Strategic rejection of newborns is associated with substantially extended lifespan in parents and increased lifespan in well-reared offspring in owl monkeys

Abstract

Parental rejection of apparently healthy newborns occurs across captive primate colonies, yet its adaptive significance remains unclear. In owl monkeys (Aotus nancymaae), spontaneous parental rejection, defined here as cessation of nursing leading to rescue nursery rearing, is typically lethal for offspring and is nevertheless transmitted across generations despite reducing offspring survival. Here, we investigate whether rejecting behaviour, through selective withdrawal of maternal care, confers lifespan and reproductive benefits to parents and their surviving offspring. We analysed long-term demographic records from a captive colony comprising 962 individuals, from which multiple parent and offspring-level analytic datasets were derived. Survival and reproductive outcomes were compared between rejector and non-rejector parents using survival analyses and regression-based models. Parents who became rejectors lived significantly longer than non-rejectors, exhibiting an average lifespan advantage of approximately 4 - 4.5 years, with similar effects in males and females. Survival benefits were concentrated during prime reproductive years (6-20 years). Well-reared offspring of rejector parents also showed increased lifespan expectancy, living on average 1.26 years longer than offspring of non-rejectors. Rejector parents produced more total offspring than controls overall; however, this reproductive advantage was fully explained by extended lifespan rather than increased reproductive output per year. Survival analyses stratified by rejection timing revealed that first-birth rejectors showed no longevity advantage, whereas parents rejecting later-born offspring exhibited longer survival, consistent with strategic rather than inexperienced rejection. Together, these findings indicate that parental rejection, despite being lethal for some offspring, can confer substantial fitness gains to parents and their well-reared young under captive conditions. Rather than being purely maladaptive, neonatal abandonment may represent an extreme form of adaptive reduction in parental investment under environmental stress.