and estrone-3-glucuronide (E1G) levels in juvenile and sub-adult females living in their birth group, and in solitary sub-adult females unaffiliated with any group to gain information on the mechanisms of reproductive suppression. Via the Owl Monkey Project, Corley et al studied wild free living monkeys within a 300 ha area – these monkeys are well-habituated to humans and have been observed and monitored since 1997. The long running observation means the approximate age of individuals can be reliably known within a range of a few weeks, and demographic changes such as births, deaths, and changes in group dynamic are also noted relatively quickly after they occur. Fecal samples from 11 identified individual monkeys were collected from the forest floor immediately after defecation, added to 5 ml of a 1:1 ethanol and DI water and shaken to homogenize. Samples were frozen as soon as was feasible and held at -20°C until needed. They were then extracted with diethyl ether and PDG and E1G levels measured using Arbor Assays kits (K037-H and K036-H respectively).
On average individuals were monitored for 129 ± 16 consecutive days between May and December in 2013, 2014 and 2015. This extended monitoring allowed the researchers to determine that the suppression of reproduction was not due to inhibition of ovarian function or delayed maturity. Average age for females to begin cycling was found to be 36 months, and all sub-adult females began to cycle prior to leaving their birth family group. The presence of unrelated adult males did not appear to affect the age at which cycling began in the females of a group. Cycles of the sub-adult females seemed to be in line with those of adult females in terms of both cycle length and maximum PDG and E1G levels observed, although some initial variability in luteal phase length was observed in the early cycles of some individuals. This suggests the sub-adult females are capable of breeding, even though they do not, and the mechanism of reproductive suppression is therefore likely behavioral rather than endocrinological. Further understanding of the endocrinology of these animals allows for better understanding of how to manage populations both in the wild and in a zoological setting.